Broken Homes

I've been watching the wren box from my perch on the back porch and I think something has gone wrong.

Robin on nest, 2019 (Margo D. Beller)

By now there should've been signs of a mother house wren feeding young. I had put the box in the dogwood rather than the apple tree as I have in the past in order to keep the squirrels (and me) from bothering the birds when we went to pick the fruit.

I thought the move had worked. The male wren has been singing up a storm from around the property, including the locust trees in front and the apple tree in back. One time I happened to see it singing from the dogwood and going into and out of the box as if feeding a mate.

Admittedly, I do not spend as much time sitting on the enclosed porch as I'd like every day - things like work intervene - so I may be missing something. It would not be the first time. 

It started me thinking about why nests fail.

I'm no scientist, and there are plenty of scientific papers on the subject, but from my observations I have a few theories.

Cooper's hawk nest after it was abandoned, 2020
(Margo D. Beller)

One of the parent birds might've been killed, either because of a predator (cat, snake, another bird, hit by a car) or disease. Maybe all the heavy rain we've had in the last few weeks created an unhealthy condition in the wooden box that made the bird sick or caused it to leave. 

Maybe the nest was in the wrong location. Was the nest more exposed to the sun because the more open canopy of the dogwood did not shade the box enough during the July-like heatwave we had in late May into early June? Or was the female wren intimidated by the larger and potentially dangerous catbirds and jays flying into the dogwood to survey the ground for food?

I think of the robin nest I discovered in my pear tree in 2019. I watched the female build it high enough off the ground that it was partially blocked by the porch roof, and I'd have missed it entirely had I not seen the pair going back and forth to the tree. Once built, the female settled down, presumably on eggs. The male would come by every so often to relieve her so she could get food. Then, the pair was gone. Squirrels would climb the tree and look in and around the nest. Eventually, it fell apart and out of the tree. 

Eggs in the wrong place, Montezuma
National Wildlife Refuge, NY, 2019
(Margo D. Beller)

Were there ever eggs? Did squirrels get at them? More recently I found a robin in her nest on a window ledge in a bird blind built high off the ground. The windows had screens on them so I could look out at her but not disturb her. This nest would be safe. (The one I and, no doubt, others found on a staircase leading to a platform at a nature center in upstate New York might not have been so fortunate.)

Another reason for nest failure? One or both birds were immature. Instinct told them to mate and build a nest but they might not have known what to do next.

An immature male Cooper's hawk may have been the reason last year's nest failed. I watched - with some trepidation - as the male called and swooped as he helped the larger female bring sticks to the top of a very tall maple tree where branches had created a V that would hold a nest. They mated - noisily - and built. The female started spending time on the nest. Then fish crows discovered it. The female chased them off, with some help from the male. Eventually, both took off and the nest was taken by squirrels.

Nest box in the apple tree, 2020
(Margo D. Beller)

Recently an immature redshouldered hawk has been flying and calling around the area and at one point I saw it flying with nesting materials. I have a rough idea where the nest is but I'm not going to search for it. If the nest is successful there will be enough stress without my hanging around.

As for the wrens, I know for sure another cause of nest failure - parasitism, such as when a cowbird puts one of its eggs in the nest and kicks out the resident eggs - is not involved because the box entrance is too small for a female cowbird to get inside, much less lay an egg. Some birds, such as the yellow warbler, build a new nest on top of the old one when it sees a foreign egg, even tho' there may be its own eggs, too.

Nature can be cruel.

House wrens are not considered an endangered species, and there is always the possibility the singing male will find another mate and the various conditions needed for a successful brood may come together. Or they may not.

I'll have to keep watching.