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Reuse of Woodpecker cavities in the breeding and non-breeding seasons in old burn habitats in the Black Hills, South Dakota.

INTRODUCTION

The composition of a biological community is influenced by a number of factors including sudden changes in habitat structure (Terborgh, 1989; Vitousek, 1994; Sillett et al., 2000), and the presence of important species that provide resources to the rest of the community (Mills et al., 1993; Power et al., 1996; Kotliar, 2000). In forest ecosystems, cavity-excavating birds (e.g., woodpeckers and nuthatches) are considered a keystone guild because the cavities they create are important to a diverse group of secondary cavity users (Daily et al., 1993; Newton, 1994; Martin and Eadie, 1999; Bonar, 2000). The interactions of primary cavity users (cavity excavators) and secondary cavity users (non-excavators), as illustrated by the concept of a nest web (Martin and Eadie, 1999), have been well studied in western North America during the breeding season (Li and Martin, 1991; Daily et al., 1993; Dobkin et al., 1995; Aitken et al., 2002; Martin et al., 2004; Saab et al., 2004), but less is known about cavity use dynamics during the non-breeding season and in a multitude of different habitats.

Breeding season studies suggest that cavities are a limiting factor for breeding populations of secondary cavity users (Scott, 1979; Newton, 1994; Holt and Martin, 1997). Cavity selection for breeding is influenced by cavity volume (Dobkin et al., 1995; Bonar, 2000; Aitken et al., 2002; Aitken and Martin, 2004; Martin et at., 2004; Saab et al., 2004), placement within the landscape (Peterson and Gauthier, 1985; Dobkin et al., 1995; Aitken and Martin, 2004; Martin et al., 2004), tree and nests height (Saab et at., 2004), tree and snag species (Dobkin et al., 1995; Aitken et al., 2002; Aitken and Martin, 2004; Martin et al., 2004), diameter at breast height (dbh; Dobkin et al., 1995) and the original cavity excavator or most recent breeding user (Dobkin et al., 1995; Aitken et al., 2002; Aitken and Martin, 2004; Martin et al., 2004; Saab et al., 2004).

Roosting and other non-breeding season cavity uses have been less thoroughly studied for many taxa. Piliated Woodpeckers appear to be labile in their cavity use; they sometimes roost in different cavity than they breed in (Bull et al., 1992) while others use the same cavity for both activities (Bull, 1987), as will Lewis's Woodpeckers (Bock, 1970). Some avian species are thought to change their roost site every few weeks (Bull et al., 1992; Covert-Bratland et al., 2007), but others may reuse roosting cavities for longer periods (Kilham, 1971). Roosting behavior is better understood amongst bats, many of which change their non-breeding season roost site frequently (Lewis, 1995). This tendency is thought to reflect the need to avoid disturbances and predators, disrupt parasite life cycles, adjust to variations in microclimate and reduce the distance to foraging habitats (Russo et al., 2005). However the likelihood that a bat changes their roost site is directly related to roost site permanency and inversely related to roost availability. Whether a similar relationship exists for birds is unknown. During the non-breeding season, cavities are frequently used as refugia from severe weather and predation (Mayer et al., 1982; Bull et al., 1992; Cooper, 1999), thus the importance of suitable cavities within a habitat may also be underestimated with respect to the persistence of a population within a habitat.

Post-burn habitats generally contain more cavities than do unburned habitats (Kotliar et al., 2002). However, post-burn habitats also contain different communities of primary and secondary cavity users than do unburned forests. The influence of wildfire on breeding communities of cavity users is well documented (Raphael et al., 1987; Hutto, 1995; Kotliar et al., 2002), but most post-fire research to date in both breeding and non-breeding seasons has focused upon community makeup with little attention to cavity use (but see Saab et al., 2004). Our objectives for this study were to describe cavity use dynamics in old-burn habitats in the breeding and non-breeding season in the Black Hills, SD. Specifically, we examined the variables influencing breeding season occupancy of cavities previously used by Lewis's and Red-headed Woodpeckers and Northern Flickers, and we examined the relative importance of their cavities and the breeding cavities used by other primary and secondary cavity users during the non-breeding season.

STUDY SITES AND METHODOLOGY

STUDY SITES

The Black Hills of South Dakota and Wyoming are dominated by ponderosa pine (Pinus ponderosa) and contain patches of aspen (Populus tremuloides), paper birch (Betula papyrifera), bur oak (Quercus macrocarpa) and white spruce (Picea glauca; Shinneman and Baker, 19971. Short-grass prairie and shrub-steppe ecosystems isolate the Black Hills from similar habitats in the Rocky Mountains. The isolation of this forest, and its unique avian community, provide an ideal location for study of bird communities in forested habitats. The woodpecker community in the Black Hills contains North America's easternmost Lewis's Woodpecker (Melanerpes lewis) population and the westernmost population of Red-headed Woodpecker (M. erythrocephalus). Likewise, the avian secondary cavity user (SCU) community is different from nearby Rocky Mountain forests. The Mountain Chickadee (Parus gambeli) is absent, Pygmy Nuthatches (Sitta pygmaea) are less abundant, and White-breasted Nuthatches (Sitta carolinensis) are more abundant than in Rocky Mountain ponderosa pine forests (Tallman el al., 2002).

The historical disturbance regime of ponderosa pine forests in the Black Hills is diverse. The southern portion of the Black Hills, where our study sites were located, is dry and is dominated by short grass prairie and homogeneous ponderosa pine forests. Historically, low-intensity ground fires burned the southern Black Hills on a regular basis, which reduced understory growth and maintained lower stand density in pine dominated forests. However, European American settlement of the Black Hills has resulted in prevention of these low intensity wild fires, effectively increasing fuel loads and thus, the frequency of stand-replacing wild fires.

We established study sites in Wind Cave National Park (WICA) and Custer State Park (CUSP) to examine the effects of wild fire upon avian communities (Fig. 1). These parks are found in the southeastern portion of the Black Hills. Although portions of CUSP also include significant patches of white spruce and aspen, the study site within CUSP did not contain these species to facilitate comparison between the sites. Each park contained a study site, 200-400 ha in size, within an area burned 11-14 y prior to initiation of the study. The WICA site (43[degrees]34'N, 103[degrees]29'W) was burned by the Shirttail fire in 1991. The portions of this site within WICA were not logged, but portions of this study site found in adjacent National Forest land were partially salvaged post-burn. The CUSP site (43[degrees]47'N, 103[degrees]21'W) was burned by the Galena fire in 1988 and was also partially salvaged. Neither study site experienced any clear-cut salvage logging. All of the sites contained homogeneous ponderosa pine forests with some bur oak in wetter draws. The shirttail site was searched during the breeding seasons of 2002-2005 and the galena site was searched during the summers of 2003-2005. The post-burn habitats found within these two study sites were influenced by varying fire intensities, the result of which is a mosaic of habitats including areas dominated by mature trees with a reduced understory to habitats with little to no living trees and a high snag density. At this stage of post-fire succession, most of the small diameter snags have fallen over and the remaining snags have a larger diameter, a broken top and are extensively decayed (Gentry pets. obs.). The sites were similar in canopy cover, vegetative diversity and disturbance history. Density of trees and snags with a dbh [greater than or equal to] 23 cm did not differ between the two sites (trees: WICA 39.6 [+ or -] 12.5, CUSP 77.9 [+ or -] 24.7, [F.sub.1,38] = 1.9, P = 0.17; snags: WICA 9.5 [+ or -] 3.7, CUSP 21.0 [+ or -] 6.4, [F.sub.1,38] = 2.4, P = 0.13). The similarities in vegetation structure and composition in green forest and old-burn sites allowed us to pool data from both parks.

NEST SEARCHING AND MONITORING

We followed nest-searching methods described by Martin and Geupel (1993) and the Birds and Burns Network (2005) and used the survey design of Dudley and Saab (2003) to comprehensively survey the study area, and reduce the chance of missing active woodpecker nests. Transects were established every 200 m on each survey unit and labeled alphabetically. We initiated nest searching on 20 May and continued until all transects had been surveyed. Although some early nesters (e.g., bluebird spp., nuthatch spp., Northern Flicker) may have initiated nests before this date, most nest initiations take place after this date (Tallman et al., 2002) and we were limited by scheduling of our undergraduate field assistants and training time and thus, 20 May was the earliest possible starting date. Nest searchers walked each transect and included the 100 m on each side of the transect line, looking for adult cavity nesters until the entire unit was surveyed. We followed adult cavity nesters in hopes of witnessing breeding behaviors such as carrying food or fecal sacs, collecting nesting material, excavating a cavity, copulation or courtship behavior. We followed adults to their cavities and once found, we used a Tree Top Peeper #2 (manufactured by Sandpiper Technologies Inc., Manteca, CA) to view the contents of each cavity. Occupancy was established after at least one egg was laid. We monitored each active nest twice each week until the nest failed or the young fledged. On a nest card, we described the cavity and microhabitat characteristics including: snag and cavity height, cavity age (new if excavation was witnessed, old or unknown), snag species, decay class, diameter at breast height (dbh) of cavity containing snag, condition of the snag top, original cavity excavator (if known), compass orientation of the cavity entrance and coordinates using a GPS.

[FIGURE 1 OMITTED]

We attempted to find nests of all strong and weak cavity excavators (primary cavity users), and some secondary cavity users present on our plots. Strong cavity excavators are those species who generally excavate a new cavity every year, weak excavators can excavate new cavities, but frequently reuse old cavities when they are available, and secondary cavity users are those species that use cavities, but can not excavate their own. The species whose nests we monitored included weak excavators Lewis's Woodpeckers (LEWO; Melanerpes lewis), Red-headed Woodpeckers (RHWO; M. erythrocephalus), Northern Flickers (NOFL; Colaptes auratus), Red-breasted Nuthatches (RBNU; Sitta canadensis), and White-breasted Nuthatches (WBNU; S. carolinensis), the strong excavator Hairy Woodpecker (HAWO; Picoides villosus), and the secondary cavity users (non-excavators) Eastern (EABL; Sialia sialis) and Mountain (MOBL; S. currucoides) Bluebirds. In accordance with the Birds and Burns Network protocols (2005) and time limitations, we did not follow the nests of some cavity nesters that were more common or whose populations are not a primary concern; those species included American Kestrels, (AMKE; Falco sparverius), Black-capped Chickadees (BCCH; Poecile atricapilla), and House Wrens (HOWR; Troglodytes aedon).

CAVITY USE MONITORING

Past studies have found that the original cavity excavator was an important factor influencing the likelihood of a cavity being reused (Dobkin et al., 1995; Aitken et al., 2002; Aitken and Martin, 2004; Martin et al., 2004; Saab et al., 2004). While we could not always confidently identify the original excavator of each cavity, the most recent breeding occupant might also be important with respect to understanding cavity dynamics because cavity entrance and volume are often modified by successive occupants. Therefore, original excavator was documented if it could be determined, but for both breeding and non-breeding season cavity dynamics, cavities were classified by their most recent breeding occupant. We monitored breeding and non-breeding season reuse only in cavities that contained an active nest during one of the previous breeding seasons. We measured the most likely relevant microhabitat variables to help determine which factors influence the use or non-use of each cavity. We checked the occupancy of each cavity twice during each of the four seasons of the year. The two breeding season checks were timed to ensure that early nesters, such as Northern Flickers, nuthatches and bluebirds would not be missed, and the second check was late enough to ensure that birds that initiate nests later in the season, such as Lewis's and Red-headed Woodpeckers, would not be missed. Although some nesting attempts may have been missed, checks of old cavities were being conducted concurrently while searching for new nests in the same sites and thus we suspect that few nesting attempts were undocumented. If cavities were found to contain active nests, we continued to monitor that nest to determine success or failure. We checked cavities during the non-breeding seasons beginning at dawn and continued through the daylight hours in hopes of finding both nocturnal and diurnal species roosting. Non-breeding season cavity checks were done roughly during the middle of each season. Fall checks were completed between 20 Aug. and 1 Oct., spring checks between 15 Mar. and 1 May and winter checks were completed between 15 Nov. and 15 Feb. Each pair of seasonal cavity checks was completed within a week of each other (except for breeding season) with at least one day between checks to ensure the independence of each check. Cavity checks were conducted during all weather conditions except during precipitation events because the cavity peeper was not waterproof.

We scratched or knocked on all cavity trees before inspection with the Tree Top Peeper #2. When cavities were occupied, the animal usually responded to the scratching and knocking, as was found by Bonar (2000), so we pooled observation data for trees that were too tall to peep with those that were not. Cavities with small entrance diameter, such as those excavated by Red-breasted (Sitta canadensis) and White-breasted (S. carolinensis) Nuthatches were peeped with the smaller diameter Tree Top nuthatch camera (Sandpiper Technologies) when it fit, and were monitored by scratching and knocking when it would not.

At each check, we noted the time and duration of each visit, date, temperature and if the cavity was occupied. When a cavity was occupied or showed evidence of recent occupancy, we attempted to identify the species of the occupant. When we could not identify the animal to the species level, we pooled the observation into larger taxonomical group (e.g., bat or mouse). We noted whether the evidence of occupancy was by directly viewing the animal or by indirect evidence of recent use such as the presence of feathers, fur, scat, fresh excavation, addition or subtraction of nest material or other.

OCCUPANCY MODELING

Occupancy modeling could not be conducted on non-breeding season data because we could not prove that a given cavity was never used. Therefore, occupancy modeling was only conducted on breeding season data. There was no difference in breeding season cavity reuse among years ([chi square] = 0.2, df = 2, P > 0.05), so breeding season data were pooled between years. We used logistic regression to model the probability of cavity reuse based upon observation of cavities during the breeding season. We generated an a priori candidate global model for each of the common nesting species in the old burns (Northern Flickers, Red-headed Woodpeckers and Lewis's Woodpeckers) based upon uncorrelated biotic and abiotic factors likely to influence cavity occupancy noted elsewhere in the literature (Bonar, 2000; Li and Martin, 1991; Aitken et al., 2002; Martin et al., 2004; Saab et al., 2004). Covariates used in the various models included cavity height, diameter at breast height (dbh) of the cavity containing snag, decay class of the cavity containing snag during the year of cavity occupancy (0 = live, 4 = most decayed), density of live trees with a dbh [greater than or equal to] 23 cm, density of snags with a dbh [greater than or equal to] 23 cm, year postfire (each year is a single parameter) and study unit (either Galena or Shirtail). We used a Hosmer-Lemeshow goodness-of-fit test (Hosmer and Lemeshow, 2000) to assess how well the global model fit the data for each species. The overdispersion parameter e was calculated to determine if a quasi-likelihood correction was necessary (Burnham and Anderson, 2002). Additionally, we examined the deviance residuals for the characteristics measuring temporal (year) and habitat (unit) patterns to test for evidence of temporal or spatial autocorrelation. We ranked candidate models using Akaike's Information Criterion (AICc) corrected for small sample size (Burnham and Anderson, 2002) to determine which factor or combination of factors influenced cavity occupancy. Ranking was based upon the AAICc, which is the difference between a candidate model's AICc score and the lowest AICc score of all the models in question. As a general rule, a AAICc score between 0-2 shows high support for that model, a [DELTA]AICc score from 4-7 indicates considerably less support, and a [DELTA]AICc score higher than 7 indicates almost no support for that model (Burnham and Anderson, 2002).

RESULTS

NESTS FOUND--BREEDING SEASON CAVITY REUSE

We located 179 nests of 9 cavity nesting species in 137 cavities dining the summers of 2002-2005. Nest searching in 2002 yielded 30 nests. In 2003, we found 49 nests, nine of which were in cavities that contained active nests in 2002 (18%). Seventeen of the 51 active nests found in 2004 (33%) and 16 of the 49 active nests found in 2005 (30%) were in old cavities that were active in one or more of the previous study years. In total, 42 of the 179 nesting attempts (23%) were in 30 of the old cavities that we monitored in previous years. Of the remaining 137 nests, 28 (20%) were in freshly excavated cavities and the remaining nests were in cavities that were obviously old (wood around cavity entrance was heavily weathered) or where the cavity age could not be determined. Four of the 30 reused cavities contained active nests in 3 of 4 y and two of the 30 contained an active nest during all 4 y of the study. Fifteen percent of the cavity containing snags (21 of 137) had fallen over by the end of the breeding season in 2005.

Weak cavity excavators were the dominant guild of cavity nesters in these old-burn habitats; the nests of Red-headed Woodpeckers, Lewis's Woodpeckers and Northern Flickers comprised 78% of the nests found. Active nests of only one strong excavator, the Hairy Woodpecker, were found throughout the duration of this project (Table 1). The only non-excavators that we monitored the nests of were bluebirds; we found six Eastern Bluebird nests and 18 Mountain Bluebird nests. Of the strong and weak cavity, excavators, Hairy Woodpeckers were the only species that nested exclusively in freshly excavated cavities. Conversely, we never found strong evidence of a Lewis's Woodpecker excavating a new cavity. Red-headed Woodpeckers freshly excavated at least 32% (12 of 38) of their cavities and Northern Flickers excavated at least 20% (9 of 46) of their cavities. Lewis's and Red-headed Woodpeckers nested in the highest cavaties. The median cavity heights for these two species were not significantly different from each other (Kruskal-Wallis Test, [H.sub.1] = 0.02, P = 0.88), but were higher than Northern Flicker and Hairy Woodpecker cavities ([H.sub.3] = 38.0, P < 0.001). Northern Flickers nested in the largest dbh cavities (Table 1).

Reuse of cavities tended to take place in consecutive years. Of the 42 nests in previously active cavities, seven (17%) were in non-consecutive years. Cavities were generally reused by the same species. Only seven of the 42 cavities used in multiple years were occupied by two different species. In five of the seven cases, Northern Flickers nested in cavities that previously contained the nests of Hairy Woodpeckers (n = 2), Lewis's Woodpeckers (n = 2) or Red-headed Woodpeckers (n = 1). Lewis's Woodpeckers were the secondary user in the other two cavities; one of which was previously used by a Northern Flicker, the other by a Red-headed Woodpecker (Fig. 2). No cavity that contained a failed nest in 1 y was ever reused in the following year, but eight of the 42 (19%) nests that were in previously used cavities failed.

[FIGURE 2 OMITTED]

BREEDING SEASON CAVITY USE MODELING

We tested 14 models made up of various combinations of seven variables to determine how those variables influenced the likelihood that cavities previously occupied by Redheaded and Lewis's Woodpeckers and Northern Flickers were reused by any cavity nesting species for breeding purposes (Table 2). The Hosmer-Lemeshow goodness of fit test indicated that the global model adequately fit the data for all three species (Red-headed Woodpecker P = 0.62, Lewis's Woodpecker P = 0.85, Northern Flicker P = 0.43). We did not apply a quasi-likelihood correction to the ranking criterion because the overdispersion parameters for the global models were near 1. Additionally, there were no strong indicators of spatial or temporal autocorrelation so, we lumped data from all years and study units into one dataset. Re-occupancy of Lewis's Woodpecker cavities was explained by a single model that including positive relationships with cavity height, wildlife snag density and snag decay (Tables 2 and 3). There was support for two models for re-occupancy of Red-headed Woodpecker and Northern Flicker cavities. For Red-headed Woodpeckers, both models showed a positive relationship with cavity height and a negative relationship with DBH. The model with best support also showed a positive relationship with decay class and the second best model showed a positive relationship with snag density (Tables 2 and 3). The two best models for re-occupancy of Northern Flicker cavities contained a positive relationship with snag density; the best fitting model also showed a negative relationship with cavity height and the second best model showed a negative relationship with DBH (Tables 2 and 3).

NON-BREEDING SEASON CAVITY OCCUPANCY

Beginning in the fall of 2002 and concluding in the spring of 2005 we completed 880 checks of non-breeding season cavity occupancy. Throughout the duration of the study, we found active nests in 137 cavities, 33 of which were found during the last summer and, therefore, never received non-breeding season checks. Forty percent (41 out of 104) of the cavities were occupied or showed indirect evidence of recent occupation (e.g., presence of fresh material) at one or more of its checks, but only 8% (67 of 880) of the actual checks were positive. Frequency of occupied cavities checks corrected for equal effort did not significantly differ between non-breeding seasons (uncorrected fall = 23, winter = 16, spring = 19; [chi square] = 1.0, df = 2, P = 0.61) or among study years ([chi square] = 1.9, df = 2, P = 0.39) so, we pooled data from all years and non-breeding seasons. Of the 41 cavities that showed evidence of occupancy, 22 were also re-used at least once for breeding purposes.

Occupant guild could be determined in 51% of positive observations (34 of 67). The one non vertebrate was a relatively large insect that appeared to be a member of the coleoptera order and the rest were of birds (n = 13) and mammals (n = 22). Mammals were found most frequently (21 of 22 observations) in the breeding cavities of the three large bodied woodpeckers (RHWO, LEWO, NOFL) whereas, avian cavity users showed less of a trend, but tended to be in similar sized cavities to what they would likely breed in (e.g., American Kestrels in large bodied woodpecker cavities, bluebird spp. in bluebird cavities). In cases where the species could be identified, the most common occupants were Northern Flying Squirrels (n = 9; Glaucomys sabrinus; GLSA), although bats were also relatively common (n = 6; Table 4). Of the 33 observations where guild or species could not be determined, the observations were of fecal material, hair or feathers, addition or removal of nest material or enlargement modification of the cavity entrance.

We constructed a nest web (Martin and Eadie, 1999) to display the relationship existing between cavities and secondary cavity users during the non-breeding season (Fig. 3). Cavities excavated by Hairy Woodpeckers had the highest percentage of use (15%), but Northern Flicker cavities were used by the most diverse group of secondary cavity users (Fig. 3).

In general, the more common secondary cavity users were restricted to the cavities of large-bodied woodpeckers. American Kestrels, Northern Flying Squirrels and bats (unknown species) were all found using only Lewis's Woodpecker, Red-headed Woodpecker and Northern Flicker cavities. Mice were also detected most frequently in the cavities of these species; 6 of the 7 mouse observations were in their cavities with the only exception occurring in a Hairy Woodpecker cavity. Smaller avian cavity users (e.g., nuthatch spp. and Black-capped Chickadees) were relatively uncommon as non-breeding season cavity users in old-burn habitats. Our only observation of them potentially using a cavity in old-burn habitats was an instance when the cavity peeper was aggressively attacked by a mixed flock of Black-capped Chickadees and White-breasted Nuthatches while peeping an otherwise unoccupied Red-headed Woodpecker cavity. This type of behavior was rare during non-breeding season checks which led us to believe that this cavity may have been a roost cavity. However, in nearby unburned forest habitats where these species are more likely to be observed (Kotliar et al., 2002), we observed direct and indirect evidence of these species using cavities excavated by Red-breasted Nuthatches (Gentry unpubl.).

DISCUSSION

BREEDING SEASON CAVITY USE

The old-burn habitats examined in this study contained a unique community of cavity users dominated by three species of large-bodied weak cavity excavators, Northern Flickers, Red-headed Woodpeckers and Lewis's Woodpeckers. These species generally nested in existing cavities and their cavities were generally reused by the same species for breeding purposes every year.

[FIGURE 3 OMITTED]

Snag density was a factor that appears to have influenced reuse of breeding cavities for all three woodpecker species. Lewis's and Red-headed Woodpeckers are both aerial insectivores and use habitats with an abundance of perches with an open view for flycatching (Tobalske, 1997; Smith et al., 2000), therefore, post-burn habitats that contain an abundance of snags would facilitate foraging. Conversely, Northern Flickers are ground feeders and generally forage in open savannah like habitats (Moore, 1995). Thus, these species may be found in post-burn habitats because high intensity wildfires reduce the forest understory, creating open habitats in a mosaic with dense shrubby understory. Post-burn habitats with a high snag density provide not only snags for nesting, but also create a matrix of open foraging habitat for all three woodpecker species.

Cavities in heavily decayed snags are thought to be more vulnerable to predators (Nilsson, 1984; Li and Martin, 1991; Christman and Dhondt, 1997), and thus, are selected against by some species. Weak cavity excavators on the other hand, appeared to preferentially excavate in more heavily decayed snags. Nest failure caused by predation was unusually low in these habitats (Gentry and Vierling, 2007) and, therefore, may increase the likelihood of cavities in heavily decayed snags being reused. Weak excavators likely choose heavily decayed snags because the rotten wood allows for easier excavation and enlargement of cavities; and the use of heavily decayed snags by Lewis's Woodpeckers has been noted elsewhere (Bock, 1970; Saab and Vierling, 2001).

While cavity height was positively associated with occupancy for Lewis's and Red-headed Woodpeckers cavities, it was negatively associated with occupancy for Northern Flicker cavities. The tendency of Northern Flickers to use lower cavities in this study may have been related to habitat selection or possibly interspecific interactions. Both Red-headed and Lewis's Woodpeckers are noted cavity usurpers (Tobalske, 1997; Smith et al., 2000; Saab et al., 2004) and because lower cavity height has been negatively associated with nest success (Nilsson, 1984; Li and Martin, 1991; Christman and Dhondt, 1997) it is possible that Redheaded and Lewis's Woodpeckers prevented Northern Flickers from nesting in the higher cavities. The mean cavity height we report (5.4 [+ or -] 0.5 m) is lower than nearly every other mean nest height reported for Northern Flickers in forest habitats (see citations in Moore, 1995). Although the potential interaction with Red-headed and Lewis's Woodpeckers was not clearly stated in every report, based upon location, we can be certain that in none of those studies did Northern Flickers have to compete with both species. Therefore, Northern Flickers may have been relegated to excavating and reusing lower cavities in this study because of the abundance of these two domineering species in the old-burn habitats.

The significant difference in cavity height between Hairy Woodpeckers nests, the most common strong excavator in the Black Hills (but not necessarily in this habitat), and those of the three large bodied woodpeckers suggests that at least some of the cavities used by Redheaded and Lewis's Woodpeckers were not originally excavated by Hairy Woodpeckers. The degree to which Lewis's Woodpeckers excavate their own cavities varies. Raphael and White (1984) found that Lewis's Woodpeckers were the original excavators of all of the nesting cavities they used, however, we found no evidence of this species excavating a new cavity; similar to the findings of Saab and Dudley (unpubl. cited in Tobalske, 1997) in post-burned forests in Idaho. Future studies should attempt to determine the conditions under which this species is more or less likely to excavate a new cavity, rather than using an existing one. Based upon the similarities in height, we are assuming that Red-headed Woodpeckers are the source of many of the cavities used by these two species. However, range overlap between these species is limited to the Black Hills and portions of Nebraska and Colorado, so in the western portions of the Lewis's Woodpeckers range, they are either more willing to excavate cavities of their own, or are using cavities provided by another primary excavator species.

Only seven of the 42 nesting attempts in previously monitored cavities occurred with > 1 y between attempts. The tendency of cavities to be reused in consecutive years or not at all was also documented by Aitkin et al. (2002), and suggests that cavities continue to be used for breeding until they are no longer suitable for nesting. The reason that cavities become less likely to be used with age may be due to physical deterioration of the cavity, build of up parasites, modification of the cavity or entrance size or they may be stuffed with nesting material from a previous nesting attempt (Newton, 1994).

NON-BREEDING SEASON CAVITY USE

This is the first study to examine the non-breeding season cavity use from a community perspective. The post-burn habitats used in this study likely influenced what secondary cavity user species would be found and, therefore, we do not suggest our study as a survey of potential secondary cavity users, but rather a relative comparison amongst different cavity types found within a post-burn mosaic. We focused on relative use of cavities during the non-breeding season rather than an absolute determination of cavity use, which could not be done without video monitoring of each cavity. We are basing our examination on a large number of cavity checks and a relatively large number of cavities, but checks were relatively infrequent compared to the time period covered. Our methods may have missed some indirect signs of cavity occupancy, but those errors were likely consistent between all cavity checks and among all species.

The only other study addressing non-breeding season cavity use was Bonar's (2000) study of non-breeding season use of Pileated Woodpecker (Dryocopus pileatus) cavities in the Northwest. Bonar (9000) showed a higher percentage of cavity use (53.5%) than we found (40%). However he inspected cavities in every month of the year, and climbed cavity trees and manually inspected many of his cavities, the combination of which likely would have yielded more observations than did the our method of seasonal inspections and use of the cavity peeper. Additionally, Bonar (9000) may also have found a higher occupancy rate because he was only examining cavities excavated by Pileated Woodpeckers, which is one of the largest primary cavity excavators in North America and cavity volume is thought to have a positive influence upon cavity use in the non-breeding and breeding seasons (Peterson and Gauthier, 1985; Dobkin et al., 1995; Aitken et al., 9002; Aitken and Martin, 9004; Martin et al., 2004; Saab et al., 2004).

Our inspection of cavities included only those that had been used for breeding so we did not include shelf cavities or partially excavated cavities that, while not suitable for breeding, may be used by some secondary cavity users (Bonar, 2000). Larger secondary cavity users and most mammalian cavity users were found in the cavities of large bodied cavity excavators, presumably because of the larger entrance diameter and internal volume. Landscape variables may also have influenced which cavities were occupied for some species, for instance, Northern Flying Squirrels are associated with unburned forests (Wells-Gosling and Heaney, 1984) and in this study they were found only in cavities that were <25 m from burn mosaic borders where live mature trees were present (Gentry unpubl.).

Motivation for cavity occupancy clearly is impossible to determine. The presence of nocturnal species like bats, flying squirrels and mice in cavities during the day can be assumed to be daytime roosts. However the daytime presence of diurnal species in cavities is open for speculation. The bluebirds found using cavities in the spring may have been prospecting for nest sites. Similarly, the chickadee and nuthatch winter observations were not of a roosting bird, but of a mixed species flock of 5 individuals who all entered a Red-breasted Nuthatch cavity one at a time and then flew off. This happened prior to the observers approach to the cavity and was thought to be an example of these birds searching for suitable roost cavities (Gentry pers. obs.). One of the kestrel spring observations was during inclement weather during which we guessed the cavity was being used as a thermal refuge. Two of the observations of woodpeckers in cavities during the fall were of a single juvenile Red-headed Woodpecker that was twice found in a cavity that had been newly excavated and successfully nested ill by Hairy Woodpeckers the preceding summer. We are guessing that the cavity was being used as a resting spot by the juvenile as they were resting or awaiting a delivery of food from a parent.

Cavity volume appears to have been an important factor influencing non-breeding season cavity use in the Black Hills. The cavities of the three large-bodied woodpeckers commonly found on our sites (LEWO, RHWO, NOFL) were generally similar in size to each other and were larger than the cavities of most other cavity users in these habitats (Gentry unpubl.). Among the secondary cavity users we detected, American Kestrels, Northern Flying Squirrels, bat spp. and mouse spp. were found exclusively using the cavities of these three species. This apparent reliance upon cavities excavated or enlarged by large bodied woodpeckers supports the suggestions that cavities with a large internal volume are important to maintaining community diversity (Peterson and Gauthier, 1985; Dobkin et al., 1995; Bouar, 2000; Aitken et al., 2002; Aitken and Martin, 2004; Martin et al., 2004; Saab et al., 2004). Therefore, at least in the Black Hills, species that enlarge cavities may be performing a keystone role just as important as are the original cavity excavators.

Northern Flickers are generally considered strong excavators (Moore, 1995), but in our study they choose to enlarge or reuse existing cavities more frequently than they excavated fresh cavities. The original excavator of most Northern Flicker cavities was unclear, but Northern Flickers enlarged two of the six Hairy Woodpecker cavities we monitored tot more than 1 y. Additionally, Northern Flicker cavities were used by more species of secondary cavity user than were the cavities of any other species, and we speculate that this may be due to the lower height of Northern Flicker cavities in our study. There is relatively little literature covering the non-breeding season roosting ecology of secondary cavity users, but because roost cavities can be changed more often than can nesting cavities (i.e., daily), the higher predation risk associated with lower cavities may be less relevant than is cavity accessibility. Bakker and Hastings (2002) found that Northern Flying Squirrels primarily used lower cavities and they speculated that this was due to low predation pressure in the Alaskan forests they studied. Although our study sites are structurally different from their forests, there is some evidence that the Black Hills have a similar paucity of cavity nest predators (Gentry and Vierling, 2007). Therefore, Northern Flicker cavities which are relatively large in volume and low in height (in this study) appear to be important to the secondary cavity user community. Because Northern Flickers are serving an important role as both a cavity excavator and enlarger, they should receive further attention regarding their possible role as a keystone cavity excavator in this system.

Much is left to be learned about reuse of cavities in the breeding and non-breeding seasons. Old-burn habitats in the Black Hills contain a unique community of cavity users and potential predators (Gentry and Vierling, 2007), therefore, similar studies should be conducted in myriad other habitats and regions to get a better understanding of how the local community influenced the cavity use patterns we documented. The observation that lower cavities received greater use in the non-breeding season needs extensive testing in order to determine if this function varies by season, region and guild. Additionally, the role of cavity excavators vs. enlargers also is somewhat inconclusive and future studies should strive to determine the relative importance of cavity height, age, volume and micro and macro habitat variables to cavity use in the non-breeding season. In order to draw these conclusions, researchers will have to take advantage of new nest monitoring technologies that determine definitively use patters for each individual cavity. The different cavity use patterns observed in the non-breeding season, compared to the breading season suggests that the role of keystone cavity provider may change with use patterns associated with breeding and non-breeding seasons.

Acknowledgments.--This work was supported by NSF grant DEB 0133854 to KTV. We thank Wind Cave National Park and Custer State Park for allowing use of their lands for research. We are particularly grateful to Barbara Mencheau and Gary Brundidge for their assistance in providing housing and logistical support for our staff of fieldworkers every summer. Lastly, we thank the 15 field assistants who assisted in data collection.

SUBMITTED 29 JANUARY 2007

ACCEPTED 6 MARCH 2008

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DALE J. GENTRY (1)

Atmospheric and Environmental Sciences, South Dakota School of Mines and Technology, 501 East Saint Joseph Street,

Rapid City 57701

AND

KERRI T. VIERLING

Fish and Wildlife Resources, University of Idaho, Moscow 83844

(1) Current address: Teton Science Schools, P.O. Box 68, Kelly, Wyoming 83011
TABLE 1.--Cavity and snag characteristics (mean [+ or -] SE)
for nests of primary and weak cavity excavator species in
old-burn habitats in the breeding seasons of 2002-2005

                            Nesting
                            density        Cavity height
Species               n    (nests/ha)           (m)

Lewis's Woodpecker   55      0.027        8.6 [+ or -] 0.4
Red-headed
  Woodpecker         38      0.018        8.7 [+ or -] 0.5
Nortern Flicker      46      0.022        5.4 [+ or -] 0.5
Hairy Woodpecker     9       0.004        3.9 [+ or -] 1.0
Red-breasted
  Nuthatch           2       0.001        2.1 [+ or -] 0.5
White-breasted
  Nuthatch           4       0.002        1.3 [+ or -] 0.3
Pygmy Nuthatch       1      <0.001              5

                         Snag decay            Snag dbh
Species                    class                 (cm)

Lewis's Woodpecker   2.3 [+ or -] 0.09     38.5 [+ or -] 1.0
Red-headed
  Woodpecker         2.1 [+ or -] 0.07     38.2 [+ or -] 1.4
Nortern Flicker      2.1 [+ or -] 0.06     38.4 [+ or -] 1.2
Hairy Woodpecker     2.1 [+ or -] 0.1      27.6 [+ or -] 2.1
Red-breasted
  Nuthatch           2.2 [+ or -] 0.1      26.6 [+ or -] 2.2
White-breasted
  Nuthatch           2.0 [+ or -] 0        24.7 [+ or -] 4.4
Pygmy Nuthatch              2                      24

                          % of cavities
Species                   newly excavated

Lewis's Woodpecker              0%
Red-headed
  Woodpecker                   32%
Nortern Flicker                20%
Hairy Woodpecker              100%
Red-breasted
  Nuthatch                      0%
White-breasted
  Nuthatch                      0%
Pygmy Nuthatch                100%

TABLE 2.--Model selection results based upon logistic
regression modeling of cavities re-occupied in the breeding
seasons of 2002-2005. All models with a DAIC score of G2 are
reported and are ranked from best fitting ([DELTA]AIC = 0) to
less likely plus the global model. k is the number of
parameters in each model. [w.sub.I]/[sub.w]t is the ratio of
Akaike weights and describes the plausability of the best
fitting models compared to the other models. Number in
parentheses represents the number of samples input into
each model

                                         Log         [DELTA]
Candidate model                      likelihood        AIC        k

Lewis's Woodpecker--Melanerpes
  lewis (n = 56)

Model 1: cavity height, large
  snag density, decay                  -22.264        0.000       3
Global model: dbh, decay, cavity
  height, large snag density,
  tree density, year, unit             -20.082       14.063      10

Northern Flicker--Colaptes
  auratus (n = 31)

Model 1: cavity height, large
  snag density                         -15.241        0.000       2
Model 2: dbh, large snag density       -15.576        0.670       2
Global model: dbh, decay, cavity
  height, large snag density,
  tree density, year, unit             -13.140       22.370      10

Red-headed Woodpecker--Melanerpes
  nythrocephalus (n = 35)

Model 1: decay, dbh, cavity height     -12.274        0.000       3
Model 2: large snag density,
  dbh, cavity height                   -12.700        0.852       3
Global model: dbh, decay, cavity
  height, large snag density,
  tree density, year, unit              -9.805       17.455      10

                                       Akaike
                                       Weight      ([w.sub.I])
Candidate model                      ([w.sub.t])   ([w.sub.t])

Lewis's Woodpecker--Melanerpes
  lewis (n = 56)

Model 1: cavity height, large
  snag density, decay                   0.89          1.00
Global model: dbh, decay, cavity
  height, large snag density,
  tree density, year, unit              0.00          0.00

Northern Flicker--Colaptes
  auratus (n = 31)

Model 1: cavity height, large
  snag density                          0.29          1.00
Model 2: dbh, large snag density        0.21          0.72
Global model: dbh, decay, cavity
  height, large snag density,
  tree density, year, unit              0.00          0.00

Red-headed Woodpecker--Melanerpes
  nythrocephalus (n = 35)

Model 1: decay, dbh, cavity height      0.36          1.00
Model 2: large snag density,
  dbh, cavity height                    0.24          0.67
Global model: dbh, decay, cavity
  height, large snag density,
  tree density, year, unit              0.00          0.00

TABLE 3.--Parameter estimates [+ or -] SE, and adjusted odds
ratios from the best supported models (Table 4) for predicting
probability of reoccupancy of nest cavities by Lewis's
Woodpeckers, Northern Flickers and Red-headed Woodpeckers in
old burned forests in the southern Black Hills, South Dakota
during 2003-2005. Odds ratios indicate the odds of occupancy
for every 1-unit change in the variable. Confidence limits that
do not contain 1 are thought to be statistically significant

Cavity user                        Parameter estimate
species        Parameter value        [+ or -] SE

Lewis's        Decay class         1.94 [+ or -] 0.60
Woodpecker     Cavity height       0.74 [+ or -] 0.26
               Snag density        0.03 [+ or -] 0.01

Northern       Model 1
Flicker        Snag density        0.02 [+ or -] 0.01
               Cavity height      -0.12 [+ or -] 0.15
               Model 2
               Snag density        0.02 [+ or -] 0.01
               DBH                -0.02 [+ or -] 0.06

Red-headed     Model 1
Woodpecker     Cavity height       0.52 [+ or -] 0.27
               DBH                -0.23 [+ or -] 0.12
               Decay class         2.04 [+ or -] 1.76
               Model 2
               Cavity height       0.51 [+ or -] 0.27
               DBH                -0.18 [+ or -] 0.09
               Snag density        0.01 [+ or -] 0.02

                                   Adjusted odds ratio

Cavity user                                   Confidence
species        Parameter value    Estimate      limits

Lewis's        Decay class          7.0        2.1-22.7
Woodpecker     Cavity height        2.1        1.3-3.5
               Snag density         1.0        1.0-1.1

Northern       Model 1
Flicker        Snag density         1.0        0.9-1.1
               Cavity height        0.9        0.6-1.2
               Model 2
               Snag density         1.0        1.0-1.1
               DBH                  1.0        0.9-1.1

Red-headed     Model 1
Woodpecker     Cavity height        1.7        1.0-2.9
               DBH                  0.8        0.6-1.0
               Decay class          7.7        0.2-241.9
               Model 2
               Cavity height        1.7        1.0-2.8
               DBH                  0.8        0.7-1.0
               Snag density         1.0        1.0-1.1

Table 4.--Seasonal occurrence (number of observations) of
cavity user guilds found during non-breeding season checks of
previously active cavity nests in old burn habitats in the
Black Hills, SD

                                      Season

Guild                        Fall     Winter    Spring    Totals

Bat                           3         2         1         6
Terrestrial Small Mammal      9         5         2        16
Woodpecker                    1         0         1         2
Bluebird                      3         0         2         5
Nuthatch /chickadee           0         1         0         1
American Kestrel              1         0         2         3
Insect                        0         0         1         1
Totals                       18         8         8        34
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Author:Gentry, Dale J.; Vierling, Kerri T.
Publication:The American Midland Naturalist
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Date:Oct 1, 2008
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