SPATIAL AND TEMPORAL MIGRATION PATTERNS OF WILSON’S WARBLER (WILSONIA PUSILLA) IN THE SOUTHWEST AS REVEALED BY STABLE ISOTOPES

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  SPATIAL AND TEMPORAL MIGRATION PATTERNS OF WILSON’S WARBLER (WILSONIA PUSILLA) IN THE SOUTHWEST AS REVEALED BY STABLE ISOTOPES
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  162 SPATIAL AND TEMPORAL MIGRATION PATTERNS OF WILSON’S WARBLER ( WILSONIA PUSILLA ) IN THE SOUTHWEST AS REVEALED BY STABLE ISOTOPES K L. P, 1,2,4  C V R III, 2  T C. T, 1   E H. P 3 1 Department of Biological Sciences, Northern Arizona University, Flagstaff, Arizona 86011, USA; 2 U.S. Geological Survey Southwest Biological Science Center, Sonoran Desert Research Station, University of Arizona, Tucson, Arizona 85721, USA; and 3 U.S. Geological Survey Southwest Biological Science Center, Colorado Plateau Research Station, Northern Arizona University, Flagstaff, Arizona 86011, USA A.—We used stable hydrogen isotopes (δD) to identify the breeding loca-tions of Wilson’s Warbler ( Wilsonia pusilla ) migrating through five sites spanning a cross-section of the species’ southwestern migration route during the springs of 2003 and 2004. Determining the temporal and spatial paerns of migration and degree of population segregation during migration is critical to understanding long-term population trends of migrant birds. At all five migration sites, we found a significant negative relationship between the date Wilson’s Warblers passed through the sampling station and δD values of their feathers. These data were con-sistent with a paern of “leap-frog” migration, in which individuals that bred the previous season at southern latitudes migrated through migration stations earlier than individuals that had previously bred at more northern latitudes. We docu-mented that this paern was consistent across sites and in multiple years. This finding corroborates previous research conducted on Wilson’s Warbler during the fall migration. In addition, mean δD values became more negative across sampling stations from west to east, with the mean δD values at each station corresponding to different geographic regions of the Wilson’s Warblers’ western breeding range. These data indicate that Wilson’s Warblers passing through each station repre-sented a specific regional subset of the entire Wilson’s Warbler western breeding range. As a result, habitat alterations at specific areas across the east–west expanse of the bird’s migratory route in the southwestern United States could differentially affect Wilson’s Warblers at different breeding areas. This migration information is critical for management of Neotropical migrants, especially in light of the rapid changes presently occurring over the southwestern landscape. Received 1 June 2005, accepted 25 January 2006. Key words: deuterium, migration, migratory connectivity, Neotropical migrant, stable isotopes, Wilsonia pusilla  , Wilson’s Warbler. Patrones Espaciales y Temporales de la Migración de Wilsonia pusilla  en el Sudoeste Detectados Mediante Isótopos Estables R.—Usamos isótopos estables de hidrógeno (D) para identificar las localidad de cría de individuos de Wilsonia pusilla  que se encontraban migrando a través de cinco sitios abarcando una sección de la ruta migratoria del sudoeste de la especie durante las primaveras de 2003 y 2004. Determinar los patrones 4 E-mail: kristina.l.paxton@usm.edu The Auk  124(1):162–175, 2007© The American Ornithologists’ Union, 2007. Printed in USA.   Migration Paerns of Wilson’s Warbler  January 2007] 163 T  , when birds move  between their wintering and breeding grounds, has not received aention proportional to the role that it plays in the population dynamics of Neotropical migrants (Gauthreaux 1979). Only within the past 10 years aention has been given to the importance of conservation of migrants along migration pathways. This is in spite of the fact that migration represents a critical period when birds can be at the edge of their physi-ological limits (Blem 1980) and may suffer the largest amount of annual mortality (Keerson and Nolan 1982, Moore et al. 1995, Sille and Holmes 2002). Both the spatial and temporal paerns of migration have important implica-tions for understanding the ecology and evolu-tion of migrants and factors influencing overall population dynamics (Moore and Simons 1992, Moore et al. 1995, Sherry and Holmes 1995). Whether breeding populations use distinctive migratory routes or mix across a broad migra-tory front directly determines how alteration of habitat along the route could potentially affect  breeding populations. Likewise, differences in the timing of migration by different populations could suggest that different selective forces are acting on those populations (Cox 1985).Research examining the dynamics of migration in the United States has primarily focused on migration systems in the eastern part of the country (Moore and Simons 1992, Moore et al. 1995, Simons et al. 2000), whereas our understanding of migration in the west remains rudimentary (Kelly and Huo 2005). Increasing habitat alterations and loss of criti-cal stopover habitat in the southwestern United States (Rosenberg et al. 1991) make it essential that we begin to understand the temporal and spatial distribution paerns of Neotropical migrants across southwestern migration routes and how habitat alterations might affect differ-ent breeding populations. Traditional meth-ods, such as banding, have yielded lile data to answer these questions, but recent studies have shown that naturally occurring stable isotopes in animal tissues can be used to delineate geo-graphically distinct populations (Chamberlain et al. 1997, Hobson and Wassenaar 1997, Marra et al. 1998). Specifically, stable hydrogen iso-topes (δD) are a powerful research tool in connecting different parts of migratory birds’ annual cycle, because predictable continental paerns of δD in precipitation are highly cor-related with δD in body tissues of birds, owing to trophic-level interactions (Chamberlain et al. 1997, Hobson and Wassenaar 1997, Hobson et al. 2001, Meehan et al. 2001, Kelly et al. 2002, Rubenstein et al. 2002). This relationship is temporales y espaciales de migración y el grado de segregación poblacional durante la migración es esencial para entender las tendencias poblacionales a largo plazo de las aves migratorias. En los cinco sitios de migración encontramos una relación negativa significativa entre la fecha en la que los individuos pasaron por la estación de muestreo y los valores de D de sus plumas. Estos datos fueron consistentes con un patrón migratorio del tipo “salto de rana”, en donde los individuos que crían en la estación previa a latitudes más meridionales migraron más temprano a través de las estaciones migratorias que los individuos que criaron en latitudes más septentrionales. Documentamos que este patrón fue consistente a través de los sitios y en múltiples años. Este hallazgo corrobora investigaciones previas realizadas con W. pusilla  durante la migración de otoño. Adicionalmente, los valores medios de D se volvieron más negativos a lo largo de las estaciones de muestreo desde el oeste hacia el este, correspondiendo los valores medios de D de cada estación a diferentes regiones geográficas del rango oeste de cría de W.  pusilla . Estos datos indican que los individuos de W. pusilla  que pasaron por cada estación representaron un subconjunto específico regional de la parte oeste del rango de cría de W. pusilla . Como resultado, las alteraciones de hábitat en áreas específicas a lo largo de la extensión este-oeste de la ruta migratoria de la especie en el sudoeste de Estados Unidos podrían afectar de modo diferencial a W. pusilla  en las diferentes área de cría. Esta información es crucial para el manejo de las aves migratorias neotropicales, especialmente a la luz de los cambios actuales rápidos que se presentan en el paisaje del sudoeste.  P  . 164 [Auk, Vol. 124 primarily associated with latitude, given that southern latitudes are more enriched in δD than northern latitudes, but elevation and con-tinental factors can also influence δD values (Ingraham 1998). Most studies examining migratory connec-tivity (e.g., links between breeding and non- breeding areas; Webster et al. 2002) using stable isotopes have focused on linking the breeding and wintering grounds of Neotropical migrant  birds (see review by Hobson 2003), with few studies focusing on migration (Meehan et al. 2001, Wassenaar and Hobson 2001, Kelly et al. 2002, Mazerolle et al. 2005). Yet determining how the migration period is linked to other stages of the annual cycle and the degree of population segregation during migration is also critical to understanding long-term popu-lation trends of migrants (Moore et al. 1995, Sherry and Holmes 1995). This is especially important in light of evidence of population declines of migratory passerine birds in recent decades (Robbins et al. 1989, Askins et al. 1990, Hagan and Johnston 1992, DeSante and George 1994). Using stable hydrogen isotopes, we exam-ined temporal and spatial migration paerns of Wilson’s Warblers ( Wilsonia pusilla ; hereaer “warblers”) during spring migration at mul-tiple migration stations spanning their south-western migration route. Warblers molt their flight feathers once on the breeding grounds  before migration (Pyle 1997). Because the feathers’ δD value reflects the diet of the bird only during the period of growth (Mizutani et al. 1990), the isotope value of the warbler feathers collected during migration should represent the isotopic signature of its breed-ing location. The two western subspecies of Wilson’s Warbler ( W. p. pileolata  and W. p. chryseola ) are abundant migrants throughout the southwestern United States, migrating north to breeding locations across western North America (Ammon and Gilbert 1999), and have a wide range of δD values. Kelly et al. (2002) and Clegg et al. (2003) showed that there is a strong negative relationship between δD values of warbler feathers collected on their  breeding grounds and the latitude of the col-lection site. On the basis of this information, Kelly et al. (2002) found that during fall migra-tion of warblers at one site in New Mexico, the northernmost breeders passed through first, ahead of more southern-breeding warblers,  but the same temporal paern was not found for this site during spring migration. Our study expands on the earlier work by Clegg et al. (2003) and Kelly et al. (2002) by examining the relationship between δD val-ues of warbler feathers collected across their  breeding range and the δD values of local precipitation at the collection site. This allows for a more precise connection between δD val-ues of feathers collected during migration and their breeding locations. We also examined the temporal paern of spring migration for war- blers at multiple sites and in multiple years to determine whether a similar “leap-frog” migra-tion paern found during the fall (Kelly et al. 2002) is exhibited during the spring migration, and whether such a paern is consistent across sites. In addition, we examined spatial paerns of warbler migration by comparing δD values among multiple migration stations spanning a cross-section of the species’ southwestern migration route. This information provides insight into whether warblers migrate in broad fronts across the southwest or whether dif-ferent breeding populations use distinctive migratory routes. Finally, both spatial and temporal migration paerns were tested in multiple years to determine the consistency of the paerns. M Study areas. —To determine the relationship  between δD values of warbler feathers col-lected on the breeding grounds and local pre-cipitation where they were captured, δD values were determined from a single rectrix collected from 63 warblers across their western breeding grounds (Fig. 1) between 1996 and 2002 (feath-ers were supplied by the University of California at Los Angeles [UCLA] Conservation Genetics Resource Center). All feathers were from adult male and female birds. To ensure that feathers were from breeding individuals and not poten-tial migrants, only warblers captured between 15  June and 1 August (15 August for sites in Alaska) were included. A geographic-information-system (GIS) derived map of δD values for growing-season precipitation across North America (Meehan et al. 2004) was used to obtain δD values of local precipitation where breed-ing warblers were sampled. In addition, we   Migration Paerns of Wilson’s Warbler  January 2007] 165  examined the relationship between δD values of the feathers collected on breeding grounds and collection site latitude, distance from coast, and elevation, all of which affect precipitation δD values in North America (Ingraham 1998). Warblers were captured during spring migration between 15 March and 1 June 2003 at four sites: Colorado River Delta in Baja California, Mexico; Lower Colorado River at Cibola National Wildlife Refuge (NWR) in southwestern Arizona; Arivaca Creek at Buenos Aires NWR in southeastern Arizona; and San Pedro River at San Pedro Riparian National Conservation Area in southeastern Arizona. In 2004, warblers were again captured during the same period at the Lower Colorado River and Arivaca Creek sites, and at an additional site, Big Sur, California (Fig. 1 and Table 1). Warblers were caught by passive mist neing and banded. Standard morphological measurements were taken, and an outer rectrix from each side of the tail was pulled for stable isotope analysis. Feathers were stored in labeled, sealed enve-lopes until analyzed. Stable isotope analysis. —Feathers were washed in detergent and thoroughly rinsed to remove oil, dirt, and residual detergent (Chamberlain et al. 1997, Kelly et al. 2002) and then air dried at room temperature. Feather material from the distal end (0.33–0.37 mg) was removed and wrapped in a silver capsule for isotopic analy-sis. Because of the problem of uncontrolled isotopic exchange between ∼ 13% of noncarbon- bound hydrogen in feathers and ambient water vapor (Chamberlain et al. 1997), we used a com-parative equilibrium approach with calibrated keratin standards to correct for this effect. As a result, values presented here are nonexchange-able feather hydrogen only. Details of this method and standards used are described in F. 1. Location of breeding and migration stations where Wilson’s Warblers’ feathers were col-lected (see Table 1 for site information). Light- and dark-gray shaded regions indicate Wilson’s Warblers’ breeding and wintering ranges, respectively. Triangles indicate sites where feathers were collected on the breeding ground, and circles indicate migration collection sites.  P  . 166 [Auk, Vol. 124 Wassenaar and Hobson (2003). Briefly, several replicates of three keratin standards whose nonexchangeable δD values were known and that spanned the range of expected feather val-ues were measured within the same analytical run as the unknown feather samples. A least-squares regression was generated for each run, relating measurements of raw δD values of the standards to their expected nonexchangeable δD values. This regression equation was then applied to all unknown samples within that run. Unlike past methods to control for non-exchangeable feather hydrogen, this method allows for comparisons of δD values between years and among laboratories.All stable isotope analyses were conducted at the Colorado Plateau Stable Isotope Laboratory at Northern Arizona University, Flagstaff. Stable hydrogen isotope ratios for both feathers and keratin standards were determined on H 2  gases, produced by high-temperature flash pyrolysis of feathers using a Thermo Finnigan high-tempera-ture conversion elemental analyzer (1,400°C) interfaced through an open split (Finnigan Conflo II, Thermo Electron Corporation, Waltham, Massachuses), with a continuous-flow isotope-ratio mass-spectrometer (Finnigan Delta Plus XL). Stable hydrogen isotope ratios ( 2 H/ 1 H) are reported in delta (δ) notation, in per-mil units (‰), where δD sample  = [( R sample / R standard ) – 1] ×  1,000, relative to a standard (Vienna standard mean ocean water [VSMOW]). Repeat analyses of internal hydrogen isotope standards yielded an external repeatability of ±2.3‰. Duplicates of the same feather sample ( n  = 23) and comparison of values from two rectrices from the same warbler ( n  = 23) had mean standard deviations of ±2.3‰ and ±1.5‰, respectively. Statistical analysis. —To determine where warblers migrating through the migration sta-tions occurred within their breeding range, we used the regression equation that expressed the relationship between δD of feathers collected on the breeding grounds (δD f ) and δD of local pre-cipitation (δD p ) to estimate the δD of warblers caught at the migration stations (δD x ). Values of δD x  were used for all analyses.We used linear regression to examine the relationship between timing of migration and putative breeding location based on δD x . Analysis of covariance (ANCOVA) was used to test whether the same paern was exhibited in 2003 and 2004 at the Lower Colorado River and Arivaca Creek sites. We examined differences in T 1. Feather collection sites sampled during the breeding and spring migration of Wilson’s Warblers. Leers correspond to those in Figure 1. Mean stable hydrogen isotope values for feathers (δD f ) and stable hydrogen isotope values for precipitation (δD p ) are given for each  breeding site. Mean predicted stable hydrogen isotope values for feathers (δD x ) are given for each migration station. δD x Sampling site Latitude, longitude n  (mean ± SD) δD p Migration sites A. Big Sur, California 36°16’N, 121°49’W 112 –62.9 ± 15.9 B. Colorado River Delta, Mexico 32°18’N, 115°20’W 99 –77.7 ± 24.7 C. Lower Colorado River, Arizona 33°18’N, 114°41’W 186 –95.9 ± 24.7  a  D. Arivaca Creek, Arizona 31°33’N, 111°33’W 77 –92.7 ± 22.4  b  E. San Pedro River, Arizona 31°34’N, 110°07’W 113 –102.7 ± 24.5 Breeding sites F. Mother Goose Lake, Alaska 57°12’N, 157°19’W 4 –111.2 ± 9.1 –90G. Denali National Park, Alaska 63°25’N, 150°26’W 8 –151.7 ± 8.4 –118H. 100 Mile House, British Columbia 51°39’N, 121°17’W 6 –148.8 ± 6.0 –113I. Wenatchee National Forest, Washington  46°56’N, 121°04’W 10 –115.3 ± 9.4 –96 J. Willamee National Forest, Oregon 44°15’N, 122°00’W 14 –84.5 ± 5.6 –72K. Tahoe National Forest, California 39°37’N, 120°31’W 5 –87.5 ± 4.8 –64L. Flathead National Forest, Montana 48°23’N, 114°02’W 5 –123.7 ± 3.3 –97M. Grand Mesa, Colorado 39°02’N, 107°57’W 6 –103.8 ± 3.6 –88 a 2003 value; for 2004, δD x  ± SD = –106.9 ± 36.6.  b 2003 value; for 2004, δD x  ± SD = –99.9 ± 34.3.
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