Adoption of non-related offspring is widespread in all vertebrates with parental care. I studied the life history costs and benefits of chick adoption in the pied avocet (Recurvirostra avosetta), a mid-sized shorebird breeding on sea coasts and in a peculiar inland population in southern Hungary. In a three-year study of 848 nests, 360 families and 800 color-ringed chicks, we found that artificial habitats are better for nesting but worse for brood-rearing than natural habitats.

Results/Conclusions

We found that chick adoption, which occurs in more than 20% of the families, can provide benefits to parents by increasing the size of the brood. Under high predation (artificial habitats), larger broods containing non-filial young experienced decreased risk of predation on the parent's own chicks. Under low predation (natural habitats), larger broods enabled parents to access better resources for chick-rearing. Why don't parents simply lay more than four eggs then? In a clutch enlargement experiment, we found that incubating one extra egg led to increased incubation costs for parents and that experimentally enlarged families were less successful than controls. Chick adoption may thus serve to reap the benefits of larger families without the costs of having to produce and incubate extra eggs.