Cowbirds (genus Molothrus) are brood parasites in North and South America. As a brood parasite, they parasitize the nests of other birds. Instead of building their own nests and tending to their eggs themselves, cowbirds lay their eggs in the nests of other species, letting this host species care for the parasitic eggs. Understandably, cowbirds have to time their parasitism well. If the cowbirds lay their eggs before the host has laid any, then the host would obviously know something was wrong. But if the cowbirds lay their eggs too late after the host has laid its own, then the host eggs may hatch before the cowbird eggs, and the host may stop caring for the cowbird eggs, or outright destroy them. Fortunately – for the cowbird – if the host’s nest is destroyed, then it may start another nest if enough time remains in the season. Cowbirds apparently take advantage of this behavior, destroying too-developed host nests in the hope that the host will start a new nest, which the cowbird may then be able to parasitize.
A recent study by Swan et al. (2015) investigated this behavior in brown-headed cowbirds (Molthrus ater), hoping to discover whether cowbirds actually farm their hosts, or whether the cowbirds just appreciate wanton destruction. Swan et al. (2015) found that when they presented female cowbirds with a choice of young vs. old nests, females destroyed the old nests more than the young nests. Specifically, the females were equally likely to attack both nest types, but would only destroy one egg from young nests, and all eggs from old nests. The authors hypothesized that the cowbirds used the first egg to assess egg age, and if the egg was developed, then the cowbirds destroyed the rest of the eggs. The authors further found that cowbirds were more likely to attack nests with a greater number of eggs, as a greater number of eggs also indicates that the host has finished laying eggs (meaning the current eggs are highly developed or the host would not accept further eggs, such as those laid by a cowbird). Swan et al. (2015) also compared these lab results with field data collected over 10 years, which demonstrated a positive correlation between host nest age and cowbird attacks, that as host nest age increased, cowbird attacks increased.
Swan, D.C., Zanette, L.Y. & Clinchy, M. (2015). Brood parasites manipulate their hosts: experimental evidence for the farming hypothesis. Animal Behaviour, 105, 29-35.