Cowbirds are some of nature’s shrewdest freeloaders: Instead of caring for their young, they lay their eggs in the nests of unsuspecting songbirds, who rear the foster chicks as their own. Now, scientists have found another way the wily birds may be harming their hosts—their extrathick eggshells can crack the hosts’ own eggs when they plop into the nest.
For years, the particularly thick shells of cowbird eggs and those of other parasitic birds puzzled scientists. Some researchers suggested they provided the orphaned embryos with added calcium or protected the eggs from the pointy beaks of their surrogate parents. Suspecting something else was going on, one team of scientists launched an egg-dropping experiment.
They gathered 157 freshly laid cowbird eggs and dropped them onto host eggs from a height of 6 centimeters. The scientists also reversed the scenario, dropping host eggs from wrens, mockingbirds, and blackbirds onto cowbird eggs. Cowbird eggs were almost immune to cracking in either scenario, with a mere 3.6% chance of getting damaged, the researchers reported last month in Behavioral Ecology. But the cowbird eggs managed to crack their hosts’ eggs about 10 times as often. In the wild, laying these deadly, thick-shelled eggs cunningly eliminates would-be food competitors from the nest before they even hatch.