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Acadian flycatcher (Empidonax virescens) nest tail structure and function in temperate forests.

INTRODUCTION

Flycatchers of the family Tyrannidae build a greater variety of nests than any other family of birds except the ovenbirds (Furnariidae; Skutch, 1997; Zyskowski and Prum, 1999; Fitzpatrick et al., 2003). Among the nest forms constructed by flycatchers are a variety of open cups, domes with side entrances, transitional partial domes, ovoid purses with narrow side openings, cavity nests placed in holes, transitional cavity nests placed in natural crevices, and a variety of pendant nests which vary in size and complexity (Skutch, 1997; Fitzpatrick et al., 2003). Nest construction appears to be a conserved life history trait within Tyrannid lineages (yon Ihering, 1901; Lanyon, 1978, 1986; Traylor and Fitzpatrick, 1982; Fitzpatrick et al., 2003).

Many Tyrannid nests, especially pendant forms, incorporate hanging debris (hereafter, nest tails) that dangles below the nest chamber (Skutch, 1997). Presumably, nest tails function in nest concealment and are likely effective amidst the numerous vines, epiphytes, and hanging vegetative debris characteristic of many tropical habitats. Hansell (2000) considers nest tails to be a form of adornment, similar to the lichen flecks and cocoons added to camouflage the nests of many North American songbirds. Such modifications to the nest cup represent physical manifestations of construction behaviors and should be subject to evolutionary forces (Collias and Collias, 1984; Zyskowski and Prum, 1999).

The Acadian flycatcher (Empidonax virescens) is one of 35 species of Tyrannid flycatcher that have expanded their distribution from the Neotropics to temperate latitudes of North America. It builds a flimsy, suspended open cup nest (a slightly pendant "pensile" form) that is the least substantial of all nests built by North American Empidonax flycatchers (Headstrom, 1970). It is also the only North American flycatcher to regularly incorporate one or more tails (Christy, 1942; Newman, 1958; Headstrom, 1970; Harrison, 1975) that do not appear to be simply the result of "sloppy" nest construction. Early in their northward range expansion, Acadian flycatchers presumably would have encountered trees festooned with Spanish moss (Tillandsia usneoides) in the southeastern U.S. where they remain the only breeding Empidonax flycatcher. In such habitats, nest tails incorporating the moss likely provided effective nest concealment as they do today (Stockard, 1905; Christy, 1942). Farther to the north, in temperate forests with fewer vines and epiphytes, such adornment may not confer the same degree of camouflage and could even make nests more conspicuous.

Given the conserved nature of Tyrannid nest features, it is perhaps not surprising that this species has retained the behavior of incorporating nest tails. The wide variety of material incorporated into nest tails across the species' range (Headstrom, 1970; Whitehead and Taylor, 2002) raises the question of whether nest tails are a relictual feature or an adaptive characteristic. Because the likelihood of nest depredation is one of the major determinants of avian reproductive fitness (Wilcove, 1985; Powell and Frasch, 2000; Lahti, 2009), construction behaviors that make the nest more conspicuous to predators should be actively selected against (Collias and Collias, 1984; Hansell, 2000). Therefore, if this behavior is in evolutionary transition, we would expect to see lower rates of nest survival (i.e., more nest depredation) for nests with the most conspicuous tails. Alternatively, variation in conspicuousness could be regulated simply by the availability of materials in the vicinity of the nest site.

Here we describe Acadian flycatcher nest tail structure in terms of composition mad prominence, a measure involving both the number and length of tails, as a prerequisite to more formal experimental approaches to answering the question of evolutionary significance raised above. The following questions are addressed: (1) are nest tails similar in length and composition to available debris from nearby branches; (2) what environmental factors (e.g., site, nest height, habitat type) best explain variation in tail prominence; and (3) does tail prominence influence nest survival?

METHODS

STUDY AREAS

Eight linear study sites (0.9-3.7 km long, ~150 m wide) were located on five headwater streams in Westmoreland County, southwestern Pennsylvania (40.16[degrees]N, 79.22[degrees]W). All streams drained lower slopes of Laurel Ridge (maximum elevation 848 m) and were located on or adjacent to Powdermill Nature Reserve, the 891 ha biological field station of the Carnegie Museum of Natural History located near Rector, Pennsylvania. Site selection was part of a larger study investigating the effects of eastern hemlock (Tsuga canadensis) decline on Acadian flycatcher populations (Allen, 2008; Allen et al., 2009; Allen and Sheehan, 2010) and was based upon stream size (small headwater streams) and forest structure (mature forests dominated either by deciduous trees or eastern hemlock). Site length was determined by the maximum amount of stream reach occurring in appropriate habitat on accessible lands. Data were collected from 15 May-20 Aug. 2006 and 2007.

NEST SEARCHING

Sites were searched thoroughly for nests every 2-5 d by walking up one side of the stream and down the other, listening and looking for singing males and calling females. During searches, locations of territorial birds were marked on schematic maps of the site to aid in territory and nest site identification. We believe that these methods, combined with the high frequency of searches (25-35 per site, per season), left few, if any, Acadian flycatcher nests undiscovered at the sites. The great majority of nests were located by following calling adults to the nest, with only a small number found by chance sightings. Thus, we do not believe that the structure of nests (e.g., length of tails) had any influence on the likelihood of their discovery.

NEST MONITORING

Active nests were visited every 2-5 d (more often as fledging approached). Nests were checked with a pole-mounted mirror or by remotely observing adult behavior (for nests [greater than or equal to] 7 m high; Martin and Geupel, 1993). A nest was considered successful if it was empty on/ after the expected fledge date (14 d after hatching; Mumford, 1964). Typically, successful nests (i.e., those that fledged at least one young) had adults and fledgling(s) active in the vicinity and moderate to high amounts of fecal material below the nest. Nests empty before their expected fledge date, and lacking evidence associated with successful nests, were assumed to have failed.

The Julian date of the start of incubation was estimated for each nest using one of the following three methods: (1) back dating from estimated dates of stage changes (e.g., 14 d before hatching, 28 d before fledging; Mumford, 1964); (2) comparing nestlings to written descriptions (Mumford, 1964) and/or photos of known-age nestlings (when nests were found or failed during the nestling stage); and (3) back dating by assuming that the midpoint of incubation was the midpoint of our observation period for that nest. The latter method was necessary for nests that were found during incubation and failed before hatching. Egg candling was not practical as most nests were not easily accessible. The estimated start-of-incubation date was subsequently used to estimate nest age at each check (in days, beginning with the first egg laid).

NEST STRUCTURE

Tail characteristics at each nest were measured on the last nest check or soon thereafter, with the exception of five nests that were not measured due to time constraints. Plant materials comprising nest tails were classified into one of five categories (see Table 1). Since nests often had > 1 tail, the number of tails of each material type was tallied within 5 cm length classes (e.g., 0-5, 5-10) and the length of each tail was estimated as the midpoint of its class (e.g., 2.5, 7.5). The length of the longest tail at each nest was also separately estimated to 1 cm. All lengths were visually estimated (viewed through binoculars if necessary), and a centimeter ruler was referenced frequently to calibrate estimates.

Descriptive variables derived from these measurements included: (1) length of the longest tail; (2) number of tails; (3) number of tails >5 cm long and; (4) the summed lengths of all tails (total length). The variable "number of tails >5 cm" was intended to exclude any short non tail fibers protruding from the nest cup. Tails that were fused together (e.g., by arthropod silk) were considered to be one tail if the fusion occurred within 5-10 cm of the bottom of the nest cup.

At fourteen nests that were easily reached, we also took measurements of the nest cup to the nearest millimeter. These included the depth and diameter (i.e., height and width) of

the nest exterior, as well as those of the inner cup (see Hansel, 2000). As nests were often oval-shaped, diameters were measured as an average of the maximum and minimum widths.

DEBRIS SURVEY

In the 2007 field season, we collected data to determine: (1) the relative abundance of tail-like debris in the environment; and (2) if Acadian flycatchers may be selecting more debris rich branches as nest sites. On each nest branch, we recorded the number and length of all pieces of fibrous plant debris [greater than or equal to] 1 cm long (not including nest tails) hanging from the branch (hereafter "hanging debris'). Material categories were the same as in Table 1, plus an additional category ("leaves") including dead deciduous leaves [greater than or equal to] 5 cm in maximum width. Branch area was visually estimated by multiplying the length and average width of the foliated portion of the branch. Random branch selection was done by choosing the lowest branch occurring between 3 to 7 m in height (the range of the majority of nests) that was at least 1 m long (as were the majority of nest branches), on the nearest tree of each of the eight nest substrate species depending upon how many of these species were present.

VEGETATION ANALYSES

Vegetation was quantified within randomly located circular plots (0.04 ha) using methods recommended by Martin et al. (1997). The number of plots per site was roughly proportional to site length (ca. one plot per 200 m) and ranged from 5 to 13. We counted and measured the diameter at breast height (dbh) of all woody stems > 5 cm in diameter (hereafter "trees"). Multiple stems arising from the same base were treated as separate if the branching occurred below 1.4 m (Martin et al., 1997). Stem frequencies from these plots were used to calculate percent species composition at each site and to classify sites as either hemlock (n = 5 sites) or deciduous (n = 3) dominated. Hemlock sites contained 16--40% eastern hemlock, while deciduous sites contained < 1%. After termination of each nest (i.e., fledging or failure), we recorded the tree or shrub species supporting it (hereafter "nest tree") and visually estimated the percent concealment of the nest by foliage from a vantage point 1 m directly above it ("overhead concealment"; Martin et al., 1997). The height of each nest was measured to 0.1 m using a 7 m graduated pole or, for higher nests, a clinometer at a measured distance of 5 m from the nest.

DATA ANALYSES

Variation in daily nest survival rate (DSR) was modeled using the logistic exposure model of Shaffer (2004) within the program R (glm, R Development Core Team, 2010). Nests with missing tail data were excluded from DSR analysis. Nests that were suspected to be renesting attempts by a single pair were included and treated as independent. This was justified because the logistic exposure method treats each nest check interval (time between successive visits) as an independent binomial trial (Shaffer, 2004). Therefore, check intervals associated with renesting attempts were not deemed to be any less independent than those within a single nesting attempt. For renesting attempts that involved the reuse of an existing nest, we remeasured tail variables following each attempt to reflect any structural changes that may have occurred.

We used a hierarchical approach to modeling DSR involving two stages (similar to Dinsmore and Dinsmore, 2007): (1) we first ran a series of models exploring temporal variation in the data; and (2) we then constructed a final model set by using the best-performing temporal model (based on AI[C.sub.c]) as a base to which other variables of interest were added singly. Effective sample size (Rotella et al., 2004) was used to calculate AI[C.sub.c]. Temporal models were identical to those used by Grant et al. (2005) and included all 24 combinations of the variables year, Julian date (linear and quadratic terms), and nest age (linear, quadratic and cubic terms), including a null (intercept only) model. The Julian date and nest age variables were calculated for the midpoint of each check interval (Shaffer, 2004). Higher order polynomial models of these variables were included to allow for potential nonmonotonic responses of DSR (e.g., Grant, et al., 2005; Dinsmore and Dinsmore, 2007). Variables added to the best-performing temporal model included tail prominence, overhead concealment (arcsine transformed), nest height, habitat, and site. We used the Hosmer-Lemeshow goodness-of-fit test to assess the fit of the global model (i.e., one including all explanatory variables) for both the temporal and the final model set (Hosmer and Lemeshow, 2000). The null model was used to produce an overall DSR estimate and confidence interval.

Preliminary exploration of the data revealed that the four nest tail variables were positively correlated with one another. Thus, principal components analysis (princomp, R Development Core Team, 2010) was used to create a single index of tail prominence (i.e., ranging from numerous/long tails to few/short tails). Variables were first log- transformed to improve normality, a practice recommended for principal component analysis of allometry data (i.e., size and shape) and when statistical inference on scores is desired (Jolliffe, 2002; McCune and Grace, 2002). Based on loadings, the first principal component (PC1) was used to represent tail prominence. Signs of the scores, which are arbitrary for this procedure, were reversed to make larger numbers equal to longer, more numerous tails (princomp documentation, R Development Core Team, 2010).

We used linear models (lm, R Development Core Team, 2010) to assess variation in tail prominence (-PC1) by site, habitat (eastern hemlock or deciduous), Julian date of nest initiation, nest height, nest tree (eastern hemlock, American beech (Fagus grandifolia), American witchhazel (Hamamelis virginiana), or "other") and various combinations of these variables.

Nests with incomplete tail data (see above) were excluded from analysis, as were three nesting attempts involving the reuse of an existing, apparently unaltered nest within the same season. Other nests that (based on territory mapping) were suspected to be renesting attempts by a single pair, but that involved the building of a new nest, were included; however, they were first evaluated to determine if they could be reasonably treated as independent (i.e., as if individual pairs do not consistently build nests with similar tail characteristics). This was done by assessing the degree of correlation between tail prominence of initial nests and their suspected renests. Model selection was based on Akaike's Information Criterion adjusted for low sample sizes (AI[C.sub.c]; Burnham and Anderson, 2002) and models [less than or equal to] 2[DELTA]AAI[C.sub.c] were considered to be equally plausible.

RESULTS

DEBRIS SURVEY

Hanging debris found on nest branches was composed of catkin/silk (95%), bark shreds (3%) and fine plant fibers (2%; n = 108 pieces, 83 branches). Composition of debris on random branches was 88% catkin/silk and 12% bark shreds (n = 50 pieces, 207 branches). Dead deciduous leaves, not included in the above percentages, were the most common form of debris found on branches with 3.5 leaves per nest branch (1.02*[m.sup.-2]) and 1.1 leaves per random branch (0.50*[m.sup.-2]). The overall density of hanging debris was 0.38 pieces*[m.sup.-2] on nest branches and 0.11 pieces*[m.sup.-2] on random branches. Densities varied widely (0.010.40*[m.sup.-2]) among species on random branches but not as widely on nest branches (0.310.49.[m.sup.-2]; Table 2). Hanging debris had a median length of 4.5 cm on nest branches and 6 cm on random branches (maximum = 31 and 24 cm, respectively).

NEST STRUCTURE

Tails were composed of catkin/silk (43%), fine plant fibers (26%), bark shreds (15%), twigs (11%), and rootlets (4%; n = 1116 tails; see Table 1 for tail material descriptions). The longest tails were composed of catkin/silk (44%), bark shreds (23%), fine plant fibers (20%), rootlets (7%), and twigs (6%; n = 145). The maximum tail length per nest averaged 21 [+ or -] 14 cm (SD) and ranged from 2 to 72 cm (n = 145; Fig. 1A). The total tail length per nest averaged 72 [+ or -] 57 cm and ranged from 3 to 370 cm (n = 145; Fig. 1C). The mean number of tails per nest was 7.7 [+ or -] 3.3 (range 1-21; n = 145; Fig. 1B), and the mean number over 5 cm was 4.5 [+ or -] 3.2 (range 0-16; n = 145; Fig. 1D). The relative abundance of major nest tail constituents varied somewhat by habitat and throughout the breeding season. Notably, nests in hemlock sites had fewer fine plant fibers than those in deciduous sites and more bark shreds earlier (May/Jun. vs. Jul.) in the season. Nests in both habitats had fewer catkin/silk tails when built later in the season (Jul. vs. May/Jun.; Fig. 2). Measurements of nest cup dimensions averaged as follows ([+ or -]SD; n = 14): outer depth = 42 [+ or -] 7 mm; inner depth = 27 [+ or -] 4 mm; outer diameter = 73 [+ or -] 5 mm; inner diameter = 51 [+ or -] 3 mm.

TAIL PROMINENCE

The first principal component (PC1) of the nest tail variables explained 79% of the total variation (Eigenvalue = 3.2). All variables were positively correlated with tail prominence (-PC1), as well as with each other (Table 3). No correlation was found between tail prominence of initial nests and their suspected renests (F = 1.08, df = 1 and 27, P = 0.31); therefore, we did not exclude these nests from analyses. The top-ranked model of tail prominence included the predictor variable nest height ([w.sub.i] = 0.89, Tables 4 and 5), predicting less prominent nest tails at greater heights. The performance of all other models was relatively poor (>5 [DELTA]AI[C.sub.c]). Average tail prominence by site was not related to the average total debris length per branch at a site (F = 0.09, df = 1 and 6, P = 0.77).

[FIGURE 1 OMITTED]

NEST SURVIVAL

Nest survival analysis included 148 nesting attempts and 1021 nest check intervals (median length = 3 d), yielding 2580 exposure days, and an effective sample size (Rotella et al., 2004) of 2436. Thirty six percent of nests were found during nest building, 6% during egg laying, 46% during incubation, and 12% during the nestling stage. Overall daily nest survival rate was 0.96888 [+ or -] 0.00348 SE, indicating an approximately 39% chance of surviving a typical 30 d nest cycle (95% CI = 31--47%).

[FIGURE 2 OMITTED]

Global model fit was adequate for both the temporal [chi-square] = 5.3, df = 8, P = 0.73) and final ([chi square] = 12.0, df = 8, P = 0.15) DSR model sets. The top-ranked temporal model of DSR included a quadratic effect of date and this model was also top-ranked in the final model set (Tables 4 and 5). The model indicated higher nest survival rates early and late in the season, and lower rates midseason (Fig. 3A). All models in the final set (including the null) were within 2 [DELTA]AI[C.sub.c] units suggesting that they are equally plausible and all had relatively low Akaike weights ([w.sub.i] [less than or equal to] 0.25). The model including tall prominence indicated that nests with more prominent tails had lower survival rates (Fig. 3B), though it did not perform well ([DELTA]AI[C.sub.c] = 1.61, [w.sub.i] = 0.13; Table 4) and had 95% parameter confidence intervals that overlapped zero (Table 5). In contrast, parameter confidence intervals of the top-ranking model did not overlap zero (Table 5).

VEGETATION

Tree densities at the sites averaged 555*[ha.sup.-1] [+ or -] 98 SD (deciduous sites mean = 591, hemlock sites = 533). Mean dbh at sites averaged 23 cm [+ or -] 3 SD (deciduous sites = 20, hemlock sites = 25). Mean overhead concealment at nests was 82% [+ or -] 18 SD and mean nest height was 5.2 m [+ or -] 2.6 SD. More information on vegetation structure and nest site characteristics can be found in Allen (2008).

DISCUSSION

Little quantitative information on the composition, length or number of Acadian flycatcher nest tails exists in the literature, though there are many qualitative descriptions (e.g., Stockard, 1905; Christy, 1942; Newman, 1958; Mumford, 1964; Walkinshaw, 1966; Headstrom, 1970; Harrison, 1975). We found that nest tails in southwestern Pennsylvania were composed primarily of catkins and other debris entangled in arthropod silk as well as fine plant fibers and bark shreds. Other authors from northern states have described use of similar materials. Christy (1942), specifically referencing southwestern Pennsylvania, referred to hanging threads of cankerworm silk, withered staminate flowers, and scales of opening buds as constituents of nest tails. Newman (1958) described a nest from Ohio as having streamers of grapevine bark while in Michigan Walkinshaw (1966) commented on nests having long pieces of material hanging below them. Southern nests occurring in forests draped with Spanish moss are described as having long "beards" of this material (Christy, 1942). Two Mississippi nests collected by Stockard (1905) were composed entirely of Spanish moss and had 18 inch streamers, giving "the exact appearance of ordinary bunches of this gray moss." Thus, tail constituents vary widely across the Acadian flycatcher's range indicating flexibility in using a variety of materials. Collias and Collias (1984) state that birds generally select nest materials based on availability, but specific cues and early experience also contribute to the flexibility displayed by birds when choosing nest materials.

In the present study, we found that composition can also vary, though more modestly, on a local scale (e.g., fewer fine fibers but more bark shreds used in hemlock dominated forest) and over the breeding season (e.g., catkins used more often in earlier nests; see Fig. 2). Interestingly, the most common debris available on branches, dried deciduous leaves, was ignored as a tail material. Percent composition of other debris materials on branches did not closely match those comprising nest tails which could be indicative of both selectivity and the fact that some materials are collected off of the ground and not from branches (TLM and MCA, pers. obs.). Skutch (1930) commented that locality was an important influence on the variety of materials used to construct nests in the Northern (Common) Tody Flycatcher (Todirostrum cinereum) that also incorporates nest tails.

We documented wide variation in tail length and number (prominence) in our study area (Fig. 1). Nests had between 1 and 21 tails, and had maximum tail lengths ranging from 2 to 72 cm, or 0.5 to 17 times the average exterior nest cup height (4.2 cm). Variation in tail prominence was best explained by nest height above the ground, with the highest prominence characteristic of lower heights. It is logical that lower forest strata would collect more debris for use in tails. However, it is also possible this relationship was influenced by the highest nests, for which the sample size was relatively low. Tail prominence at our sites did not appear to depend strongly on factors at the stand-level, as models incorporating habitat type and site were not well supported. Similarly, we found little evidence that tail prominence changed over the course of the season (Table 4), despite apparent changes in composition (Fig. 2). It is possible that the variation in tail prominence observed was driven mainly by a patchy, localized availability of desired materials during the period of nest construction. These questions would benefit from quantitative behavioral studies of Acadian flycatchers during the nest construction process.

Variation in nest survival rates was best explained by a quadratic effect of date, indicating that nests were most likely to fledge young early and late in the breeding season (Fig. 3A). This effect could logically be a result of food availability and/or predation though it remains a subject for future inquiry as our study did not collect data on food resources or predator activity. Brown and Roth (2002) found that earlier nests recruited more wood thrush (Hylocichla mustelina) fledglings into a population, but later nests experienced higher nesting success. Neither food resources nor predators were implicated as factors in explaining temporal productivity patterns, which were attributed in this species to winter-related survival costs. The model including tail prominence predicted lower survival for nests with the most prominent vs. the least prominent tails (roughly ten percentage points lower; Fig. 3B), though confidence limits for this effect overlapped zero (i.e., "no effect"; Table 5). If future data lend more support to this effect, it would suggest that nest tails may be serving as visual cues for nest predators, contrary to their presumed original function as camouflage. Regardless, our study suggests that nest tails do not convey dramatic (i.e., easily detectable) survival costs or benefits to Acadian flycatchers, at least in the forests we studied. This supports the hypothesis that nest tails in southwestern Pennsylvania may be a relictual feature that is not under intense selective pressure. It is possible that the Acadian flycatcher, having evolved, like all flycatchers, in the epiphyte and vine rich neotropics, has retained the behavior of adding tails to their nests simply due to the conserved nature of nest form and a lack of significant survival costs. Studies involving larger sample sizes, and possibly experimental manipulation of tail prominence, would be useful to more rigorously evaluate potential survival consequences.

[FIGURE 3 OMITTED]

Our failure to reach definitive conclusions regarding the function of Acadian flycatcher nest tails is shared by other investigations into the adaptive significance of camouflaging nest adornments (Hansell, 1996; Leader and Yom-Tov, 1998; McGuire and Kleindorfer, 2007). However, there is value to these inquiries nonetheless. By documenting the physical structure of Acadian flycatcher nest tails in southwestern Pennsylvania, we have laid the groundwork towards a better understanding of their function and evolutionary history. More data from other regions and habitat types will be needed, perhaps incorporating experimental approaches, to better assess causes of variation, survival costs/benefits, and evolutionary implications of nest tails in the species as a whole.

Acknowledgments.--We wish to thank the Carnegie Museum of Natural History's Powdermill Avian Research Center, especially Bob Mulvihill, for encouraging us to become research associates, for logistical and technical support and stimulating conversations about Acadian flycatcher ecology and behavior. Cokie Lindsay provided logistical help and friendly support. Our study was funded by a Rea Internship in Applied Ecology from Powdermill and a Faculty Development Grant from East Stroudsburg University and the Pennsylvania State System of Higher Education. Mike Allen, Fisheries Manager, generously granted us access to the Rolling Rock Farms property. D. Mejila, M. Paulino, C. Meny, S. Wolbert, C. Effinger, D. Detwiler, B. Romano, J. Sheehan, J. Morgan, and C. Lindsay all generously assisted with field work. Manuscript style and content benefited greatly from reviews provided by Howard (Sandy) Whidden and Douglas Gross.

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SUBMITTED 26 MARCH 2010

ACCEPTED 27 MAY 2011

TERRY L. MASTER (1)

Department of Biological Sciences, East Stroudsburg University of Pennsylvania,

East Stroudsburg 18301

AND

MICHAEL C. ALLEN

New Jersey Audubon, Wattles Stewardship Center, 1024 Audubon Road, Port Murray

07865

(1) Corresponding author: Telephone: (570) 422-3709; FAX: (570) 422-3724; e- mail: tmaster@ esu.edu
TABLE 1.-Descriptions of nest tail and debris materials used
by Acadian flycatchers in southwestern Pennsylvania

Material           Description

Catkin/silk        Catkins and/or bud scales of oak
                   (Quercus sp.), American beech (Fagus
                   grandifolia), or birch (Betula sp.),
                   often bound together with arthropod silk

Fine plant fiber   Thin (<one cm) strands of grass or forb
                   stems

Bark shred         Usually thicker (>one cm) strand of
                   inner tree bark or grape vine (Vitis
                   sp.) outer bark

Rootlet            Fine rootlet of a woody plant

Twig               Fine woody twig (mainly eastern hemlock,
                   Tsuga canadensis)

TABLE 2.--Hanging debris densities on nest and random branches of
different nest tree species used by Acadian flycatchers in
southwestern Pennsylvania. The number of branches sampled is shown
in parentheses

                                            density (pieces x
                        Hanging debris         [m.sup.-2])
Species                Nest branches (n)   Random branches (n)

Eastern hemlock            0.31 (38)           0.40 (35)
(Tsuga canadensis)

American beech             0.49 (20)           0.07 (38)
(Fagus grandifolia)

Other species (a)          0.49 (25)           0.01 (116)

(a) Other species include American witchhazel (Hamarnelis
virginiana), sugar maple (Acer saccharum), striped maple (A.
pensylvanicum), red maple (A. rubrum), black birch (Betula lenta),
and yellow birch (B. lutea)

TABLE 3.--Pearson correlation coefficients among Acadian flycatcher
nest tail variables, including the  first principal component (tail
prominence)

                  Longest tail   No. tails   No. tails >5 cm

Longest tail         1             --             --
No. Tails            0.332         1              --
No. Tails >5 cm      0.629         0.745          1
Total length         0.820         0.613          0.874

                  Total length   Tail prominence

Longest tail           --             0.850
No. Tails              --             0.756
No. Tails >5 cm        --             0.945
Total length           1              0.984

Coefficients among tail variables are for untransformed values,
except for tail prominence, which is  the correlation of log-
transformed tail variables with--PC1

TABLE 4.--Model rankings for Acadian flycatcher nest tail
prominence and daily nest survival based
on Akaike's Information Criterion ([AIC.sub.c])

Response variable, model            k (a)   RSS/deviance (b)

Tail prominence
  Nest height                         3          420.1
  Habitat + Initiation date           4          430.8
  Initiation date                     3          442.1
  Habitat                             3          446.9
  (Initiation date) (2)               4          441.9
  Nest tree + Initiation date         6          430.5
  Null                                2          457.6
  Nest tree                           5          430.5
  Site + Initiation date             10          420.1
  Site                                9          435.6
  Site + Nest tree                   12          432.9
Daily nest survival
  Date (2)                            3          536.0
  Site + Date (2)                    10          522.7
  Null                                1          541.1
  Overhead concealment + Date (2)     4          535.6
  Tail prominence + Date (2)          4          535.6
  Nest height + Date (2)              4          535.8
  Habitat + Date (2)                  4          535.8

Response variable, model            [AIC.sub.c]   [DELTA][AIC.sub.c]

Tail prominence
  Nest height                          571.9              0.0
  Habitat + Initiation date            577.7              5.8
  Initiation date                      579.3              7.4
  Habitat                              580.9              9.0
  (Initiation date) (2)                581.3              9.4
  Nest tree + Initiation date          581.9             10.0
  Null                                 582.2             10.3
  Nest tree                            584.5             12.6
  Site + Initiation date               587.4             15.5
  Site                                 590.3             18.4
  Site + Nest tree                     596.4             24.5
Daily nest survival
  Date (2)                             542.0              0.0
  Site + Date (2)                      542.8              0.8
  Null                                 543.1              1.1
  Overhead concealment + Date (2)      543.6              1.6
  Tail prominence + Date (2)           543.6              1.6
  Nest height + Date (2)               543.8              1.9
  Habitat + Date (2)                   543.9              1.9

Response variable, model            [w.sub.i.sup.c]

Tail prominence
  Nest height                            0.89
  Habitat + Initiation date              0.05
  Initiation date                        0.02
  Habitat                                0.01
  (Initiation date) (2)                  0.01
  Nest tree + Initiation date            0.01
  Null                                   0.01
  Nest tree                              0.00
  Site + Initiation date                 0.00
  Site                                   0.00
  Site + Nest tree                       0.00
Daily nest survival
  Date (2)                               0.25
  Site + Date (2)                        0.17
  Null                                   0.15
  Overhead concealment + Date (2)        0.11
  Tail prominence + Date (2)             0.11
  Nest height + Date (2)                 0.10
  Habitat + Date (2)                     0.10

(a) Number of model parameters

(b) Residual sum of squares (RSS) is given for tail prominence
models, and deviance is given for nest survival models

(c) Model Akaike weights (Burnham and Anderson, 2002)

TABLE 5.--Parameter estimates (with 95% confidence intervals)
for selected models of Acadian flycatcher nest tail prominence
and daily nest survival

Parameter                 Estimate (a)   95% Confidence interval

Tail prominence
(top-ranked model)
  Intercept                0.99435          0.37631, 1.61238
  Nest height             -0.19306         -0.29990, -0.08622
Daily nest survival
(top-ranked model)
  Intercept               28.94384          4.74481, 56.13679
  Date                    -0.28180         -0.57854, -0.01932
  Date (2)                 0.00077          0.00006, 0.00158
Daily nest survival
(tail prominence model)
  Intercept               28.42976          4.24578, 55.61267
  Tail prominence         -0.04620          -0.19141, 0.09722
  Date                    -0.27542         -0.57216, -0.01301
  Date (2)                 0.00075          0.00005, 0.00156

(a) Parameter estimates for Daily Nest Survival models
apply to the logit of daily survival rate
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Author:Master, Terry L.; Allen, Michael C.
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2012
Words:6342
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