Dear Bird Folks,
We decided to look into our bluebird boxes today to make sure they were all set for this year’s occupancy. Much to our surprise we found a dead female bluebird sitting on top of a nest with four eggs. (See attached photos.) Except for the fact that she was dead, the bird seemed healthy. Any thoughts about what might have done her in?
– Stan, Brewster, MA
Thanks for the note, Stan,
I enjoy hearing from you. And it’s a good thing I do because next to my mother and companies demanding money, you write to me the most. However, I’m not happy to read that you are just getting around to checking the inside of your nest boxes. It’s the end of April. Those boxes should have been checked six weeks ago. You should know better than that. Don’t you ever read the answers I send you? Or do you just put them in the “read someday” pile, like I do with the notes I get from my mother and those companies demanding money?
After I received the photos of your mysterious dead bluebird I contacted CSI: Cape Cod for help. That was a waste of time. It turns out that locally “CSI” stands for the Cape Sailing Institute. (They didn’t know a thing about dead birds, but they did sell me a new bilge pump and I was glad to get it. My bilge has needed pumping for years.) With no help in sight I did something I rarely do; I tried to figure out the problem myself. With the aid of a borrowed pair of eyeglasses (I’m much too young to have my own pair), I studied your photos more carefully. It turns out your “healthy” dead bird had one other problem…in addition to being dead. Its eyes were missing. The bluebird’s eyes had been completely plucked out of its head. That can be a problem. With this new piece of evidence I have an idea of what happened to the mother bluebird. The answer is kind of gruesome so you may want to send the kids to bed right now or they may be up all night with nightmares about eyeless bluebirds. Okay, you’ve been warned.
In the avian world the Eastern Bluebird is the one bird that all the others like to pick on. Even non-bullies line up to have whack at this bird. But the one bird that really has it in for them is the non-native House Sparrow. Amazingly, House Sparrows are smaller and weigh less than bluebirds, but none of that seems to matter. House Sparrows, which also like to nest in birdhouses, are tough, streetwise birds and will do whatever it takes to steal a birdhouse from bluebirds. They’ll harass them, chase them, or kill them, and won’t even feel bad about it. House Sparrows have the nasty habit of landing on the back of a mother bluebird while she sitting on her eggs, and pecking right through the top of her head. That’s not a good thing. Pecking out the bluebird’s eyes is also something that this sparrow is known to do. Sweet, eh? I wasn’t kidding about those nightmares.
Another bird that bluebirds have problems with is the even smaller House Wren. While the tiny House Wren, which also likes to nest in birdhouses, is no threat to adult bluebirds, it will go after their eggs or chicks. If you come home to find broken eggs or nestlings lying on the ground, you most likely have discovered the lovely handiwork of a House Wren. Another birdhouse nester, the Tree Swallow, is no direct threat to bluebird eggs or chicks, but they will sometimes drive adult bluebirds away from a box. A sign that Tree Swallows have moved into your bluebird house is if the box contains a number of white feathers. (No one knows why, so don’t ask.) And sometimes even the sweet Black-capped Chickadee can be aggressive enough to drive away bluebirds (Talk about embarrassing). Indications that chickadees have moved into your bluebird box are fur and moss. Chickadees love a comfy nest.
Non-avian predators are also a problem for bluebirds. If you discover that either eggs or nestlings have disappeared without a trace, your neighborhood snake could be the culprit. Snakes can easily climb any tree or pole, and will do so without disturbing the nest or leaving fingerprints. The dreaded house cat or agile raccoon will also steal chicks, but they usually will disturb the nest or leave a trail of nesting material hanging out of the hole. Abnormally wet weather can cause chicks to succumb to hypothermia, while some chicks may die after being fed insects tainted with pesticides, probably from that neighbor who loves to spray everything. Starvation is another reason why chicks mysteriously die in the nest. That usually happens after both parents fall victim to a predator (see house cat above).
I’m not 100 % sure if House Sparrows killed your bluebird, Stan, but your note does highlight the need for us to constantly monitor and clean our nest boxes. Nest boxes should be checked regularly and not put in the “someday” pile. I’m also not sure why bluebirds are picked on so much. Perhaps the other birds are simply jealous of their striking good looks. Those poor little bluebirds must think they are cursed to be so handsome. I know exactly how they feel.