Brood Parasitism in Action

We spent the weekend in Canmore and, while enjoying a patio dinner, spotted a Dark-eyed Junco busy providing dinner to a juvenile.  The young bird seemed altogether too big- perhaps 20% larger than the "parent" - and on closer investigation turned out to be a Brown-headed Cowbird.

Dark-eyed junco - the unsuspecting foster parent

All three species of cowbird that breed in North America are brood parasites - they lay their eggs in the nest of other breeding species.  Some birds such as Redhead and Ruddy Ducks as well as the two North American Cuckoo species do this on an occasional, opportunistic basis.  Brown-headed Cowbirds however, like the Old World Cuckoos, have entirely done away with nesting and lay a dozen or more eggs per season  in the nests of over 200 different species.  Unlike the European cuckoos, cowbird hatchlings do not push their foster siblings out of the nest but are usually larger and born sooner so the host species offspring only survive if they are not outcompeted for food.

Young cowbird waiting for its next meal...

...and having a little nap!

The impact of this breeding behaviour on songbird populations is significant.  Some species have defences against the cowbird.  For example, the Gray Catbird can recognize the parasitic eggs and eject them from the nest while the Yellow Warbler simply builds a new nest right on top of the parasitized nest and starts with a new batch of eggs.  Most birds do not have this mechanism and, while that's all well and good for common species such as the above Junco, it can be a real threat to rarer species such as Kirtland's Warbler and Black-headed Vireo.  In fact there's a pernicious one-two punch at work here, as some species are threatened by clearing and fragmentation of forest and the cowbirds thrive in marginal habitat - the border between grassland and forest created when forest is cleared.

Information in this post is from The Sibley Guide to Bird Life and Behaviour and the excellent Birdwatcher's Companion to North American Birdlife by C.W. Leahy

Update July 28th - I just discovered that there was a similar post on another Calgary bird blog a couple of weeks ago.  You can find it here