Geographic variation of strontium and hydrogen isotopes in avian tissue: implications for tracking migration and dispersal

Abstract

Background: Isotopes can provide unique solutions to fundamental problems related to the ecology and evolution of migration and dispersal because prior movements of individuals can theoretically be tracked from tissues collected from a single capture. However, there is still remarkably little information available about how and why isotopes vary in wild animal tissues, especially over large spatial scales.

Methodology/principal findings: Here, we describe variation in both stable-hydrogen (deltaD(F)) and strontium ((87)Sr/(86)Sr(F)) isotopic compositions in the feathers of a migratory songbird, the Tree Swallow (Tachycineta bicolor), across 18 sampling sites in North America and then examine potential mechanisms driving this variation. We found that deltaD(F) was correlated with latitude of the sampling site, whereas (87)Sr/(86)Sr(F) was correlated with longitude. deltaD(F) was related to deltaD of meteoric waters where molting occurred and (87)Sr/(86)Sr(F) was influenced primarily by the geology in the area where feathers were grown. Using simulation models, we then assessed the utility of combining both markers to estimate the origin of individuals. Using 13 geographic regions, we found that the number of individuals correctly assigned to their site of origin increased from less than 40% using either deltaD or (87)Sr/(86)Sr alone to 74% using both isotopes.

Conclusions/significance: Our results suggest that these isotopes have the potential to provide predictable and complementary markers for estimating long-distance animal movements. Combining isotopes influenced by different global-scale processes may allow researchers to link the population dynamics of animals across large geographic ranges.

Figure 1. Map of 18 Tree Swallow sampling sites.

Sites are black dots, which are overlaid on the relative breeding abundance (based on data from the Breeding Bird Survey; [37]). The intensity shading represents breeding density where lightest grey is lowest density (<1 individual) and black is the highest density (>100 individuals; following [37]).

Figure 2. Relationship between δD and ⁸⁷Sr/⁸⁶Sr values in Tree swallows sampled across North America.

Values are mean±s.e. from primary flight feathers. Abbreviations are: AB = Red Deer, Alberta; AK = McCarthy, Alaska; IA = Ames, Iowa; MA = Amherst, Massachusetts; MB = Brandon, Manitoba; MI = Allendale, Michigan; MT = Monarch, Montana; NS = Wolfville, Nova Scotia; NW ON = Thunder Bay, Ontario; NY = Ithaca, New York; OR = Portland, Oregon; SE ON = Elgin, Ontario; SK = Saskatoon, Saskatchewan; SW ON = Guelph, Ontario; TN = Lenoir City, Tennessee; VA = Waynesboro, Virginia; WI = Saukville, Wisconsin; WY = Big Horn, Wyoming.

Figure 3. Relationship between geographic location and isotopes in flight feathers of Tree swallows.

Isotope values are mean±s.d. from 18 sites. (a) latitude versus δD_F (r_s = −0.47, P = 0.05), (b) longitude versus ⁸⁷Sr/⁸⁶Sr_F (r_s = 0.62, P = 0.009).

Figure 4. Geographic variation of (a) δD and (b) ⁸⁷Sr/⁸⁶Sr values in Tree Swallow feathers.

Contour maps were produced by ordinary kriging and are based on mean values in primary flight feathers at 18 breeding sites (denoted by black circles).

Figure 5. Predictors of δD and ⁸⁷Sr/⁸⁶Sr values in Tree swallow primary flight feathers.

Each point represents an individual (n = 79). (a) δD growing season precipitation (δD_GS) versus δD_F (b) age of under lying bedrock versus ⁸⁷Sr/⁸⁶Sr_F. δD_GS data are from waterisotopes.org . Bedrock ages represent the estimated age of the components that make up the rocks in the area that contribute to the strontium reservoir of the area (see appendix S2). Results from the GLMM are presented in table 1.

Figure 6. The proportion of Tree swallow correctly assigned to their original site of origin.

(a) δD, (b) ⁸⁷Sr/⁸⁶Sr, and (c) δD and ⁸⁷Sr/⁸⁶Sr. Proportion of correct assignments are based on 5000 simulations at each of the 18 breeding sites.

Figure 7. The proportion of Tree swallows correctly assigned to their original site of origin.

Same as figure 6, except 18 sites were collapsed to 13 sites based on geographic proximity. The Great Lakes group (GL) includes Ithaca, NY, Elgin, ON, Allendale, MI, and Saukville, WI, the Mid-west group (MW) includes Ames, IA, Big Horn, WY, and Brandon, MB. All other sites are the same as in figure 6.