A Method for Investigating Population Declines of Migratory Birds Using Stable Isotopes: Origins of Harvested Lesser Scaup in North America

Abstract

BackgroundElucidating geographic locations from where migratory birds are recruited into adult breeding populations is a fundamental but largely elusive goal in conservation biology. This is especially true for species that breed in remote northern areas where field-based demographic assessments are logistically challenging.Methodology/FindingsHere we used hydrogen isotopes (δD) to determine natal origins of migrating hatch-year lesser scaup (Aythya affinis) harvested by hunters in the United States from all North American flyways during the hunting seasons of 1999–2000 (n = 412) and 2000–2001 (n = 455). We combined geospatial, observational, and analytical data sources, including known scaup breeding range, δD values of feathers from juveniles at natal sites, models of δD for growing-season precipitation, and scaup band-recovery data to generate probabilistic natal origin landscapes for individual scaup. We then used Monte Carlo integration to model assignment uncertainty from among individual δD variance estimates from birds of known molt origin and also from band-return data summarized at the flyway level. We compared the distribution of scaup natal origin with the distribution of breeding population counts obtained from systematic long-term surveys.Conclusions/SignificanceOur analysis revealed that the proportion of young scaup produced in the northern (above 60°N) versus the southern boreal and Prairie-Parkland region was inversely related to the proportions of breeding adults using these regions, suggesting that despite having a higher relative abundance of breeding adults, the northern boreal region was less productive for scaup recruitment into the harvest than more southern biomes. Our approach for evaluating population declines of migratory birds (particularly game birds) synthesizes all available distributional data and exploits the advantages of intrinsic isotopic markers that link individuals to geography.

A multi-isotope (δ13C, δ15N, δ2H) approach to connecting European breeding and African wintering populations of barn swallow (Hirundo rustica)

“…Similar to our treatment of the multi-isotope assignments to molt origins in Africa using clusters, we also used the ringrecovery DFA to derive prior probabilities, and used Bayes' theorem to estimate posterior probabilities of origin given the observed δ 2 H f and the prior probability of wintering north versus south of 6.7° S. Thus, for each individual, we obtained one probability of origin map. To portray the distribution of origins for a population sample, origin maps of individuals were combined by selecting the cells within the isoscape that represented the upper 67% of likelihoods and coding these as likely (1) and all other cells as unlikely (0), representing 2:1 odds of being correct versus incorrect [34][35][36]. We then used simple addition of the maps to depict the geographic distribution of assigned origins.…”

Models to predict the distribution and abundance of breeding ducks in Canada

“…Annual abundances were correlated with the number of available wetlands , but only for regions with annual pond counts. Spatial interpolation of survey data has been used to predict abundances of Scaup (Aythya affinis, Aythya marila) pairs across their range (Hobson et al 2009). However, no SAMs and their derived predictive abundance maps have been published for any waterfowl species at national extent.…”

Shorebird hunting in Barbados: Using stable isotopes to link the harvest at a migratory stopover site with sources of production

“…First, we converted an amount-weighted growing season precipitation surface from Bowen et al (2005) (Hobson et al 2009b). We used digital range maps obtained from BirdLife International and NatureServe (2015) to delineate known breeding areas for all 4 species; breeding ranges for American Golden-Plover in northern Quebec, Canada, and Stilt Sandpiper in eastern Nunavut and western Northwest Territories, Canada, were modified based on secondary sources (Québec Breeding Bird Atlas: http:// www.atlas-oiseaux.qc.ca/index_en.jsp, Andres 2006) and other regional surveys (Andres 2006).…”

“…Similarly to Hobson et al (2009b), we used an odds ratio of 2:1 to assign individuals to their potential natal origin, where cells in the isoscape in the upper 67% of probabilities were considered likely (1) origins and all others were considered unlikely (0). Assignments conducted for feather samples resulted in a spatially explicit binary surface for each individual; surfaces were summed across assignments for all individuals of a species to represent potential origins for that species.…”