Nesting success of grassland birds in shinnery oak communities treated with tebuthiuron and grazing in Eastern New Mexico.
In New Mexico, sand shinnery oak (Quercus havardii) communities were historically co-dominated by shrubs and grasses (Martin, 1990; New Mexico Partners in Flight, http://www.nmpartnersinflight.org/bcp.html). There is no evidence that shinnery oak invades overgrazed rangeland, but oak is an effective water-gatherer, and when given an advantage may almost eliminate associated plants due to effects of shading and competition for moisture (Peterson and Boyd, 1998). Thus, unmanaged grazing can change community composition, resulting in decreased grass production and greater frequency of shinnery oak (Peterson and Boyd, 1998). In many areas, shinnery oak has become essentially a monoculture, and there is interest in restoring communities to a more historic grass-shrub balance.
Grazing and herbicide application, common management practices used to restore sand shinnery oak communities, may affect nest success through influence on available nesting substrate. In New Mexico, the predominant method of control of shinnery oak is application of herbicide (Peterson and Boyd, 1998). During 1981-1993, the United States Bureau of Land Management treated 40,469 ha of shinnery oak in New Mexico with the herbicide tebuthiuron (N-[5-(1,1-dimethylethyl)-1, 3, 4-thiadiazol-2-yl]-N, N'-dimethylurea; United States Department of the Interior, 1997). Depending on rate of application, treatment with tebuthiuron tends to decrease vertical screening immediately after application as the shinnery oak dies, but as bunchgrasses recover, vertical screening in treated plots may surpass that in untreated plots (Doerr and Guthery, 1983). Likewise, canopy cover eventually can be greater in tebuthiuron-treated plots than in untreated plots (Jones, 1982; Doerr and Guthery, 1983).
Birds in grassland communities evolved in conjunction with grazing; however, effects of grazing on bird communities and their habitats are neither uniform nor easily defined (Wiens and Dyer, 1975). Knopf et al. (1988) argued that grazing has the greatest influence on nongame birds as it alters the horizontal patterning of lower vegetational layers; similarly, Kantrud and Kologiski (1982) stated that grazing by domestic livestock can alter use of grasslands by breeding birds through changes in height and density of vegetation. Roseberry and Klimstra (1970) detected an inverse relationship between intensity of grazing and use of habitat by nesting eastern meadowlarks (Stumella magna).
In shinnery oak communities, data on nesting success are available only for the lesser prairie-chicken (Tympanuchus pallidicinctus). Riley et al. (1992) reported that in shinnery oak and bluestem (Andropogon and Schizachyrium) communities in southeastern New Mexico, nests were more successful in taller vegetation. They suggested that tall, wide clumps of vegetation with spreading stems provided better concealment from predators, both overhead and laterally (Riley et al., 1992). Haukos and Smith (1989) reported that vertical screening cover and percentage of overhead cover were the most important features in selection of nest sites by prairie-chickens. However, there is little information available on density, site selection, or success of nests of passerines in shinnery oak communities. Also, existing studies assume that shinnery oak communities are pristine, when historically these communities supported a greater grass component (Peterson and Boyd, 1998). Our objective was to determine if treatment with tebuthiuron or short-duration grazing used to restore sand shinnery oak communities to historic shrub and grass mixes impacted reproductive success of grassland birds. We accomplished this by testing four combinations of tebuthiuron and grazing treatments to determine if the subsequent vegetational response affected density, site selection, or daily rate of survival of nests of grassland birds.
MATERIALS AND METHODS--Our study site in Roosevelt County, New Mexico, consisted of 16 ca. 65-ha plots (one plot was ca. 80 ha). Tebuthiuron was applied at 0.75 kg/ha to 532 ha of private land in 2000, which was adjacent to 518 ha of untreated State Game Commission-owned land (North Bluit Prairie Chicken Area) representing the extant shinnery oak-grassland community. This rate of application rate was <50% of previously recommended rates to ensure that sand shinnery oak was not completely eliminated from the community. The control area had not been grazed for [greater than or equal to] 7 years before the study began; tebuthiuron-treated areas had not been grazed for [greater than or equal to]5 years before the study began, 2 years pre-tebuthiuron treatment and 3 years post-tebuthiuron treatment. Grazing treatment was a short duration system in which plots were grazed once during the dormant season (January and February) and once during the growing season (July). Stocking rate was calculated each season based on measured forage production and designed to take 25% of available herbaceous material per season. During 2003-2005, stocking rate was 584-2,224 animal-days on the tebuthiuron-treated plots and 147-556 animal-days on the untreated plots (Smythe, 2006). Higher rates of stocking occurred during the dormant season, while lower rates occurred during the growing season. Plots consisted of two treatments arranged in four combinations: tebuthiuron with grazing, tebuthiuron without grazing, no tebuthiuron with grazing, and a control of no tebuthiuron or current grazing. Average yearly precipitation in the region is 31.5 cm (United States Department of Commerce, http://www.ncdc.noaa.gov/oa/climate/research/cag3/z3.htm1); however, 2003 represented the end of a 15-year drought at 26.15 cm of precipitation for the year in the study area. In 2004, cumulative precipitation was 85.85 cm, the second highest ever recorded in the region. Precipitation in 2005 was closer to average at 27.5 cm (C. Dixon, unpublished data).
In each of the 16 treatment plots (four replications of four treatment combinations), we randomly placed four 4-ha subplots. We conducted searches for nests on each plot in March, April, May, and June 2004. Because no nest was found in March 2004, we searched for nests only in April, May, and June 2005. We randomly selected two subplots in each treatment plot to be searched each month, but we were limited by time and available labor to searching only one-half of each subplot. Each search consisted of walking through either the east or west one-half of the subplot with 2-4 people, for a total of 4 ha searched/treatment replicate/month. Searches and monitoring of nests were conducted within the guidelines of Winter et al. (2003). Nests were located by systematically parting vegetation with a hockey stick (Berthelsen and Smith, 1995; Koford, 1999) and watching for flushed adults. Baxter and Wolfe (1973) reported that 94% of dummy nests were found when searching plots with hockey sticks.
We found active and inactive nests during searches. Active nests usually were found by flushing an adult; these nests contained eggshells, eggs, or young. Inactive nests also were discovered while searching; these nests did not contain eggs or young, had no associated adult, and were likely [greater than or equal to]1 year old. Although we had no way of determining true age of inactive nests, we recorded the number found. At each active nest, we recorded CPS coordinates, identified species of plant in which the nest was located, marked the nest with a flag, and rechecked each nest every 4 days to determine fate of nest (Ralph et al., 1993). At failed nests, we checked for evidence of depredation and identified cause of failure if possible. Determining type of predator from evidence left at the nest can be unreliable (Marini and Melo, 1998; Pietz and Granfors, 2000), so we did not categorize depredations. We calculated density of nests by dividing number of nests (active or inactive) found during all searches by total size of area searched. March was excluded from calculations of density in 2004 because no nest was found.
Success of nests was calculated using the Mayfield (1961, 1975) approach with the maximum-likelihood method in program MARK (White and Burnham, 1999; Williams et al., 2002). We considered nests of altricial species to be successful if [greater than or equal to]1 young fledged, whereas nests of precocial species were considered successful when [greater than or equal to]1 egg hatched. Daily rates of survival were modeled among species and treatments using program MARK. We selected the best supported model using Akaike's Information Criterion for selection of model adjusted for small samples ([AIC.sub.c]; Anderson and Burnham, 2002). We made pairwise comparisons of daily rates of survival between 2004 and 2005 and between incubation and the nestling period with a [[chi square].sub.2] test using program CONTRAST as described in Sauer and Williams (1989). We compared density of nests among treatments and years using analysis of variance (ANOVA) in a mixed-linear model after data were tested for normality and heterogeneous variances (Cochran and Cox, 1957). Treatment (tebuthiuron and grazing combination) and year were analyzed as fixed effects. We separated means with pair-wise comparisons of least-squares means using the least-significant-difference (LSD) test if the F-test on marginal means was significant (P < 0.05).
To determine if birds were selecting specific structural characteristics at nest sites, we measured vertical density and overhead cover at each nest site and compared these measurements to those made at a random point within 50 m of the nest site. We measured vertical density by taking profile-board readings from each cardinal direction (Nudds, 1977; Guthery et al., 1981). The profile board consisted of 10, 10-cm strata. We recorded percentage of each stratum obscured by vegetation and maximum height of vegetation. Readings were taken at a distance of 7 m from the nest site at a height of 1 m (Guthery et al., 1981). We took digital photographs of each nest from a height of 1 m and then quantified concealment of the nest by using a photo software program to outline the nest and divide the number of vegetational pixels by number of total pixels of nest (Ortega et al., 2002). We compared these measurements to random points by importing the outline of each nest into a corresponding photo of a random point and dividing the number of vegetation pixels by the same total number of corresponding pixels of nests.
Comparison of vertical structure in different strata between nest sites and random points for vertical cover was performed using multivariate analysis of variance (MANOVA; Manly, 1994). Overhead cover between nest and random sites was analyzed as a completely randomized design in a mixed-linear model in ANOVA (Cochran and Cox, 1957). Treatment (tebuthiuron and grazing combination), hatching success (hatch or fail), and year were analyzed as fixed effects. We separated means using a LSD-test if the overall F-test was significant (P < 0.05). Statistical analyses were performed using SAS statistical software (SAS Institute, Inc., 2003).
RESULTS--During March June 2004, 256 ha were searched for nests (64 ha/month). Across all treatments in 2004, 26 active nests and 97 inactive nests of various species were found (Smythe, 2006). During April-June 2005, 192 ha were searched (64 ha/month). Across all treatments in 2005, we found 59 active nests and 72 inactive nests of various species (Smythe, 2006). The first nest was found 17 days earlier in 2005 (1 April) than in 2004 (18 April). Density of nests was calculated using only nests found in subplots during searches. Nests found incidentally outside of searches were used in all other comparisons.
Density of Nests--Density of nests (nests/ 10 ha) for all species was similar among treatments ([F.sub.3,24] = 2.07, P = 0.13), but density of nests in 2005 was more than twice that in 2004 (mean for 2004 = 0.70 [+ or -] 0.20 SE, mean for 2005 = 1.90 [+ or -] 0.30; [F.sub.1,24] = 9.06, P < 0.01; Table 1). Cassin's sparrows (Aimophila cassinii), mourning doves (Zenaida macroura), lesser prairie-chickens, and meadowlarks (Sturnella) were most common (Smythe, 2006). These species were categorized as resident or migrant based on whether they were present on the study site year-round or only during the breeding season. Cassin's sparrows and mourning doves were migrants and meadowlarks and lesser prairie-chickens were residents. Density of nests of Cassin's sparrows (nests/10 ha) was not affected by treatment ([F.sub.3,24] = 1.32, P = 0.29), but density was six times greater in 2005 than 2004 (mean for 2004 = 0.20 [+ or -] 0.10, mean for 2005 = 1.20 [+ or -] 0.20; [F.sub.1,24] = 13.44, P < 0.01). We found too few nests of other species to make comparisons of densities among treatments or years.
Success of Nests--Because there was no difference in daily rates of survival for any species between 2004 and 2005 (Smythe, 2006), we combined data from both years for further analysis. Daily rate of survival across treatments did not differ between incubation and nestling period for any species or within any treatment (Table 2).
Across treatments, models with the lowest [AIC.sub.c], values indicated that daily rates of survival were different among species during incubation but not during the nestling period (Table 3). Among treatments, daily rates of survival differed during both incubation and the nestling period (Table 4). During incubation, the model with the lowest [AIC.sub.c], indicated that daily survival of nests differed between tebuthiuron-treated and untreated plots (Table 4); survival was 6.3% higher in untreated plots than in treated plots (Table 2). During the nestling period, the same model indicating that daily survival of nests was the same among tebuthiuron-treated plots and among untreated plots was slightly less supported by data as the model with the lowest [AIC.sub.c], but we chose this model because it was more logical in regard to effects of treatment than the model with the lowest [AIC.sub.c], (Table 4). However, during the nestling period the opposite trend manifested; daily survival of nests was 17.3% higher in tebuthiuron-treated plots than in untreated plots (Table 2).
Only four nests were considered successful in 2004: one meadowlark, two Chihuahuan ravens (Corpus cryptoleucus), and one mallard (Anas platyrhynchos). In 2005, six nests were successful: one Chihuahuan raven, one northern bobwhite (Colinas virginianus), one Cassin's sparrow, and three lesser prairie-chickens. Depredation accounted for 84% of all failures of nests.
Vegetation at Nest Sites--We were able to collect vegetational-composition data for 73 of the 85 active nests. The majority of nests of Cassin's sparrow (76%) and meadowlarks (90%) were in little bluestem (Schizachyrium scoparium; Smythe, 2006). We found too few nests of other species to make categorizations, although sand sagebrush (Artemisia filifolia) appeared to be another important nesting substrate. All nests found in sand sagebrush were in untreated plots; the tebuthiuron treatment removed all sand sagebrush in treated plots.
We analyzed profile-board data from 66 active nests and 66 associated random points. We excluded nests in trees, nests found by other researchers that we did not personally monitor, and nests that already had failed when found. Although there was a difference in vegetational structure at sampling points (nest and random) among treatments (Wilks' [lambda] = 0.54, P < 0.01) and between years (Wilks' [lambda] = 0.38, P < 0.01), there was no difference in vegetational structure between nest and random sites (Wilks' [lambda] = 0.87, P = 0.17). Among treatments, visual obscurity differed only in strata 5-8 (41-80 cm) and maximum height (Table 5). In these strata, the treated, ungrazed plots consistently had the greatest visual obscurity, whereas untreated, grazed plots had the lowest (Table 5). Vegetation was tallest in tebuthiuron-treated plots (Table 5). Visual obscurity was greater in these strata in 2005 and maximum height was nearly twice that in 2004 (Table 6). Across years, vegetational structure at hatched nests did not differ from vegetation at failed nests (Wilks' [lambda] = 0.91, P = 0.92) regardless of treatment (Wilks' [lambda] = 0.63, P = 0.85).
Across treatments, vertical structure did not differ between hatched and failed nests in either 2004 (Wilks' [lambda] = 0.07, P = 0.07) or 2005 (Wilks' [lambda] = 0.83, P = 0.73). We were unable to test for an interaction between hatching success, treatment, and year because of the lack of nests that hatched in certain treatment-year combinations.
We analyzed overhead-cover data from 55 nests of grassland birds and 55 associated random points. Meadowlark nests were excluded because they modify the nest site by pulling grass over the top. Overhead cover was greater at nest sites than at random sites (n = 110; mean of nest sites = 72.8 [+ or -] 3.3%, mean of random sites = 44.0 [+ or -] 4.5%; [F.sub.1,96] = 15.41, P < 0.01). This relationship was true in both years ([F.sub.1,96] = 0.0, P = 0.98), regardless of treatment ([F.sub.3,96] = 1.74, P = 0.16). Overhead cover at all sample points (nest and random) did not differ among treatments ([F.sub.3,96] = 0.92, P = 0.43), although it was greater in 2005 than 2004 (mean for 2004 = 41.0 [+ or -] 7.6%, mean for 2005 = 62.2 [+ or -] 3.3%; [F.sub.1,96] = 7.49, P < 0.01). There was no difference in overhead cover between nests that hatched and nests that did not hatch (n = 56; mean number that hatched = 70.6 [+ or -] 5.7%, mean number that failed = 74.2 [+ or -] 3.8%; [F.sub.1,41] = 1.86, P = 0.18), regardless of treatment ([F.sub.3,41] = 0.78, P = 0.51).
Average overhead cover for all open-cup and ground-nesting species was 72.8 [+ or -] 3.2% (n = 56). Average overhead cover at nests of Cassin's sparrows was 74.8 [+ or -] 3.8% (n = 34). Average overhead cover at nest sites of mourning doves was 73.8 [+ or -] 6.0% (n = 16).
DISCUSSION--Use of tebuthiuron and grazing management to restore shinnery oak communities had little effect on initial density of nests of grassland birds. Despite greater avian density in tebuthiuron-treated areas (Smythe, 2006), density of nests was similar among treatments; i.e., more birds in a treatment did not translate to more nests. Overall density of nests was greater in 2005 than 2004, and the effect was most pronounced in migrant species. Equal numbers of nests of residents (lesser prairie-chicken, meadowlark) were found in both years, in contrast to the more numerous nests of migrants (Cassin's sparrow, mourning dove) found in 2005 than 2004. Migrant species appeared to respond to improved conditions of vegetation in 2005 by initiating more nests. Resident species did not initiate more nests but had higher rates of survival. The first nest was found 17 days earlier in 2005 than in 2004, possibly because of better initial conditions in 2005 than 2004.
Precipitation likely had an important influence on results. This study was conducted over years of unusual precipitation in the region; beginning in drought conditions in 2003, followed by twice the average yearly precipitation in 2004 (C. Dixon, unpublished data). These precipitation patterns created atypical vegetational conditions throughout the study site, and these results may not hold in dry-to-average years.
Grassland birds did not exhibit selection among nest sites based on vertical density, nor did vertical density affect hatching success. However, differences in vertical density among treatments and years occurred only at heights >40 cm. At lower levels of vegetation, those most important for concealment of nests, there was no difference in vertical density among treatments and no need for birds to select nest sites. Average height of shinnery oak on the study site was 46.4 cm (C. Dixon, unpublished data). This indicates that at lower vegetational strata, untreated shinnery oak provides similar vertical screening as the predominantly little bluestem communities that replace them after treatment with tebuthiuron. In this study, the moderate grazing regime did not significantly impact vertical density.
Grassland birds selected nest sites based on overhead cover, presumably as a defense against avian predators. Although average overhead cover did not differ among treatments, and ranged from 41 to 61% from 2004 to 2005, nesting birds selected sites with 72% cover on average. Overhead cover was greater at nest sites than at random points, but this did not influence whether or not the nest hatched.
Greater horizontal and vertical cover in 2005 versus 2004 did not translate to higher daily rates of survival of nests. Likewise, greater vertical cover in tebuthiuron-treated plots did not always result in higher daily rates of survival of nests; in fact, untreated plots had higher daily rates of survival of nests during incubation, although tebuthiuron-treated plots had higher rates during the nestling period. This may indicate that grasses and shrubs are needed during different periods of brood rearing and, thus, both are required in a restored shinnery oak community. Several other studies documented a similar disassociation between success of nest and vegetation. Newton and Heske (2001) detected no relationship between vertical density in a field and number of artificial nests depredated in that field; Rivers et al. (2003) reported no consistent difference in side and overhead concealment between depredated and undisturbed artificial nests. Winter et al. (2005) did not find recognizable effects of vegetational structure on nesting success in two of three grassland species, and Kershner et al. (2004) detected no difference in nest-site vegetational characteristics between successful and failed nests of meadowlarks. The inconsistent effects of concealment of nests may be related to ecology of predators; medium-sized predators (e.g., striped skunk Mephitis mephitis, Chihuahuan raven) may be less likely to find a well-concealed nest, but small predators (e.g., hispid cotton rat Sigmodon hispidus; spotted ground squirrel Spermophilus spilosoma) may be more likely to depredate a well-concealed nest if the concealment offers protection from secondary predators (Dion et al., 2000; Rivers et al., 2003). Losses of nests to predation were high, but this has been documented in other grassland birds (Martin, 1993; Davis, 2003). In addition to improving habitat for grassland birds, the abundant rainfall may have improved conditions for predators of grassland birds.
Resident birds (lesser prairie-chicken, meadowlark) had higher daily rates of survival during incubation than did migrants (Cassin's sparrow, mourning dove). However, all species had low daily rates of survival from hatching to fledging. We did not find a difference in daily rates of survival between incubation and nestling periods, although our nestling-period calculations were based on a relatively small sample. Stake and Cimprich (2003) documented more-frequent predation during the nestling stage than during incubation. Predation tends to be higher during the nestling stage than during incubation, possibly because noise, movement, and scent of nestlings attracts predators (Eichholtz and Koenig, 1992), although Davis (2003) did not detect differences between incubation and rates of survival of nestlings of prairie songbirds.
With the exception of the lesser prairie-chicken, rates of nest success for all species to fledging were low compared to those of several other studies in grasslands (Lanyon, 1957; Roseberry and Klimstra, 1970; Haukos, 1988; Riley et al., 1992; Berthelsen and Smith, 1995; Hughes et al., 2000; Kershner et al., 2004). This is discouraging considering the optimal conditions during the study of light grazing and abundant rainfall. Low rate of success of nests may indicate that overall quality of nesting habitat is low (Johnson and Temple, 1986), but does not necessarily translate to reduced annual productivity, as females may compensate by renesting and double-brooding (Murray, 2000). Powell et al. (1999) theorized that small increases in daily rates of survival of adults and juveniles could have a large positive effect on breeding-season productivity. In spite of preventative efforts (Winter et al., 2003), it was possible that disturbance by humans increased predation on monitored nests. However, the low rate of success of nests suggests the possibility that this community may currently be a population sink or ecological trap.
Restoring shinnery oak communities using tebuthiuron and grazing treatments clearly altered vegetational structure, but impacts of these treatments on avian reproduction remain unclear. The above-average rainfall may have masked some effects of treatment; birds may be more selective in dry years. It also is important to consider that control areas did not represent shinnery oak communities in pristine condition, and there may also be a lag in reproductive response of birds to treatments. Our results indicate that carefully managed application of tebuthiuron and grazing in shinnery oak communities do not adversely impact density or success of nests of grassland birds; however, current high rates of depredation and low rates of nest success overall do not bode well for grassland birds in this community.
We thank C. Dixon of Wildlife Plus Consulting for sharing data on vegetation and rainfall, and M. Patten of the G. M. Sutton Avian Research Center for sharing data on nests of lesser prairie-chickens. We also thank the hardworking field technicians who assisted with data collection: A. Andrei, K. Bailey, H. Coin, D. Ferris, J. Hull, B. Rigby, D. Strain, and P. Whiting. P. McDaniel of Phalarope Consulting also provided assistance on this study. B. Vizcarra and C. Vigil translated the abstract into Spanish. We thank the anonymous reviewer who provided comments to improve the manuscript. This study was funded by a grant from the National Fish and Wildlife Foundation, with support from the Region 2 Migratory Bird Office of the United States Fish and Wildlife Service, the Department of Natural Resources Management, Texas Tech University, and Grasslands Charitable Foundation. This is manuscript T-9-11143 of the College of Agricultural Sciences and Natural Resources, Texas Tech University.
Submitted 4 December 2006. Accepted 1 September 2008. Associate Editor was Michael S. Husak.
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LINDSAY A. SMYTHE AND DAVID A. HAUKOS *
Department of Natural Resources Management, Box 42125, Texas Tech University, Lubbock, TX 79409
Present address of LAS. Kofa National Wildlife Refuge, 9300 East 28th Street, Yuma, AZ 85365
* Correspondent. firstname.lastname@example.org
TABLE 1--Total number of nests of birds and average density ([+ or -] SE) among four combinations of tebuthiuron treatment and grazing in sand shinnery oak (Querces havardii) communities in Roosevelt County, New Mexico, 2004-2005. Treatment Number of active nests 2004 2005 Untreated, grazed 5 10 Untreated, ungrazed 7 15 Treated, grazed 3 15 Treated, ungrazed 11 16 Treatment Number of inactive nests 2004 2005 Untreated, grazed 15 9 Untreated, ungrazed 13 6 Treated, grazed 34 31 Treated, ungrazed 35 24 Treatment Density of active nests (nests/10 ha) (a) 2004 2005 Untreated, grazed 0.21 [+ or -] 0.21 A 1.25 [+ or -] 0.24 Ab (b) a (c) Untreated, ungrazed 0.83 [+ or -] 0.48 Aa 1.46 [+ or -] 0.93 Ab Treated, grazed 0.21 [+ or -] 0.21 Aa 2.08 [+ or -] 0.87 Ab Treated, ungrazed 1.46 [+ or -] 0.63 Aa 2.71 [+ or -] 0.40 Ab (a) Density includes only nests found in subplots during searches. (b) Uppercase letters indicate differences (P < 0.05) among treatments within a year. (c) Lowercase letters indicate differences (P < 0.05) between years within a treatment. TABLE 2--Daily rates of survival of nests during incubation and nestling period for four species of grassland birds and among restoration treatments in sand shinnery oak (Querces havardii) communities in Roosevelt County, New Mexico, 2004-2005. Incubation (n) By species All species 0.915 [+ or -] 0.013 (64) as Cassin's sparrow 0.892 [+ or -] 0.023 (30) Aba Lesser prairie-chicken 0.963 [+ or -] 0.018 (8) B Meadowlark 0.926 [+ or -] 0.036 (7) Ca Mourning dove 0.864 [+ or -] 0.039 (16) Da By treatment Untreated, grazed 0.962 [+ or -] 0.018 (11) Acad Untreated, ungrazed 0.922 [+ or -] 0.023 (18) Aa Treated, grazed 0.916 [+ or -] 0.026 (15) Ba Treated, ungrazed 0.856 [+ or -] 0.035 (20) Ba Hatching to fledging (n) By species All species 0.854 [+ or -] 0.034 (19) a Cassin's sparrow 0.780 [+ or -] 0.067 (10) Aa Lesser prairie-chicken N/A Meadowlark 0.933 [+ or -] 0.046 (3) Aa Mourning dove 0.723 [+ or -] 0.126 (4) Aa By treatment Untreated, grazed 0.688 [+ or -] 0.122 (4) Aa Untreated, ungrazed 0.805 [+ or -] 0.106 (4) Aa Treated, grazed 0.939 [+ or -] 0.034 (5) Ba Treated, ungrazed 0.816 [+ or -] 0.076 (5) Ba (a) Lowercase letters indicate differences (P < 0.05) between incubation and nestling period within species. (b) Uppercase letters indicate differences (based on MARK models) within incubation or nestling period among species. (c) Uppercase letters indicate differences (based on MARK models) within incubation or nestling period among treatments. (d) Lowercase letters indicate differences (P < 0.05) between incubation and nestling period within a treatment. TABLE 3--Comparison of models for daily rates of survival using program MARK among species of birds across treatments in sand shinnery oak (Querces havardii) communities in Roosevelt County, New Mexico, during April-June 2004 and 2005. Model AIC, [Delta] Number of Deviance [AIC.sub.c] parameters Incubation among species A--all species different 160.402 0.00 4 152.294 B--all species same 161.586 1.18 1 159.576 Nestling period among species B--all species same 29.233 0.00 1 27.131 A--all species different 31.276 2.04 2 26.960 TABLE 4--Comparison of models for daily rates of survival of birds using program MARK among treatments across species in sand shinnery oak (Querces havardii) communities in Roosevelt County, New Mexico, April June 2004 and 2005. Model AIC.sub.c] [Delta] [AIC.sub.c] Incubation among treatmentsa A--treatments 1 and 2 same, 3 and 4 same 171.112 0.00 B--all treatments different 171.437 0.33 C--treatments 1 and 3 same, 2 and 4 same 172.200 1.09 D--all treatments same 173.056 1.94 E--treatments 1 and 4 same, 2 and 3 same 174.947 3.84 Nestling period among treatments E--treatments 1 and 4 same, 2 and 3 same 51.698 0.00 A--treatments 1 and 2 same, 3 and 4 same 51.892 0.19 B--all treatments different 53.071 1.37 D--all treatments same 53.391 1.69 C--treatments 1 and 3 same, 2 and 4 same 54.604 2.91 Model Number of Deviance parameters Incubation among treatmentsa A--treatments 1 and 2 same, 3 and 4 same 2 167.083 B--all treatments different 4 163.341 C--treatments 1 and 3 same, 2 and 4 same 2 168.171 D--all treatments same 1 171.046 E--treatments 1 and 4 same, 2 and 3 same 2 170.919 Nestling period among treatments E--treatments 1 and 4 same, 2 and 3 same 2 47.562 A--treatments 1 and 2 same, 3 and 4 same 2 47.756 B--all treatments different 4 44.606 D--all treatments same 1 51.346 C--treatments 1 and 3 same, 2 and 4 same 2 50.467 (a) Treatment 1 = untreated, grazed; 2 = untreated, ungrazed; 3 = treated, grazed; 4 = treated, ungrazed. TABLE 5--Means of visual obscurity and maximum height by treatment at nests of 66 grassland birds and associated random points in sand shinnery oak (Querces havardii) communities in Roosevelt County, New Mexico, 2004-2005. Visual obscurity (%) Stratum cm Untreated, grazed 1 0-10 94.1 [+ or -] 3.9a (a) 2 11-20 87.3 [+ or -] 4.3a 3 21-30 65.2 [+ or -] 4.7a 4 31-40 41.9 [+ or -] 4.9a 5 41-50 23.3 [+ or -] 4.6c 6 51-60 13.6 [+ or -] 2.8b 7 61-70 6.8 [+ or -] 1.5c 8 71-80 4.3 [+ or -] 1.0c 9 81-90 2.0 [+ or -] 0.5a 10 91-100 1.6 [+ or -] 0.6a Maximum 64.1 [+ or -] 4.5 c height (cm) Visual obscurity (%) Stratum cm Untreated, ungrazed 1 0-10 96.4 [+ or -] 1.0a 2 11-20 89.8 [+ or -] 2.0a 3 21-30 67.6 [+ or -] 4.0a 4 31-40 51.7 [+ or -] 8.8a 5 41-50 27.9 [+ or -] 2.8bc 6 51-60 17.1 [+ or -] 2.5b 7 61-70 9.7 [+ or -] 1.6c 8 71-80 6.2 [+ or -] 1.0bc 9 81-90 3.1 [+ or -] 0.6a 10 91-100 2.1 [+ or -] 0.4a Maximum 72.4 [+ or -] 3.5 b height (cm) Visual obscurity (%) Stratum cm Treated, grazed 1 0-10 96.2 [+ or -] 1.6a 2 11-20 85.5 [+ or -] 3.0a 3 21-30 59.2 [+ or -] 3.4a 4 31-40 49.0 [+ or -] 3.6a 5 41-50 36.8 [+ or -] 3.7ab 6 51-60 31.5 [+ or -] 23.7 a 7 61-70 18.3 [+ or -] 2.6b 8 71-80 11.1 [+ or -] 1.9b 9 81-90 5.1 [+ or -] 1.2a 10 91-100 3.7 [+ or -] 1.0a Maximum 79.5 [+ or -] 2.5 a height (cm) Visual obscurity (%) Stratum cm Treated, ungrazed 1 0-10 97.7 [+ or -] 0.7a 2 11-20 89.2 [+ or -] 1.7a 3 21-30 69.6 [+ or -] 2.8a 4 31-40 59.1 [+ or -] 3.2a 5 41-50 44.6 [+ or -] 3.9a 6 51-60 37.0 [+ or -] 3.9a 7 61-70 26.5 [+ or -] 3.6a 8 71-80 19.6 [+ or -] 3.1a 9 81-90 10.2 [+ or -] 2.4a 10 91-100 6.5 [+ or -] 2.1a Maximum 79.2 [+ or -] 2.6 a height (cm) (a) Lowercase letters indicate differences (P < 0.05) among treatments within a stratum. TABLE 6--Means of visual obscurity and maximum height by year at nests of 66 grassland birds and associated random points in sand shinnery oak (Querces havardii) communities in Roosevelt County, New Mexico, 2004-2005. Visual obscurity (%) Stratum cm 2004 1 0-10 96.7 [+ or -] 1.5a (a) 2 11-20 87.2 [+ or -] 3.0 a 3 21-30 64.9 [+ or -] 4.3 a 4 31-40 45.4 [+ or -] 4.9 a 5 41-50 27.3 [+ or -] 5.5 a 6 51-60 18.0 [+ or -] 4.9 a 7 61-70 9.6 [+ or -] 3.7 a 8 71-80 5.6 [+ or -] 3.0 a 9 81-90 3.5 [+ or -] 2.7 a 10 91-100 3.3 [+ or -] 2.7 a Maximum 50.0 [+ or -] 3.0 a height (cm) Stratum cm 2005 1 0-10 96.1 [+ or -] 1.1 a 2 11-20 88.4 [+ or -] 1.4 a 3 21-30 66.1 [+ or -] 2.0 a 4 31-40 53.5 [+ or -] 3.2 a 5 41-50 36.8 [+ or -] 2.0 b 6 51-60 28.9 [+ or -] 2.0 b 7 61-70 18.9 [+ or -] 1.6 b 8 71-80 13.2 [+ or -] 1.4 b 9 81-90 6.4 [+ or -] 0.9 a 10 91-100 4.0 [+ or -] 0.5 a Maximum 82.5 [+ or -] 1.2 b height (cm) (a) Lowercase letters indicate differences (P < 0.05) between years within a stratum.
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|Author:||Smythe, Lindsay A.; Haukos, David A.|
|Date:||Jun 1, 2009|
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