A new form of reproductive parasitism in cliff swallows

Abstract

A common reproductive strategy among some egg-laying animals, especially birds, is to lay an egg in another individual's nest and thereby parasitize the reproductive effort of others of either the same species or a different species. Intraspecific parasitism is now known to occur regularly in some species 1–4 and sporadically in many others5,6, and may represent a strategy by which individuals augment their reproductive performance2 or succeed in reproducing when it would otherwise be impossible or too costly5. We have discovered that colonial cliff swallows (Hirundo pyrrhonota) not only lay eggs in other individuals' nests, but also physically transfer eggs between nests after the eggs are laid. Egg transfers can occur at any time after an egg is laid and before it hatches, and may represent a strategy by which an individual distributes its eggs in several nests to ensure some surviving offspring in the event of nesting failures. Sneaky transfer of eggs between nests represents a previously unknown form of reproductive parasitism in birds.