American Crow

Of all the collective nouns for birds, a “murder” of crows tends to be among the more popular. Presumably this terminology came about because of the tendency for large numbers of crows to gather at places like battlefields to feed on corpses. Thankfully we don’t see murders of American Crows in this context in New Hampshire, but during the non-breeding season it’s possible to observe huge numbers going toward their communal evening roosts. These are usually in large groups of trees, sometimes in urban areas, and are quite the avian spectacle if you happen upon one. Birds will start streaming in from all directions in the mid-afternoon and stage in trees around the periphery of the roost before converging on around dusk. Starting in late winter these roosts start to break down as birds shift to breeding territories.

American Crows are cooperative breeders, meaning that young from one or more previous years often remain in a territory to help rear the next brood of young. These helpers aid with tasks as varied as nest construction, feeding the incubating female or young, nest defense, and even occasionally incubation. The number of helpers can be as high as ten and some may remain for 5-6 years before finally going off to set up a territory of their own. The benefits of having helpers vary. In some cases, multiple helpers improve nest success but not in others, and the helpers themselves presumably benefit from gaining nesting experience.

Crows provide an excellent illustration of the potential effects of disease on bird populations. When West Nile Virus first appeared in New York City in 1999 it killed thousands of crows in a matter of months, and probably millions more as it spread outward across the Northeast and beyond. You can see a dramatic drop in crow numbers on Breeding Bird Survey data (but oddly not in NH), but as the disease spread it became less virulent. Crows are still slowly declining in most areas (including New England), but the sudden drop related to disease did not continue. Time will tell if populations of this adaptable bird will recover to the levels before West Nile was introduced to North America.