Ecological correlates of mate fidelity in two Arctic-breeding sandpipers

Monogamous birds exhibit considerable interspecific variation in rates of m ate fidelity between years, but the reasons for this variation are still po orly understood. In a 4-year study carried out in western Alaska, mate-fide lity rates in Semipalmated Sandpipers (Calidris pusilla; mate fidelity was 47% among pairs where at least one mate returned and 94% among pairs where both mates returned) were substantially higher than in Western Sandpipers ( Calidris mauri; 25 and 67%, respectively), despite the similar breeding bio logy of these sibling species. Divorce was not a response to nesting failur e in Western Sandpipers, and mate change had no effect on the reproductive performance of either species. Nor were mate-fidelity rates related to diff erential rates of breeding dispersal, because the species did not differ in site fidelity. Reunited pairs and males that changed mates showed strong s ite tenacity, while females that changed mates moved farther. Differences i n local survival rates or habitat are also unlikely to explain mate fidelit y, since the two species did not differ in local survival rates, phi (Weste rn Sandpipers: <(<phi>)over cap> = 0.57 +/- 0.05 (mean +/- SE), Semipalmate d Sandpipers: <(<phi>)over cap> = 0.66 +/- 0.06), and they bred in the same area, sometimes using the same nest cups. Although we were able to reject the above explanations, it was not possible to determine whether mate reten tion was lower in Western Sandpipers than in Semipalmated Sandpipers becaus e of interspecific differences in mating tactics, time constraints imposed by migration distance, or a combination of these factors. Western Sandpiper s exhibited greater sexual size dimorphism, but also migrated for shorter d istances and tended to nest earlier and more asynchronously than Semipalmat ed Sandpipers. Finally, we show that conventional methods underestimate div orce rates, and interspecific comparisons may be biased if breeding-dispers al and recapture rates are not considered.