Summary
The global decline in many species of migratory birds has focused attention on the extent of migratory connectivity between breeding and wintering populations. Stable-hydrogen isotope (δD) analysis of feathers is a useful technique for measuring connectivity, but is constrained by features of molt location and timing. Claws are metabolically inert, keratinous tissues that grow continuously and can be sampled at any point in the annual cycle, thus providing potentially useful clues about an individual’s previous movements. However, variation in the rate at which claws incorporate local δD values is not well described. We measured δD values in claws of two species of Neotropical-Nearctic migrant wood-warblers (Golden-winged Warbler and Cerulean Warbler) breeding in eastern Ontario, Canada to investigate the rate of δD change through the breeding season and the utility of claw δD values for estimating migratory origins. δD values of claw tips from 66 different individuals, each sampled once during the breeding season, showed an average change of -0.3‰ to -0.4‰ per day in the direction of the expected local Ontario value. There were no significant sex or species differences in the rate of change. These results suggest δD values of claw tips in Parulids may reflect those of the non-breeding area for 3–7 weeks after arrival on the breeding grounds, and are useful estimators of non-breeding migratory origin. Our results also suggest that these species may leave the breeding ground before claw tips fully incorporate a local δD signature, as claws sampled at the end of the breeding season did not match locally grown feather and claw δD values. This is the first study to examine the seasonal rate of the change in δD values of claws in long-distance, insectivorous, migratory birds.
Methodology
A total of 42 adult Golden-winged (female = 17, male = 25) and 22 Cerulean (female = 6, male = 16) Warblers were captured in mist nets throughout the breeding season (May–July) at the Queen’s University Biological Station (QUBS, 44º34’N, 76º19’W) near Chaffey’s Locks, Ontario in 2005. In 2004, we sampled claws from three Golden-winged Warbler hatch-year individuals that had recently fledged and ten adults late in the breeding cycle (1–10 July), as well as the rectrices of eight adults known to have bred at the study site in the previous year. Males were captured using conspecific song playback; females via Black-capped Chickadee (Poecile atricapillus) ‘mobbing’ call, or when flushed from the nest after 4 d of incubation. Approximately 2 mm of claw tip from the middle toe of both feet was collected using sharp dissection scissors, and deposited in a small paper envelope. Capture and sampling were carried out under permit from Environment Canada (CA 0106) and our field methods were approved by Queen’s University Animal Care Committee (Protocol-2005-017-R1). All individuals were released without injury, and no females abandoned their nests after capture. One claw per individual (from left or right foot, chosen randomly) was washed to remove lipids and particles in a 2:1 chloroform:methanol solution for 24 h and left to air dry. To account for potential exchangeable hydrogen effects between ambient laboratory water vapor and tissue, we used an equilibration technique. Claws were stored in open containers in a fume hood for at least 1 week at the Queen’s Facility for Stable Isotope Research (QFIR). Although other laboratories recommend the use of a calibration curve derived for each run using three in-house keratin standards for which an estimate of the non-exchangeable δD has been derived (Wassenaar and Hobson 2003), we used a single homogenized chicken feather keratin standard (δD = -95±5‰) and two mineral standards (Georgia clay δD = -58‰ and brucite δD = -96‰). Additionally, we assumed that ambient lab moisture δD available for exchange with our standards and unknowns remained constant throughout the year as our lab was maintained at a constant temperature, and air-conditioning water came from Lake Ontario, a large reservoir with relatively constant δD. Our laboratory results for feathers and claws are assumed to be reproducible to ±3‰, but may not be comparable to those produced by other laboratories using different calibration techniques (Wassenaar and Hobson 2006). Mineral and keratin standards and claw samples were weighed (0.08–0.22 mg), placed in silver capsules, and oven dried for 24 h at 100°C to remove any absorbed water, which can be a substantial source of exchangeable hydrogen (Algie and Watt 1962, Bowen et al. 2005a). Samples were crushed and loaded into an autosampler connected to a Finnigan TC/EA (1450°C) reduction furnace, where they passed online to a Finnigan MAT Delta Plus XL isotope ratio mass spectrometer for analysis. Stable hydrogen isotope ratios (2H/1H = R) are expressed in per mil (‰) units, where δ = [(Rsample/Rstandard)-1] x 1000 and Rstandard is the hydrogen isotope ratio of Vienna Standard Mean Ocean Water (VSMOW).