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Adoptions: the misfiring of adaptive parental care

All behaviors do not necessarily have to be directly adaptive. In fact, there are many instances where an animal unknowingly decreases its fitness when performing some action. Some cases of adoption of young birds are examples of this. Kin recognition is not an ability that all bird species possess. Parents cannot tell their own offspring from other young and will resort to certain clues to determine who to feed. For example, if a young bird is hungry and in a nest, the adults in that nest will feed it. It is better for parents to care for all young despite the possibility that some of the young may not be theirs than to risk letting one of their own offspring die by not taking care of it. Generally, the policy of caring for all young in a nest is a safe bet for most two-parent offspring-raising systems because it is not common to find non-related young there, though brood parasitism is a notable exception (Wetzel and Chandler 2008; Krueger and Davies 2002).

Infanticide in the communally breeding guira cuckoo

The South American Guira Cuckoo ( Guira guira ) is one of the few birds that breeds communally, a type of breeding where three or more individuals reproduce together either in a joint nest or nearby nests (Brown 1987; Cariello et al. 2002) (see [link] ). Guira cuckoo groups are composed of related and unrelated members, where up to 7 can be reproducing members who share a joint nest (Cariello et al. 2002). With a communal clutch of up to 26 eggs, competition occurs between these reproducing individuals, which leads to pushing other eggs out of the nest and to infanticide (Cariello et al. 2002; Macedo and Melo 1999). In larger groups where competition between the hatchlings is the most intense, guira cuckoo mothers lay larger, heavier eggs, which hatch into larger, heavier chicks that can better survive (Macedo et al. 2004). Smaller chicks are more likely to die due to infanticide than larger ones. Moreover, because older chicks begin to eat and grow before younger chicks hatch, they tend to escape infanticide more often than the younger chicks (Macedo et al. 2004). In the guira cuckoo, infanticide is a common occurrence, where nearly half of the young in the communal clutch can be killed. In most cases, an infanticidal adult picks up a few-day old nestling, tosses it out of the nest and pecks it repeatedly (Macedo and Melo 1999). Interestingly, the infanticidal adults are usually accepted members in the guira cuckoo group and not immigrant or replacement individuals. Macedo and Melo (1999) suggest that this could be a reproductive strategy used by the infanticidal adult to force the group to renest, lay more eggs, and possibly allow the adult to produce more offspring.

a Guira Cuckoo
Guira Cuckoo, Guira guira.
Courtesy of Aaron Siirila

Though not typically considered “infanticide,” the guira cuckoo’s pushing other eggs out of the nest is a form of competition and a way to get rid of young that would compete with one’s own offspring for resources and protection. Like adoption and infanticide, the cuckoos have developed this behavior after generations and generations of natural selection. The practice of this behavior increases the fitness of the actor because it allows the actor’s eggs to make up a higher percentage of the total communal clutch. Moreover, cuckoos have developed counter tactics to this practice. By making the eggs heavier and larger, cuckoos have a harder time dumping them out of the nest (Macedo et al. 2004). Thus the heavy eggs serve a double purpose: to produce big chicks and to prevent egg-dumping!

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Source:  OpenStax, Mockingbird tales: readings in animal behavior. OpenStax CNX. Jan 12, 2011 Download for free at http://cnx.org/content/col11211/1.5
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