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Parasitic Birds Plan Their Hits Like Professional Burglars

The cowbird is a master thief that “steals” the maternal care of unsuspecting victims, laying its eggs in the nests of other birds so that they unwittingly incubate and raise chicks that are not their own. In a new study, researchers reveal how, before deciding on which nests to target, cowbirds case the joint like professional criminals.

Appearing in the journal Behavioral Ecology and Sociobiology, the research concerns shiny cowbirds and screaming cowbirds, which parasitize the nests of mockingbirds and baywings, respectively. The study authors used video recordings and radio tracking to follow the movements of these sneaky avian con-artists in the periods just before and after laying their eggs.

They discovered that both species of cowbird meticulously plan their crimes by visiting prospective nests in order to keep tabs on their hosts, building up a memory bank of which birds are getting ready to lay eggs and can therefore be targeted – a technique known as “bookkeeping”.

The crafty cowbirds always conduct their reconnaissance missions before dawn, using the cover of darkness to avoid being detected.

Interestingly, screaming cowbirds tended to visit nests about twice as frequently as shiny cowbirds. The researchers say this is probably because baywings are much less predictable than mockingbirds in the timing of their egg-laying, which means screaming cowbirds have to monitor their favored hosts much more closely than shiny cowbirds do theirs.

Screaming cowbirds also continue to check up on baywing nests after they have parasitized them, to make sure that the impostor chicks have not been discovered and ejected by the host. In contrast, shiny mockingbirds do not return to nests that they have laid in, because they always peck holes in the eggs of their hosts in order to prevent them from hatching, thereby ensuring that the host provides more care for the surviving impostor chicks.

They therefore deliberately avoid all nests that they have laid in, to ensure they don’t accidentally destroy their own eggs.

While this may all seem somewhat devious and despicable, it is nothing compared to the ruthlessness of the brown-headed cowbird, which is said to engage in “mafia behavior,” whereby it repeatedly destroys the nests of hosts that have ejected its eggs, just out of spite.

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