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Linking Breeding and Wintering Ranges of a Migratory Songbird Using Stable Isotopes

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Science  08 Feb 2002:
Vol. 295, Issue 5557, pp. 1062-1065
DOI: 10.1126/science.1067124

Abstract

We used the natural abundance of stable isotopes (carbon and hydrogen) in the feathers of a neotropical migrant songbird to determine where birds from particular breeding areas spend the winter and the extent to which breeding populations mix in winter quarters. We show that most birds wintering on western Caribbean islands come from the northern portion of the species' North American breeding range, whereas those on more easterly islands are primarily from southern breeding areas. Although segregated by breeding latitude, birds within local wintering areas derive from a wide range of breeding longitudes, indicating considerable population mixing with respect to breeding longitude. These results are useful for assessing the effects of wintering habitat loss on breeding population abundances and for predicting whether the demographic consequences will be concentrated or diffuse.

In recent decades, many species of neotropical migrant birds have shown marked changes in abundance—both increases and decreases—in parts of their North American breeding range (1,2). These changes may be due to events occurring in the breeding grounds, in the wintering grounds, or during migration (3). Because these birds migrate long distances and are not easy to track year-round, it is difficult to determine at which points in the annual cycle their populations are most vulnerable (3). It is thus necessary to develop methods for differentiating among populations, for determining where birds from particular parts of the breeding range spend the winter, and for estimating the degree to which individuals from different breeding areas mix with individuals from other breeding areas in their wintering quarters (4, 5).

Determining how birds mix on the wintering grounds will be particularly important for understanding how wintering habitat loss could affect abundances of breeding birds (4). Stable isotopes can be used to examine such potential links between the breeding and wintering ranges of migratory species (6–11). We used carbon and hydrogen isotopic ratios in the feathers of a neotropical migrant bird species to identify where individuals in particular wintering locations breed (breeding origins) and to assess whether individuals from different breeding areas co-occur in local wintering sites (population mixing). We described the isotopic patterns in feathers collected from almost 700 black-throated blue warblers (Dendroica caerulescens) throughout the species' breeding range in temperate North America and its main wintering range in the Greater Antilles (Fig. 1) (12). We then used the breeding ground isotopic patterns to build models from which we infer connections between the breeding and wintering ranges of this long-distance migrant.

Figure 1

Temperate North American breeding range and Greater Antillean wintering range of the black-throated blue warbler. A few individuals also winter in the Florida everglades, on the coast of the Yucatan Peninsula, and in Belize (12). Sampling locations in the breeding grounds and wintering grounds are indicated by black circles. In the Greater Antilles, samples were collected from individuals wintering in Jamaica (six sites in central and western portions), in Cuba (one site on a small cay, Cayo Coco, 15 km off the north-central coast), in Hispaniola (two sites in the Dominican Republic near the border with Haiti), and in Puerto Rico (three sites near the eastern end).

In birds, the isotopic ratios of carbon and hydrogen (the nonexchangeable portion) are fixed permanently into inert keratin tissues such as feathers, and they reflect a bird's diet and the local environment in which it grew those tissues (13). For insectivorous birds such as the black-throated blue warbler, isotopic ratios in feathers reflect those of the insects they eat, which in turn reflect those of the plants on which the insects have fed (14). The isotopic ratios are fixed in the feathers at the time of molt, which for many migratory passerines, including the black-throated blue warbler, occurs in late summer on or near breeding locations (12). Because isotopic ratios show natural patterns of geographic variation (15), they provide useful markers for identifying breeding localities and, ultimately, for determining links between breeding and wintering sites (6).

Carbon and hydrogen isotopic ratios (expressed as δ13C and δD values, respectively) in black-throated blue warbler feathers vary systematically along a latitudinal gradient throughout the species' breeding range in temperate North America (6, 15). We determined the δ13C and δD values (16) of feathers collected from black-throated blue warblers (17) at 10 sites that span the species' breeding range in temperate North America, as well as at 11 sites on four islands across the species' wintering range in the Greater Antilles (Fig. 1) (18). δ13C and δD values in black-throated blue warbler feathers decrease with increasing breeding latitude in the temperate North American breeding grounds (Fig. 2, A and B) (19). In the Greater Antillean wintering grounds, isotopic ratios in black-throated blue warbler feathers also show a strong geographic pattern, with δ13C and δD values in feathers decreasing from Puerto Rico to Cuba (Fig. 2, C and D). Thus, δ13C and δD values decrease from east to west—that is, with increasing longitude—within the wintering range of this species.

Figure 2

Population means (±SE) of δ13C and δD in black-throated blue warbler feathers at different breeding latitudes and wintering longitudes. Sample sizes are indicated next to each site. (A) δ13C values from the temperate North America breeding range decrease with increasing breeding latitude [F 1,8 = 7.67,P = 0.024, r 2 = 0.49, latitude = −19.35 − 0.11(δ13C)]. (B) δD values from the temperate North America breeding range decrease with increasing breeding latitude [F 1,7 = 7.17,P = 0.032, r 2 = 0.51, latitude = 1.32 − 2.02(δD)]. (C) δ13C values from the Greater Antillean wintering range decrease with increasing wintering longitude [F 1,9 = 6.07, P = 0.036,r 2 = 0.40, longitude = −18.81 − 0.07(δ13C)]. (D) δD values from the Greater Antillean wintering range decrease with increasing wintering longitude [F 1,7 = 22.93,P = 0.002, r 2 = 0.77, longitude = 20.82 − 1.51(δD)].

We used these geographic patterns in breeding ground isotopic signatures to develop a regression model that predicts breeding latitude (breeding origins) of wintering warblers based on δ13C and δD values in their feathers (20). Results indicate that more birds from the northern portion of the breeding range winter on the westerly islands of Cuba and Jamaica, whereas more birds from the southern portion of the breeding range winter on the easterly islands of Hispaniola and Puerto Rico (Fig. 3, A and B). Thus, black-throated blue warblers appear to segregate on the wintering grounds with respect to breeding latitude.

Figure 3

Breeding latitudes of wintering black-throated blue warblers predicted from a regression model (20). (A) Population mean (±SE) predicted breeding latitudes from birds sampled at different wintering longitudes in the Greater Antilles (Fig. 1). Sample sizes are indicated next to each site. Predicted breeding latitude increases with increasing wintering longitude (F 1,7 = 47.99, P = 0.0002,r 2 = 0.87). (B) Predicted breeding latitudes of individuals sampled on islands in the western wintering range (solid horizontal lines) and on islands in the eastern wintering range (dotted horizontal lines) (C, Cuba; J, Jamaica; H, Hispaniola; PR, Puerto Rico). The dashed line divides the northern and southern portions of breeding range at 42.9°N, the midway point between the northernmost sampling location in the southern portion and the southernmost sampling location in the northern portion (Fig. 1). Diamonds indicate the mean predicted breeding latitudes for each island. A greater proportion of birds from the northern portion of the breeding range winter on the more westerly islands, whereas a greater proportion of birds from the southern portion of the breeding range winter on the more easterly islands (χ2 = 58.44, df = 1, n = 144, P < 0.0001).

We developed a second model (21) that relates breeding longitude to isotopic ratios in the feathers of birds collected from the four most northerly breeding populations, which span ∼26° of longitude (Fig. 1). We then used this model to estimate the range of breeding longitude values represented in feathers collected from local wintering sites. We constrained our analysis to only those feather samples from wintering birds that had previously been identified as coming from the northern portion of the breeding range (Fig. 3) (20) and to those wintering sites (four on the westerly islands) with sample sizes of ≥10 individuals. The predicted breeding longitudes of these wintering birds ranged from 65.9° to 82.5°W at Cayo Coco, Cuba (n = 25); from 70.1° to 87.1°W at Baronhall Estates, Jamaica (n = 10); from 69.3° to 89.8°W at Font Hill Nature Preserve, Jamaica (n = 15); and from 73.5° to 85.0°W at Portland Ridge, Jamaica (n = 13). Thus, individuals wintering at these four sites came from breeding areas spanning an average of 16.4° of longitude, or 63% of the ∼26° of longitude over which this species breeds in temperate North America (Fig. 1). The predicted latitudinal range for these same individuals was 4.6°, or 27% of the ∼17° of latitude over which this species breeds (Fig. 1). Thus, black-throated blue warblers in local wintering sites are well mixed with respect to breeding longitude, even though they tend to be segregated with respect to breeding latitude. Because the southern portion of the breeding range covers only a narrow longitudinal range, it was not possible to do a comparable analysis for birds wintering on the easterly islands.

Studies of other migratory songbird species have found morphological (22) and behavioral (23) evidence for segregation on the wintering grounds by breeding region, as well as morphological (24) and isotopic (10) evidence suggesting mixing of breeding populations at wintering sites. Our results from a single species sampled across its entire breeding and wintering ranges illustrate that wintering populations can be segregated with respect to one axis of their breeding distribution (latitude) even while they are mixed with respect to another (breeding longitude). Future research must determine whether other migratory species exhibit similar regional patterns of segregation on the wintering grounds, and whether their wintering populations also tend to be mixed differentially with respect to breeding latitude and longitude.

Understanding patterns of migration has important implications for the conservation of songbirds and other migratory species. Because black-throated blue warblers appear to segregate by breeding latitude on the wintering grounds, the changes in breeding abundance (2) could, in part, be a consequence of the variable rates and geographic scope of habitat loss incurred on the different Greater Antillean islands. According to our results, habitat loss in Cuba or Jamaica should have had diffuse effects on our study species' abundance throughout the northern portion of its breeding range, but similar loss in Hispaniola or Puerto Rico should have had more concentrated and visible effects on abundances in the relatively narrow southern portion. In fact, breeding bird survey data from the past 30 years indicate declines in black-throated blue warbler abundance in the southern breeding areas—particularly at the southernmost extreme—and little change, or even increases, in abundance throughout much of the northern breeding range (2). Furthermore, the most extensive deforestation in the Greater Antilles (>97% of forestland) has occurred in Haiti (25), which lies on the island of Hispaniola. Our results connecting a high proportion of birds from the southernmost portion of the breeding range to the island of Hispaniola suggest a possible correlation between this severe wintering habitat loss and the sharpest breeding population declines. Understanding such geographic links between the breeding and wintering ranges and ultimately their relation to demographic trends is necessary to develop satisfactory models of songbird population dynamics (5). Only by considering events throughout the entire annual cycle can well-informed decisions be made about how best to conserve and manage declining populations of migratory birds.

  • * Present address: Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA.

  • To whom correspondence should be addressed. E-mail: drr24{at}cornell.edu

  • Present address: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA.

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