- University of Maryland
Summary
Natal dispersal, the process through which immature individuals permanently depart their natal area in search of new sites, is integral to the ecology and evolution of animals. Insights about the underlying causes of natal dispersal arise mainly from research on species whose short dispersal distances or restricted distributions make them relatively easy to track. However, for small migratory animals, the causes of natal dispersal remain poorly understood because individuals are nearly impossible to track by using conventional mark–recapture approaches. Using stable-hydrogen isotope ratios in feathers of American redstarts (Setophaga ruticilla) captured as immature birds and again as adults, we show that habitat use during the first tropical nonbreeding season appears to interact with latitudinal gradients in spring phenology on the temperate breeding grounds to influence the distance traveled on the initial spring migration and the direction of natal dispersal. In contrast, adult redstarts showed considerable site fidelity between breeding seasons, indicating that environmental conditions did not affect dispersal patterns after the first breeding attempt. Our findings suggest that habitat occupancy during the first nonbreeding season helps determine the latitude at which this species of Neotropical–Nearctic migratory bird breeds throughout its life and emphasize the need to understand how events throughout the annual cycle interact to shape fundamental biological processes.
Methodology
Fieldwork was conducted in southwestern Jamaica at the Font Hill Nature Preserve (18°02′N, 77°57′W). In winter (January 15–February 20) of 2002–2005, we captured redstarts in mist nets, color-banded and processed them, and plucked a single tail feather for δD analysis. From March 20 to April 15 of each year, we recaptured birds to assess overwinter change in body mass. We estimated the date of departure on spring migration by searching territories of color-banded redstarts every 3 days from April 1 to May 15. When we failed to resight a bird, we rechecked the territory twice more during the 3-day period and once more in the next 3-day period. On this final visit, we broadcast a recording of redstart songs and chips for five bouts of 20 s interspersed with 30 s of silence. We considered birds to have left on migration when the playback drew no response. In each following winter, we captured color-banded redstarts returning from the previous winter to sample a second feather for δD analysis.
Isotope analyses were performed at the Queen's University Facility for Isotope Research (QFIR). Feathers were washed of surface oils and debris in a 2:1 chlorform:methanol solution and air-dried under a fume hood for 48 h. After transport to the QFIR, feathers were allowed to equilibrate with the local atmosphere for 72 h. A small sample of each feather (0.10–0.15 mg) was clipped, loaded into a silver capsule, and placed in a drying oven at 100°C for 24 h to remove potential surface water. The capsules were crushed, combusted at 1,450°C in an elemental analyzer (TC/EA; Finnegan), and introduced online to an isotope ratio mass spectrometer (MAT Delta Plus XL; Finnegan). One in-house standard was run for every five unknowns. We reported isotope ratios in δ notation relative to Vienna Standard Mean Ocean Water (VSMOW), where δD = (2H/1Hsample/2H/1Hstandard)−1) × 1,000. Analytical error (±1 SD) was 2‰ based on replicate analyses of the same feather (n = 18) and analyses of standards (kaolinite, n = 11; brucite, n = 12). We adjusted the isotope ratio of each feather by 19‰ to account for isotopic fractionation among precipitation, redstart prey, and feathers (38). The δD values reported here included both exchangeable and nonexchangeable hydrogen. To minimize any potential systematic error caused by nonexchangeable hydrogen, we analyzed all feather samples during a period of 6 days and included an approximately equal number of feathers from each habitat and age-class in each run of the mass spectrometer.
We examined the relationship between spring departure schedules of immature redstarts and date of lilac bud-burst at the latitude of their first breeding attempt by using data from World Data Center of Paleoclimatology (24) and a previously published δD base map (39). We first assigned the δD ratio of each feather sampled after the first breeding attempt to one 10‰ division on the δD base map. We then determined the average lilac bud-burst date within each 10‰ division of the δD base map. By assuming that the rate of migration was the same for individuals, we were able to assess whether immature redstarts used plant phenology as a cue for settling their first breeding territory.
We judged whether redstarts dispersed from or were faithful to their latitude of origin during the previous summer by calculating the 95% C.I. of δD in feathers sampled from separate population of redstarts (±9‰, n = 42) known to have bred at the Queens University Biological Station (44°43′N, 76°19′W) (23). In the present study, only individuals whose feathers from successive years had differences in δD in excess of ±9‰ were considered to be dispersers. This cutoff likely caused us to label as site-faithful some birds that actually dispersed short distances from their origin the previous summer. Despite the potential for such error, we believed this approach to defining dispersal events was warranted given published estimates of variation in δD in feathers molted at the same latitude (23, 26, 40).