Overlooked Costs of Coloniality: Mislaid Eggs and the Double Incubation of Separate Nests

The evolution of colonial breeding remains an outstanding question in evolutionary biology due to our limited understanding of the costs and benefits of group living. We document 85 cases of Barn Swallows (Hirundo rustica erythrogaster) laying, and subsequently abandoning, eggs in empty, unclaimed nests located adjacent to active nests during a 6 y study. The frequency of this behavior was positively correlated with total available nests, a metric that increases with colony size. In addition, two female swallows were observed alternately incubating multiple clutches after mislaying eggs in neighboring nests. We argue the potential to mislay eggs and allocate parental care across separate nests may represent an overlooked cost of colonial nesting in birds.