Probing parentage in parasitic birds: an evaluation of methods to detect conspecific brood parasitism using goldeneyesBucephala islandicaandBl. clangulaas a test case

Abstract

Conspecific brood parasitism (CBP) occurs in over 200 species of birds. Efforts to detect CBP have relied on either observational criteria, or more recently, on molecular methods. While molecular approaches are powerful, they are expensive, time consuming and may prove prohibitive for studies requiring estimates of CBP over large spatial and temporal scales involving hundreds of nests. We evaluated a series of observational methods that have been applied in previous studies to detect CBP, using two species of cavity‐nesting ducks, the Barrow's goldeneyeBucephala islandicaand common goldeneyeB. clangula, as test species. We first describe a method based on differences in egg morphology and find it to be a reliable method to detect CBP in both species in British Columbia, Canada. The application of recursive partitioning analysis was especially effective in classifying parasitized and non‐parasitized nests using differences in egg morphology. We then evaluated five additional observational criteria that have been used previously in several studies to detect CBP in birds. We show that considerable redundancy exists among all criteria, as expected, but no single method is effective at detecting all suspected cases of CBP. Subsets of criteria (2 or more eggs/d, eggs laid 2 or more days after incubation, and clutch sizes exceeding 12 eggs) were successful, in combination, in detecting 75% of parasitized nests for goldeneyes. Finally, we suggest that ecological and evolutionary analyses of the dynamics of CBP will require estimates of the frequency of the parasitic tactic in the population (rather than just the proportion of parasitized nests) and we provide a simple method to obtain such an estimate. Although our data are specific to goldeneyes, the techniques we used should have broad application to other studies of CBP.