On the morning of the 28th the Eastern Phoebe that we have on our webcam at work laid a third egg early in the morning. I grabbed this image on my way out the door at 6:53. It is hard to see as the camera was in infrared mode but there are three eggs in the nest.
When I got to work at 8:00, things had changed. There were only two eggs in the nest. The web cam caught this Brown Headed Cowbird stealing one of the phoebe eggs at 7:33.
This was sad to see but even more frustrating was when she came back the next day and destroyed the two remaining eggs. Here's what the nest looks like now.
There is just a little piece of broken shell. Cowbirds have gone after this nest in past years but they have always just added their eggs to the clutch and let the phoebe raise them. Cowbirds are brood parasites. They do not create their own nest, they lay their eggs in other birds nests and let the other birds raise their young.
A first I wondered if what we're seeing here could be an example of Mafia behavior.
Research by Jeffrey P. Hoover and Scott K. Robinson shows that cowbirds will retaliate against other species for rejecting their eggs. The researchers showed that birds who rejected a cowbird egg had their own eggs destroyed 56% of the time while those who accepted them only had their eggs destroyed 6% of the time. This is actually a really interesting example of evolution in action. This behavior of the cowbirds helps drive evolution of other bird species in a direction beneficial to cowbirds. They are selecting for those birds who can't tell the cowbird eggs are in their nest while destroying the offspring (and genetic material) of those birds that have figured out what the cowbirds are up to.
This doesn't seem to be what happened in this nest though since it appears that the cowbirds just came in and destroyed the eggs having not laid any of their own first. This could be an example of what Hoover and Robinson call "farming." The cowbirds are trying to create more opportunities to parasitise the nest by making the phoebes start over. We'll see if they do.
I'm very conflicted about this. Nest parasitism is natural. I don't have a problem with it. However, the Brown-headed Cowbird is an edge species. It is a bird that historically hung out on the edge of the forest. It does not like the thick middle of a heavily wooded habitat and so those birds that do not live in edge areas have not had to compete with cowbirds. The problem is that humans simply adore edge. It is our favorite habitat probably owing to our own genetic heritage. We're slowly turning everywhere to edge as we open up forests while at the same time planting trees in open areas. We've allowed the cowbird into areas where it historically has not been and we've pet it into contact with species who have not had to deal with it in the past.
Cowbirds are just doing what they have always done, it is becoming a problem though because we've changed the playing field.
~Kirk
This was sad to see but even more frustrating was when she came back the next day and destroyed the two remaining eggs. Here's what the nest looks like now.
Eastern Phoebes lay one egg per day, usually in the morning. This means that one of these eggs was laid this morning (Monday) and one was laid yesterday (Sunday.) Eastern Phoebes typically lay four to five eggs. It should be interesting to watch the nest for the eggs of Brown Headed Cowbirds. This particular nest site was parasitized by cowbirds last year and while walking in to work this morning I could hear cowbirds calling very near this nest.
One of the birds is banded. This bird is wearing a bird band, no doubt it is one of the bands from our active bird banding program at the nature center. We have banded the adults from this nest in the past. It would appear that one of these birds has used this nest site before though we will have to re-capture the bird at some point to be sure.
I checked an hour later and it did look like some things had been moved a bit. Interesting but not conclusive.
This time of year the phoebes are still checking out possible nest sites. Eastern Phoebes are one species that will reuse nests from previous years.
The Memorial Day holiday took me away from web access which of course means the eggs in the Phoebe Nest have hatched. According to co-workers who were in on last Friday, that is when the action first started. I was so busy today I didn't have much time to check on things but now several (if not all) of the eggs have hatched. I was able to grab a two random shots during the day, the morning one above and the afternoon one below. Sorry, they aren't the best. In the top photo I can clearly make out two birds for sure. It will be really interesting to watch over the next two weeks to see how the phoebe nestlings do with two huge fast growing cowbirds in the nest. Here's one of the parents bringing back some food. Much better pictures will be coming in the next few days. I may also monkey with the feed a little to see if we can get a little crisper image.
Eastern Phoebes are not known to remove cowbird eggs so this seemed strange. I took a look on the big monitor we have in our lab downstairs. The lab feed is a direct live video feed of the nest and the image is larger than the stills we can pull from the feed we send to Cornell. I could clearly see that the two cowbird eggs were still in the nest. It was a phoebe egg that was missing. I next looked out the window near the nest and sure enough I could see a broken smashed egg on the ground under the nest. On Saturday, I taught a group of girl scouts and at the end of class we stopped by the monitor so I could tell them about the nest monitoring project and how they can take part. How's this for weird, there's another egg missing from the nest. I could see on Saturday that there were only 5 eggs in the nest. The cowbird eggs are still there but there are only 3 phoebe eggs now.
