Are adoptions in a predatory songbird a strategy to aid mate acquisition?

. Adoption of unrelated young by adult birds is costly and therefore they should avoid it unless adoption carries compensating benefits. Loggerhead Shrikes ( Lanius ludovicianus ) have a monogamous mating system with low rates of extra-pair paternity and biparental care of young. To date, no documented instances of adoption have been published in this species. In two small nesting populations in South Carolina, we documented six cases in three years in which an unpaired Loggerhead Shrike adopted a brood of apparently unrelated nestlings or fledglings, in some cases sharing provisioning duties with the brood’s mother and in two cases taking over care completely while the mothers renested with different males. These adoptions occurred in 3.4% of all nesting attempts and in about 5% of broods that reached the nestling or fledgling stages. In at least five of the adoptions, young survived to independence. All adopters were male, and we documented a male-biased sex ratio among unpaired birds. We surveyed other Loggerhead Shrike researchers and uncovered three instances of similar alloparental care in three other populations, suggesting that this behavior is widespread but previously overlooked. We speculate that adoption by male Loggerhead Shrikes may be adaptive in male-biased populations by increasing the chances of an adopting male subsequently mating with the mother of the chicks he adopted, which happened in two of the six cases