22 Nature? Nurture? Cowbirds Know


By Sarah Wilson, Healthy Planet Staff Writer

Cowbirds are the world’s most obnoxious neighbors. They lay their eggs in other birds’ nests, forcing those birds to raise the cowbirds’ young.

As you can imagine, this is not popular with the 200+ species cowbirds use in this way.

The other birds fight back. Species such as the Yellow Warbler call out a PSA “cowbird alert.” Many birds do their best to chase them off and a few, like Red-winged Blackbirds and Bobolinks, mob them. Being mobbed is no fun, regardless of your species, so cowbirds steer clear. My guess would be that this increases other species nesting in and around those with such united fronts.

Once a cowbird finds a less-defended nest, they check it regularly, waiting for a host egg or two to be laid before adding their own. Sometimes removing a host egg when they do, presumably so the math works out when the host returns.

Naturally, hosts may simply abandon the nest to try again elsewhere. Others attempt to dispatch such eggs by piercing the shell. That has meant that tougher, thicker shells survived so, these days, cowbird shells are hard to pierce. If the egg cannot be pierced or removed, some birds, the alarm-calling Yellow Warblers for example, take the “bury them alive” approach by building a new nest on top.

All this is clever but risky. Cowbirds don’t just lay and go away. They check in. If they find their egg dead or gone, they may trash the host nest. Whether this is a tantrum, revenge, or to reset the host nesting process is not yet known. Given all this, hosts may not remove the egg and end up raising “Baby Huey” — a bigger, hungrier, fast-growing chick, who often hatches first.

But then, how do cowbirds learn to be cowbirds? One way is that, as juveniles, they start sneaking out at dusk to hang out in fields around other cowbirds then return to the host nest in the morning. No one shows them how to do this, they just do it.

Also, young cowbirds respond to female cowbirds “chatter calls.” Called a “password” of sorts, this sound triggers the young cowbirds to learn the songs they hear after that call.

So, nature or nurture? Cowbirds say nature.