It has been hypothesized that populations that are strongly connected between two periods of the year (i.e., individuals that breed in similar locations and also spend the nonbreeding season in similar locations) will be most vulnerable to population perturbations. Using stable-hydrogen isotopes in feathers and data from the North American Breeding Bird Survey, we examined this hypothesis for a vulnerable songbird, the Cerulean Warbler (Dendroica cerulea). Cerulean Warblers exhibit a parallel migration system, whereby western breeding populations are generally connected to southwestern wintering sites and eastern breeding populations are generally connected to northeastern wintering sites. As predicted, breeding populations that exhibited the strongest degree of migratory connectivity with a specific wintering region were also those populations that experienced the most severe declines over the past 40 years. Our results suggest that the strength of migratory connectivity should be an important factor when making resource-allocation decisions for the management and conservation of migratory species.
To provide an estimate of wintering location we collected crown feather samples from male Cerulean Warblers (n = 103) at five breeding locations in Ontario, Illinois, Pennsylvania, West Virginia, and Tennessee from 2001 to 2003. Cerulean Warblers molt and regrow their crown feathers during the stationary winter period prior to northward migration (G. Colorado, Universidad Nacional de Colombia, unpubl. data); hence, crown feather isotopic signatures reflect the chemical signature of where an individual spent the winter. We captured males in mist nets using song playback and painted model presentation. No individuals were sampled after mid-July to ensure that sampling occurred before the annual prebasic molt. We aged individuals as second-year (SY) or after-second-year (ASY) based on plumage coloration (Pyle 1997) and molt limits (Mulvihill 1993).
Stable-hydrogen isotopic ratios (δD) are expressed in delta notation in units of ‰, where δ = [(Rsample/Rstandard) – 1] × 1000, and Rstandard is the hydrogen isotope ratio of the international standard, Vienna Standard Mean Ocean Water (VSMOW). We washed feathers in a 2:1 chloroform:methanol mixture to remove surface contaminants and left them to air dry under a fume hood for 72 hr. Because a fraction of the hydrogen in feathers rapidly exchanges with ambient moisture (Wassenaar and Hobson 2000), feathers were equilibrated with local atmosphere for 72 hr to ensure that all samples had an equal opportunity to exchange with the local atmosphere (Norris et al. 2006). Based on controlled experiments, we found only a small proportion of exchangeable hydrogen (3%–5%) in tail feathers (MKG, DRN, and TKK, unpubl. data). To control for potential seasonal differences in the atmospheric moisture of δD values in the laboratory, all analyses were performed within a span of two months. We cut 0.10–0.15 mg from each feather (taking care to select the same portion from each), loaded each sample separately into a silver capsule, and heated the capsules at 100°C for 24 hr to remove potential surface water. After the capsules were crushed with metal tweezers, they were loaded into a reduction furnace (Finnigan TC/EA, Thermo Electron Corporation, Waltham, Massachusetts) at 1450°C, and introduced on-line to an isotope ratio mass spectrometer (Finnigan MAT Delta Plus XL). Within each run of 20–22 samples we ran three different standards (brucite, Georgia kaolinite clay, and an in-house keratin standard of domestic chicken [Gallus gallus] feathers), ensuring that at least one standard was run after every five samples (Norris et al. 2006). Previous work with these standards (Norris et al. 2006), as well as repeated measurements of the crown feathers used in this study, indicate that δD values were reproducible to ±3‰.