Biology Reference
In-Depth Information
it as their own chick. When young cowbirds start feeding on their own, they leave their
foster parents and join a flock of cowbirds.
Cowbirdsare
brood parasites.
Theyhavenoinstinct tobuildanestatallortocarefor
their babies directly. Rather, the females spend their spring and early summer searching
out bird nests in which to lay their eggs. And cowbirds lay a lot of eggs — some may lay
as many as 40 per season!
Cowbirds discover nests by sitting quietly, watching the comings and goings of other
birds, and also by landing noisily in leaves while flapping their wings, which probably is
donetostartleandflushnearbybirdsofftheirnests.Whentheyfindanesttheywaituntil
thebroodingfemaleisgoneandthenrushin.Theyusuallyeatortossoutoneeggandlay
their own. That parent sparrow is stuck raising the cowbird baby.
A Song Sparrow weighs 0.08 ounces (2.4 g) at hatching. It almost doubles its weight
on the first day out of the egg, and by day 11, at the time of fledging, weighs two-thirds
of an ounce (18.8 g). A cowbird isn't much heavier when it first emerges from the egg,
butwithinsevendaysweighsalmostafullounce(26g).Itbegsmoreloudlyandenerget-
ically, and its mouth is larger than the sparrow young, so it elicits more feedings than the
sparrow nestlings. Unless food is abundant, the sparrow parents simply cannot provide
enough food to meet the demands of the cowbird and all of their own young. Very often
at least one of their own chicks will starve.
Q Why don't birds just throw out the cowbird egg or hatchling instead of raising it?
A
Most cowbird hosts are smaller than cowbirds, and their beaks are often too small to
easily grasp a cowbird egg. In cases where they try, some of their own eggs may get
scratched or punctured. A few birds have developed other strategies to get rid of cowbird
eggs.WhenBlue-grayGnatcatchersdetectacow-birdegg,theysometimesabandontheir
nest and start anew. Yellow Warblers sometimes build a new floor on their nest, relegat-
ing the cowbird egg, and any of their own eggs that were there, into the “cellar” where
they won't be incubated. Some Yellow Warbler nests have been found with as many as
six layers, each with at least one cowbird egg!
But with these strategies, gnatcatchers and Yellow Warblers lose all the eggs they've
already laid. Song Sparrows and most other hosts seldom gain anything if they toss out a
cowbird egg in an attempt to protect their own young. Researchers have discovered that
cowbirds periodically check nests. If a cowbird egg is suddenly missing, the adult cow-
bird often destroys the remaining eggs or chicks — researchers call this behavior “mafia
retaliation.” So once a cowbird egg is in a nest, most birds have a better chance of suc-
cessfully raising at least one of their own if they raise the cowbird.
Another reason that many birds raise cowbirds is that, overall, songbirds have an in-
stinctive urge to incubate eggs and to feed young that they find in their nest. If they were
