Cowbird eggshells could double as deadly weapons

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.