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Mass change values of landbird migrants at an inland stopover site dominated by nonnative vegetation.

INTRODUCTION

Transitional, early successional habitats in eastern North America are declining (Askins, 2000; Oehler, 2003; Rich et al., 2004). To date most of the concern about this loss has focused on reduction of breeding habitat for shrub-nesting landbird species, many of which are also in decline (Askins, 2001; Hunter et al., 2001; Rich et al., 2004). However, the significance of these habitats during the postfledgling to premigratory period, as well as during spring and fall migration, is also becoming apparent not only for species that nest in shrubland habitat (Moore et al., 1990; Moore et al., 1995; Smith and Hatch, 2008) but also for those species that breed in late-successional habitats (Rodewald and Brittingham, 2004; Smith and Hatch, 2008; Vitz and Rodewald, 2007). For example Vitz and Rodewald (2007) captured both hatch year and after hatch year individuals of 32 mature forest breeding species in early-successional (regenerating clearcuts) habitat during the post-breeding period, suggesting the dense vegetation structure characteristic of these habitats may explain their high use. Moreover, using a combination of survey and netting data, Smith and Hatch (2008) demonstrated many landbird migrant species, including species that breed in late-successional habitats, used shrublands extensively during spring migration at an inland stopover site in northeastern Pennsylvania.

Unfortunately, nonnative shrubs are an increasingly common component of early successional shrublands (Askins, 2001). Exotic species, especially if they become invasive, may be a serious ecological threat to community structure and function (Wilcove et al., 1998, Levine et al., 2003) including the avian community. Of the limited avian studies to date, most have focused on the influence of exotic vegetation on breeding birds (e.g., Leston and Rodewald, 2006; Rentes, 2003; Rodewald et al., 2011; Schmidt et al., 2005), documenting higher nest predation rates and overall lower nesting success in areas dominated by nonnative vegetation. Studies on the effects of nonnative vegetation on landbirds during migration are even rarer, even though quality stop-over habitat is crucial for successful migration (Moore et al., 2005). For example although migrant abundance and diversity were found to be similar between native-dominated and exotic-dominated habitats during spring (Arizaga et al., 2013) and fall (Walker, 2008) migration, the fitness consequences of using nonnative-dominated sites may differ from native-dominated sites (Ortega et al., 2006). Arizaga et al. (2013) found higher fat deposition rates and shorter minimum stopover duration in native habitat, whereas Cerasale and Guglielmo (2010) found higher plasma triglyceride levels, indicating higher rates of fat deposition in habitats invaded by nonnative species. Collectively, these studies highlight the need for additional data on use of exotic vegetation and, especially, the need to assess habitat quality using measures in addition to abundance or density.

Species density is a commonly utilized correlate of habitat quality (Bock and Jones, 2004; Johnson, 2007; Morrison et al., 2006). However, it may be misleading if factors other than habitat quality lead species or individuals to aggregate in particular habitats (Johnson, 2007; Van Home, 1983). A better way to assess habitat quality is to examine the fitness consequences resulting from the use of a specific habitat (Sherry and Holmes, 1996). Migratory landbirds use lipid stores to fuel migration and evidence of mass gain while using a particular habitat indicates lipid store replenishment, whereas evidence of mass loss reflects loss of lipid stores (Blem, 1980; Moore et al., 1995; Newton, 2008). Hence, mass change is related to fitness and provides an index of habitat quality (Dunn, 2000; Dunn, 2002; Seewagen et al., 2011; Smith et al., 2007). Consequently, our objective was to evaluate the quality of a nonnativedominated shrubland habitat by estimating mass change in a suite of avian species using the site during spring migration.

METHODS

AVIAN CAPTURE

We captured birds in shrub-dominated habitats within Lackawanna County, northeastern Pennsylvania (41.562, -75.722), during the spring migratory period (from the 3rd week of Apr. through 14 Jun.), 2005-2007. We delineated the endpoint of spring migration as 14 Jun. because both historical records (McWilliams and Brauning, 2000) and our capture data indicate many species, including those breeding north of the study area, occur in Pennsylvania in nonbreeding habitats through the second week of Jun. (Smith and Hatch, 2008). We captured migrant landbirds using mistnets (n = 25) at both Lackawanna State Park and private lands immediately adjacent to the Park. Straight line distance between the two sites was 3.2 km. We selected mistnet locations with similar habitat ages, as well as vegetation composition and characteristics as much as possible. Net placement was greater than 50 m from the edge of shrubland habitat. Nets were opened shortly before sunrise, remaining open through early afternoon (generally 7 h post sunrise) and were checked at 30 min intervals. We did not capture birds if temperatures were below 3 C or in the event of high wind or rain. For each individual captured, we recorded capture date and time, species, age and sex where possible (Pyle, 1997), mass, unflattened wing chord length, and tarsus length. Individuals were banded with a U.S. Geological Service aluminum leg band and all recaptures were measured without reference to previous records.

HABITAT AND SHRUB PHENOLOGY

Shrub habitat was approximately 25-30 y post agriculture and was a mix of exotic [primarily honeysuckle (Lonicera spp.)] and native shrubs [primarily dogwood {Comus spp.) and arrow-wood viburnum (Viburnum dentatum)], as well as a small number of saplings of most tree species found in nearby forested habitat (see Smith and Hatch, 2008 for a more complete site description). The dominant shrub was nonnative honeysuckle [as determined using the methodology of james and Shugart (1970)], which comprised 41% of shrub and sapling stems.

We tracked timing of spring leaf-out in honeysuckle, viburnum, and dogwood using the methodology of Ewert et al. (2011). Each morning the same individual (iyS) collected phenology data from the same 10 individuals of each vegetation species at our banding locations. Phenology was categorized as: (1) leaves in bud; (2) leaves emerging from bud; (3) leaves unfurled but not fully expanded; and (4) leaves fully expanded. Phenology results indicated that honeysuckle completed leaf development some 13 d prior to 15 May, whereas the phenology of other species common to the study site were 13 to 19 d delayed relative to honeysuckle (Smith and Hatch, 2008). These dates were used to partition our data into two periods: one in which honeysuckle phenology was advanced relative to native shrubs (captures through 15 May) and a later period, after native shrubs had completed leaf-out (captures from 16 May through 14Jun.). Partitioning our data in this way permitted assessment of avian mass change in relation to nonnative honeysuckle phenology. For example during early migration, honeysuckle phenology was advanced relative to native vegetation and was therefore the predominant leafy foraging substrate; however, during later migration, leaf-out was complete in both honeysuckle and native shrubs, providing birds more choice of shrub species within which to forage.

STATISTICAL ANALYSES AND MASS CHANGE ESTIMATES

To evaluate potential differences in fitness relative to phenological period, we estimated mass change in 13 species of landbird migrants [Red-eyed Vireo (Vireo olivaceus), Ruby-crowned Kinglet (Regulus calendula), Hermit Thrush (Catharus guttatus), Veery (Catharus fuscescens), Wood Thrush (Hylocichla mustelina), Gray Catbird (Dumetella carolinensis), Blue-winged Warbler (Vermivora pinus), Nashville Warbler (Vermivara mficapilla), Magnolia Warbler (Setophaga magnolia), Chestnut-sided Warbler (Setophaga pensylvanica), Ovenbird (Seiurus aurocapillus), Common Yellowthroat (Geothlypis trichas), and White-throated Sparrow (Zonotrichia albicollis) ]. We limited our dataset to first captures, as it resulted in a dataset containing mostly migrating individuals with local breeders making up only a small percentage of captures (Dunn, 2002; Jones et al., 2002; Smith et al., 2007). We used a mixed model approach because it is useful when analyzing data that exist in hierarchical form (Zuur et al, 2007) such as ours (effectively time series). Further, rather than evaluating directional trends across year and date, as would occur using a multiple regression approach, we sought to statistically control these effects while assessing the relative importance of the relationship of interest, i.e., how mass changed with time of day (Zuur et al, 2009).

We estimated mass change for each phenological period (early, late) and for the entire migratory period. Capture times were converted to minutes since sunrise (Dunn, 2000). We ran models in IBM SPSS version 21 (IBM, 2012), with capture year, date, and minutes since daylight considered fixed effects. Because we were interested in short-term mass change (i.e., minutes since daylight effect), we controlled for year and date by modeling them as classification variables and considered combinations of year and date to be treatment levels. Wing chord was considered a random effect (".&, a covariate that might account for variation in mass gain that could not be experimentally controlled). If birds gain mass while using the site, then there should be a positive relationship between first capture mass and the time elapsed since daylight (Dunn, 2000; Winker et al, 1992). Model residuals were screened for agreement with assumptions using techniques available in SPSS. We calculated mass change using parameter estimates. Estimates for treatment levels (year, date, wing chord) are not reported due to the many levels in the analyses and because our focus was on the effect of minutes since daylight.

It should be noted that inclusion of multiple species in our analyses may increase the likelihood of making a Type I error (Moran, 2003). However, there are logical, practical, and mathematical objections to adjusting P values for tables of multiple statistical tests (Moran, 2003; Roback and Askins, 2005). Consequently, we present uncorrected P values as well as effect magnitudes and reiterate Moran's (2003) point that while one significant effect in a large table of multiple statistical tests might be concerning, a high proportion of significant species level results are strong evidence that the observed effect is real.

RESULTS

BODY MASS CHANGE

Our results indicate a number of species gained mass while using honeysuckle-dominated shrub habitat, and mass change rates during both phenological periods (early, late) were comparable (Table 1). Further, we found no negative effects of minutes since daylight on mass during either phenological period or the entire migratory period, suggesting none of the examined species lost mass while using shrubland habitat during spring migration. Over the entire migratory period, 92% (12 of 13) of species examined had a significant positive effect of minutes since daylight on mass (Table 1). Red-eyed Vireo, Ruby-crowned Kinglet, Hermit Thrush, Veery, Wood Thrush, Gray Catbird, Blue-winged Warbler, Magnolia Warbler, Chestnut-sided Warbler, Ovenbird, Common Yellowthroat, and White-throated Sparrow gained mass while using shrub habitat. Estimated mass change ranged from 0.32% to 1.79% of mean body mass per hour (Table 1). Only one of the species examined, Nashville Warbler, did not appear to gain mass during the entire migratory period.

During the early migratory period, we found that 58% of species with sufficient sample size (7 of 12) gained mass (Table 1). Minutes since daylight was positively related to mass in Ruby-crowned Kinglet, Hermit Thrush, Wood Thrush, Blue-winged Warbler, Magnolia Warbler, Common Yellowthroat, and White-throated Sparrow, and estimated mass change ranged from 0.41% to 1.79% of mean body mass (Table 1). Species for which we did not find a significant change in mass included Veery, Gray Catbird, Nashville Warbler, Chestnut-sided Warbler, and Ovenbird.

Mass change estimates for captures during the late migratory period indicated that 76% of the species (6 of 8) gained mass (Table 1). Red-eyed Vireo, Veery, Gray Catbird, Magnolia Warbler, Chestnut-sided Warbler, and Common Yellowthroat gained mass at rates ranging from 0.29% to 1.06% of mean body mass per hour. Species for which there was not a significant mass change included Wood Thrush and Ovenbird.

DISCUSSION

Our results are consistent with the hypothesis that shrublands provide important stopover habitat and even a nonnative-dominated shrubland may provide suitable habitat for migrating landbirds. Further, shrublands appear to provide fitness benefits for species that breed in early successional habitats (Gray Catbird, Blue-winged Warbler, Chestnut-sided Warbler, Common Yellowthroat) as well as species that characteristically breed in late successional habitats (Red-eyed Vireo, Ruby-crowned Kinglet, Hermit Thrush, Veery, Wood Thrush, Ovenbird and White-throated Sparrow). While we did not control for other factors that may have varied over the course of the season (e.g, the ratio of older to younger birds or males to females), birds still appeared to gain mass during the early phenological period at rates comparable to those for both the late phenological and the entire migratory periods. That is, birds appeared to gain mass regardless of gender or age effects.

Relative to other studies that have estimated hourly mass change (e.g., Bonter et aL, 2007; Dunn, 2002; Seewagen et al, 2011; Smith et aL, 2007), our estimates indicate migrants were depositing fat stores and that most species did so quickly. For example at this study site, individuals improved their condition by ~0.91% per hour during the early phenological period, ~0.69% per hour during the late phenological period, and ~0.79% per hour over the entire migratory period; rates exceeding estimates for spring landbirds migrating through nearshore stopover sites both in upstate New York (0.62%, Bonter et aL, 2007) and Michigan's eastern Upper Peninsula (0.41% Smith et aL, 2007). Further, most species examined at this study site gained mass while none of the species examined lost mass. That is, birds increased, or at least maintained, migratory condition while utilizing shrub habitat.

Fat deposition rate depends in large part on the quality of habitats encountered while en route (Dunn, 2000; Moore et al., 1995). Previously, at the same site we documented more flying arthropods in shrubland than forested habitat in 2 of 3 y and more substrate dwelling arthropods in shrub habitat than forested habitat during the early phenological period (Smith and Hatch, 2008). We hypothesized these resource differences as a likely explanation for an apparent preference of shrub habitat by most migrant species. Our mass change results support this assertion. If birds were using shrubland for reasons other than resources, e.g., taking advantage of the structural complexity of these habitats to reduce predation risk, we would expect lower or even negative mass change. However, the positive or stationary mass gains observed in this study suggest individuals were actively foraging while using these shrub-land habitats, even as they were dominated by nonnative vegetation.

Surprisingly, few studies have either directly or indirectly examined use of nonnative vegetation by landbirds during migration. Those that report fitness measures (e.g, fat deposition rates), and not just abundance or density, have found differing results, potentially due to differences in focal species and habitats. For example Arziga et al. (2013) found Sedge Warblers (Acrocephalus schoenobaerms) had higher fuel deposition rates in native dominated control areas as compared to areas invaded with nonnative saltbush (Baccharis hamlimifolia). Similarly, Cerasale and Guglielmo (2010) found more migrants in native cottonwood-willow habitat, as well as greater arthropod biomass, but refueling rates, as measured by plasma triglycerides, were higher for Wilson's Warbler (Wilsonia pusilla) in nonnative Tamarisk- dominated habitats. Cerasale and Guglielmo (2010) hypothesized migrants replaced fat stores more quickly in the exotic habitat due to lower bird abundance and hence lower competition. Similar to the findings of Cerasale and Guglielmo (2010), our results suggest exotic-dominated habitats may provide sufficient resources to permit birds to gain, or at least maintain, body mass during the early phenological period, when the prevalent foraging substrate was honeysuckle.

Additional work is necessary to evaluate the fitness consequences of nonnative plant species such as honeysuckle on migratory landbirds. However, in doing so, it will be important to examine the cumulative effects of exotic vegetation through multiple stages of the avian annual cycle (spring migration, breeding season, fall migration). For example there is evidence early leaf flush and expansion of exotic plants may attract early nesting birds (Remes, 2003; Schmidt and Whelan, 1999) who in turn may experience higher predation rates (Schmidt and Whelan, 1999). Further, studies of habitat use by fall migrants, such as those by Parrish (1997) and Johnson et al. (1985) demonstrated migrants, including a number of previously thought obligate insectivores, incorporate fruits of exotic shrubs into their diet even as there is evidence that exotic fruits such as honeysuckle are a poor energy source because they are low in both protein and lipid (Herrera, 1987; Ingold and Craycraft, 1983). Clearly, nonnative vegetation has a potentially diverse set of consequences for avian communities and determining how exotics affect songbird populations has important management implications for efforts focused on countering the declines in many populations of landbirds.

Acknowledgments.--Birds were banded under USFWS permit # 23302 (RJS), State of Pennsylvania permit # BB00225 (RJS), and all procedures were approved by The University of Scranton Institutional Animal Care and Use Committee permit # 2-06. Funding was provided by The Pennsylvania Department of Conservation and Natural Resources Wild Resource Conservation Program and The University of Scranton. We are grateful to S. Saalfeld, D. Denton, and two anonymous reviewers for comments that significantly improved the manuscript. We thank Ms. A. Bushko and Lackawanna State Park for permission to capture birds on their properties. We also thank M. Carey for allowing us to utilize part of his Field Sparrow study area and for providing insight into the ecology of this system. Finally, numerous undergraduates from the University of Scranton contributed to this work.

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Submitted 16 September 2014

Accepted 26 September 2015

ROBERT J. SMITH (1)

Department of Biology, The University of Scranton, Scranton, Pennsylvania 18510

AND

MARGRET I. HATCH

Penn State Worthington Scranton, 120 Ridge View Drive, Dunmore, 18512

(1) Corresponding author: e-mail: Robert.Smith@scranton.edu
TABLE 1.--Mixed-model analyses of mass change for 13 common species of
landbirds captured in shrub-dominated habitat during spring migration
in northeastern Pennsylvania, 2005-2007. Birds are listed in taxonomic
order. Shown are parameter estimates (P), standard errors (SE),
F-statistics, and probability values (P) for the effect of time since
daylight. The early phenological period encompasses captures from the
3rd week in Apr. through 15 May, when nonnative honeysuckle phenology
was advanced relative to native shrubs; the late phenological period
includes captures from 16 May through 14 Jun., when leaf-out was
complete in both native and nonnative shrub species; and the entire
migratory period includes captures from the 3rd week of Apr. through
14 Jun. Other factors included in the models but not shown are year,
date, and wing chord length. Body mass is the mean mass [+ or -] SE for
that time period. Mass change estimates are from unstandardized
coefficients ([beta] weights) calculated from the appropriate mixed
models and reported as the mean estimate. An asterisk (*) denotes
insufficient sample size (n < 30). NS indicates the effect of minutes
since daylight on mass was not significant

Phenological period   n     [beta]     SE       F      df        P

Red-eyed Vireo (Vireo olivaceus)
  Early               10*
  Late                55    0.0030   0.0011    7.01    1,29    0.013
  Entire migration    64    0.0027   0.0011    6.21    1,34    0.018
Ruby-croumed Kinglet (Regulus calendula)
  Early              150    0.0020   0.0003   61.98   1,123   <0.001
  Late                 0*
  Entire migration   150    0.0020   0.0003   61.98   1,123   <0.001
Hermit Thrush (Catharus guttatus)
  Early               62    0.0038   0.0015    6.62    1,37    0.014
  Late                 6*
  Entire migration    67    0.0038   0.0015    6.72    1,38    0.013
Veery (Catharus fuscescens)
  Early               41    0.0025   0.0024    1.15    1,21    0.295
  Late                50    0.0034   0.0012    7.86    1,25    0.010
  Entire migration    91    0.0027   0.0012    4.99    1,49    0.030
Wood Thrush (Hylocichla mustelina)
  Early               50    0.0073   0.0032    5.13    1,33    0.030
  Late                41    0.0018   0.0028    0.42    1,19    0.526
  Entire migration    91    0.0044   0.0022    3.88    1,55    0.054
Gray Catbird (Dumetella carolinensis)
  Early              148    0.0033   0.0029    1.88    1,132   0.173
  Late               291    0.0017   0.0007    5.91   1,258    0.016
  Entire migration   439    0.0019   0.0009    4.10   1,394    0.044
Blue-winged Warbler (Vermivora cyanoptera)
  Early               53    0.0006   0.0003    4.00    1,34    0.053
  Late                28*
  Entire migration    81    0.0005   0.0002    5.41    1,45    0.025
Nashville Warbler (Oreothlypis rufuapilla)
  Early               57    0.0005   0.0005    1.17    1,35    0.286
  Late                 5*
  Entire migration    62    0.0006   0.0005    1.30    1,36    0.262
Magnolia Warbler (Setophaga magnolia)
  Early               54    0.0016   0.0006    9.05    1,38    0.005
  Late                55    0.0012   0.0005    4.94    1,35    0.033
  Entire migration   109    0.0014   0.0004   14.15    1,76   <0.001
Chestnut-sided Warbler (Setophaga pensylvanica)
  Early               47    0.0011   0.0006    3.15    1,29    0.086
  Late                44    0.0014   0.0005    8.07    1,52    0.010
  Entire migration    90    0.0014   0.0004   11.55    1,52    0.001
Ovenbird (Seiums aurocapilla)
  Early               65    0.0018   0.0010    3.47    1,46    0.069
  Late                40    0.0002   0.0020    0.02    1,18    0.898
  Entire migration   105    0.0016   0.0008    4.15    1,68    0.046
Common Yellowthroat (Geothlypis trichas)
  Early              120    0.0013   0.0002   38.91   1,103   <0.001
  Late               331    0.0009   0.0003   18.66   1,297   <0.001
  Entire migration   451    0.0009   0.0001   44.81   1,404   <0.001
White-throated Sparrow (Zontricia albicollis)
  Early              172    0.0030   0.0010    8.46   1,143   0.004
  Late                11*
  Entire migration   183    0.0028   0.0009    8.67   1,147   0.004
Mean for all species
  Early
  Late
  Entire migration

                                                           % mean
                                                g           mass
Phenological period     Body mass (g)      [hr.sup.-1]   [hr.sup.-1]

Red-eyed Vireo (Vireo olivaceus)
  Early              17.00 [+ or -] 0.31
  Late               16.60 [+ or -] 0.13      0.18          1.06
  Entire migration   16.71 [+ or -] 0.14      0.16          0.98
Ruby-croumed Kinglet (Regulus calendula)
  Early               6.56 [+ or -] 0.04      0.12          1.79
  Late
  Entire migration    6.56 [+ or -] 0.04      0.12          1.79
Hermit Thrush (Catharus guttatus)
  Early              30.67 [+ or -] 0.21      0.19          0.61
  Late               29.80 [+ or -] 0.22
  Entire migration   30.48 [+ or -] 0.22      0.23          0.75
Veery (Catharus fuscescens)
  Early              30.12 [+ or -] 0.36       NS
  Late               30.06 [+ or -] 0.23      0.21          0.69
  Entire migration   29.88 [+ or -] 0.24      0.16          0.54
Wood Thrush (Hylocichla mustelina)
  Early              47.04 [+ or -] 0.53      0.44          0.94
  Late               48.17 [+ or -] 0.35       NS
  Entire migration   47.64 [+ or -] 0.40      0.26          0.56
Gray Catbird (Dumetella carolinensis)
  Early              35.19 [+ or -] 0.41       NS
  Late               35.10 [+ or -] 0.13      0.10          0.29
  Entire migration   35.13 [+ or -] 0.16      0.11          0.32
Blue-winged Warbler (Vermivora cyanoptera)
  Early              8.37 [+ or -] 0.71       0.03          0.41
  Late               8.94 [+ or -] 0.20
  Entire migration   8.50 [+ or -] 0.06       0.03          0.37
Nashville Warbler (Oreothlypis rufuapilla)
  Early              8.34 [+ or -] 0.70        NS
  Late               8.70 [+ or -] 0.34
  Entire migration   8.37 [+ or -] 0.09        NS
Magnolia Warbler (Setophaga magnolia)
  Early              8.59 [+ or -] 0.10       0.10          1.14
  Late                8.86 [+ or -]0.11       0.07          0.77
  Entire migration   8.78 [+ or -] 0.08       0.09          0.97
Chestnut-sided Warbler (Setophaga pensylvanica)
  Early              9.72 [+ or -] 0.10        NS
  Late               9.71 [+ or -] 0.09       0.08          0.88
  Entire migration   9.70 [+ or -] 0.07       0.09          0.89
Ovenbird (Seiums aurocapilla)
  Early              19.45 [+ or -] 0.16       NS
  Late               19.74 [+ or -] 0.12       NS
  Entire migration   19.35 [+ or -] 0.15      0.10          0.50
Common Yellowthroat (Geothlypis trichas)
  Early              9.79 [+ or -] 0.67       0.08          0.81
  Late               9.94 [+ or -] 0.04       0.04          0.45
  Entire migration   9.90 [+ or -] 0.04       0.05          0.55
White-throated Sparrow (Zontricia albicollis)
  Early              26.97 [+ or -] 0.20      0.17          0.64
  Late               25.80 [+ or -] 0.38
  Entire migration   26.78 [+ or -] 0.20      0.17          0.62
Mean for all species
  Early                                                     0.91
  Late                                                      0.69
  Entire migration                                          0.74
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Article Details
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Author:Smith, Robert J.; Hatch, Margret I.
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:100NA
Date:Jan 1, 2016
Words:5206
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