When parents let the kids go hungry.

Scientists creating fake crises at bird nests have confirmed a theory about parents protecting themselves at their offspring's expense.

According to theorists, this type of behavior should occur in species that live relatively long and bear only a few young at a time. Species with shorter lives and that produce more offspring per batch should take the opposite path: Parents ought to risk their own lives for the sake of offspring. Field studies have confirmed these proposed behavior patterns, say Cameron K. Ghalambor of the University of California, Davis and Thomas E. Martin of the University of Montana in Missoula.

They analyzed life patterns for 182 songbirds. Southern Hemisphere species tend to live longer and lay smaller clutches than northerners, the researchers report in the April 20 SCIENCE. Ghalambor and Martin identified five pairs of species with similar ecology but living in opposite hemispheres. For example, they matched a robin with a rufous-breasted thrush.

Playing calls of hawks, which eat adult birds, near a nest prompted more self-protective behavior among the southern species. These birds, with a smaller relative investment in any particular clutch than their northern counterparts, chose to avoid feeding runs and remain hidden, even though their young went hungry. The northern birds, however, risked such runs.

When these braver parents heard calls of a jay, which kills nestlings but not adults, they curtailed feeding runs, which might have revealed their nest's location. However, their less invested counterparts hazarded such trips, the researchers observed.