Oh hi again! It has been a long time since I’ve been able to write a post. I don’t know if I will be much better with grad school starting up this month, but we shall see! On an fun note, this is my 100th post! WOOT! Anywho, if you didn’t know, I spent the summer doing avian field work for the Illinois Natural History Survey (i.e. point counts, vegetation surveys, nest monitoring, etc). I learned a lot about being a bird whisperer and I have a greater respect for birds, especially after nest monitoring! My boss said most birds have a success rate of about 30% for their nests! Consequently, I saw numerous nests fail throughout the summer. Besides predation, brown-headed cowbird parasitism was the most common reason nests failed. The link above is to an article by the Smithsonian Institution about this issue. Before I became a bird whisperer, I didn’t know about the impact brown-headed cowbirds had on other birds. I think it’s interesting, so I thought I’d share and include some pictures from my summer.
I’m just going to pick out main points from the article to summarize:
– Adult brown-headed cowbirds “parasitize” (lay their eggs in) nests of other birds, rather than build a nest themselves and raise their own young. VIDEO!
– The foster parents, “hosts,” usually either abandon their nest completely or raise the cowbird chicks as their own.
– Apparently, cowbirds can lay an egg almost every day during the peak breeding season! This trait earned them the nickname “songbird chickens.”
– The majority of the time, cowbirds will remove a host’s egg when they lay their own (as seen in the earlier video).
– Cowbird eggs usually require less incubation time than the host’s eggs. This is very influential because the foster parents will be tending to the cowbird chick first, giving it a higher chance of survival. Because cowbirds will usually be the bigger, more aggressive chick in the nest, they also out-compete the host’s actual young for food. TANGENT: I monitored an indigo bunting nest that had two cowbird eggs and one indigo bunting egg. The cowbirds hatched and were twice the size of the indigo bunting chick! The indigo bunting chick was completely naked and smashed to the bottom of the nest, being completely ignored. It was really sad to see. The nest ended up getting predated anyway though.
– “Cowbirds tend to parasitize birds smaller in size than themselves… a fledgling cowbird is larger than the adult warblers and vireos!” (By “preying” on nests of smaller birds, the cowbirds also ensure their eggs won’t be thrown out of the nest because smaller birds won’t be able to remove them.)
– Humans/ agricultural practices create the perfect environment for cowbirds because of the abundance of grassy foraging areas and open perching sites.
– Cowbirds are causing major problems for endangered bird species. Trapping and shooting are the most common methods of removal, but they are too costly and labor-intensive to be effective on a large scale.
PICTURES FROM THE SUMMER!
Some of the birds came up with different strategies other than to abandon. This willow flycatcher was able to push the cowbird egg down to the bottom/into the nest so it wouldn’t get properly incubated. The cowbird egg never hatched and the willow flycatcher successfully fledged its own young. There were also a few Bell’s Vireos who were somehow able to selectively not incubate the cowbird egg. If you are wondering, “why don’t the birds just crack the cowbird egg?” it’s because then the nest would get infested with bugs/ants.
Below is a willow flycatcher nest with two young, two flycatcher eggs, and one cowbird egg (in front). Sorry the picture is a little blurry! But, you can see the cowbird egg is notably larger than the flycatcher eggs.
Below: Cowbird egg (left) in a dickcissel nest.
Below: Same dickcissel nest as above. The cowbird and dickcissel matured at similar rates, so the size discrepancy isn’t very noticeable. This nest ended up being successful. I should’ve taken another stage picture, but I forgot. I’m not sure which bird is the cowbird because they are so young, but if you look at the beak shape/color it is apparent they are two different types of birds.