Sunday, May 9, 2010

Yellow Warblers and Cowbirds - appropriate for Mother's Day

Last week I spent a bit of my own time checking out migrants in the Stratford Great Meadows Important Bird Area. This encompasses Great Meadows Marsh, the McKinney Wildlife Refuge, Long and Pleasure Beaches, the “warehouse pools” on Long Beach Blvd, and some other habitat connected to these areas. I discovered a female Yellow Warbler already building a nest in what we call zone 3. She was using mostly natural materials, but occasionally bringing a bit of twine or string back as well. What was fascinating about the sighting was the fact that I spotted a female Brown-headed Cowbird only meters away. The species is a brood parasite, as females lay their eggs in the nest of other birds. One is egg is laid in each nest with the intention of having their offspring raised by this other species. The Brown-headed Cowbird young will out-compete nearly any other offspring in the nest for food, often leading to the death of the host species’ young.

I almost chuckled when I saw the Cowbird so close to the nest, thinking about how it may lay an egg there in a couple weeks. However, I was surprised to see it immediately hop right in the partially constructed nest, seemingly thinking about laying an egg at that very moment! I am sure it would have toppled right out of the now flimsy nest had she done so. Perhaps the bird was simply checking out the progress of the nest – either way the female Yellow Warbler came back a moment later, calling loudly and becoming animated. She aggressively flew towards the Cowbird, pecking at it and chasing it out of the nest and away from the general area. I watched her return about 10 minutes later, and I did not see the Cowbird again that day. I am almost positive it will be back.

Last year we conducted a Breeding Bird Survey in the Stratford Great Meadows IBA over a six-week period in June and July. Yellow Warblers were confirmed as breeding in several areas. Additionally, Brown-headed Cowbirds were confirmed as breeding in three locations because of three separate sightings of Yellow Warblers feeding a young Cowbird. The picture below was taken in zone 1 on July 3, 2009. A male Yellow Warbler is holding an insect he caught. He proceeded to feed it to the Cowbird chick, which has it's back to the camera.

We used handheld GPS units to record the exact location of every breeding confirmation during the Breeding Bird Survey. Fortunately, I was carrying such a unit last week and recorded the location of the Yellow Warbler nest. It would be classified as a “probable” breeder now as it was in the process of creating a nest and not having actually bred or produced young. Now that we have the GPS coordinates anyone easily return to the spot to check on its progress. Decades from now others will also be able to look at the exact spot where a Yellow Warbler, or any other species, was found breeding. Hopefully we will be able to take these vital records on a yearly basis in critical habitats across Connecticut.

Photo © Scott Kruitbosch

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