Breeding between Tree Swallows from the same brood

We provide data on three instances where Tree Swallows (Tachycineta bicolor) that fledged from the same nest (broodmates) eventually bred together in subsequent years. Two instances were detected in Ontario and a third was detected in Nova Scotia. Based on demographics at the times of each event, we estimated probabilities of these broodmate pairings as approximately 1 in 16,000, 1 in 5600, and 1 in 29,000, respectively, whereas the number of identified pairs in the populations was less than 65 in each case. Thus, inbreeding occurred at a higher frequency than expected by chance. We cannot distinguish whether these identical natal dispersal responses arose from similarity in genes or in rearing environments.