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Avian brood parasites such as the brown-headed cowbird and the common cuckoo are known as “obligate” brood parasites, meaning that they must parasitize other birds in order to reproduce. They do not build nests of their own to house their eggs or raise any of their own nestlings. As a result, whether hosts choose to reject or accept parasitic eggs controls the reproduction of these species of birds.
Parasitic birds watch for opportunities to exploit hosts in the nesting season. When host birds leave the nests, they approach the nest and lay their own eggs inside. Several differences may occur at this point depending on the species of parasite, such as the time in the laying period when the parasites act and whether host eggs are eaten or ejected. The common cuckoo, for instance, pushes out a host egg before laying its own. Its young hatch before the other nestlings and eject the other eggs, monopolizing the food source. Brown-headed cowbird hatchlings, on the other hand, do not exhibit this behavior and are raised with the host’s own hatchlings. These will result in different strategies by hosts in responding to reduce the costs imposed by parasitism.
Credits: Wikimedia Commons
Just as experience in humans helps us to avoid being duped, there is some evidence suggesting that experienced breeders are less likely to be fooled into accepting and raising parasitic chicks. In one of the first studies investigating breeder age, Lotem et al. (1992, 1995) introduced and removed eggs from nests of reed warblers ( Acrocephalus arundinaceus ) and then observed rejection of cuckoo eggs and artificial painted eggs. The authors argue for a learning-based mechanism for rejection; that is, breeders memorized their own eggs through the instinctual learning process called imprinting in order to reject dissimilar eggs. When eggs were introduced, false eggs were accepted more often by mid-season breeders, which are younger on average than early-season breeders Replacing all of a reed warbler host’s eggs rather than one, and therefore reducing the host’s exposure to its own eggs, caused the naïve mid-season breeders to accept more false eggs, but had no effect on experienced early-season breeders. As further support for learning through experience, returning an original egg to the experienced early breeders prompted rejection of an entire clutch of eggs, but had no effect on less inexperienced mid-season breeders. The experienced breeders were better able to detect sudden changes in their clutches than the naïve breeders and were able to respond accordingly.
A species may reject parasitic eggs by one of a number of methods. For example, the yellow-browed leaf warbler ( Phylloscopus inornatus ) accepts eggs based on size and then adjust their preferences based on variations within the clutch (Marchetti 1999). When clutches were entirely replaced with large artificial eggs, the birds would accept and incubate them. The birds would also completely accept the original eggs if they were exchanged for the batch of artificial eggs. However, the presence of even one large artificial egg resulted in its rejection. Although it is parasitized by cuckoos, this warbler has a very brief learning period since it judges based on size instead of appearance. This is opposite of the predictions by Strausberger and Rothstein, but the bird’s dynamic ability to adjust preference depending on the variation between its eggs at any given time offers a distinct advantage against cuckoo eggs.
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