Casualties have seemed remarkably low this spring. As a conservationist I’m grateful for that, because fewer dead birds is a good thing. As a scientist, it bothers me a bit, because every time I get complacent enough to think I might understand mortality patterns of avian window collisions at the Noble Research Center, the data throw me a curve ball. In truth, the spring of 2013 was a bit like this too: strangely low mortality during April when it seemed like the bulk of migrants were passing through, but then a flurry of casualties through May – and into June – when out birding it seemed like migration was really at an ebb.
It’s easy as well to fall prey to confirmation bias in thinking that a lot of birds passing through correlates with peaks of mortality. Often I have posted images of bird migration captured on radar after finding a bird or two dead at a window so that I can exclaim “Aha! No wonder we had casualties last night – check out the flight!” That correlation would only matter, however, if the great many heavy flights captured on radar were always associated with casualties I find. I don’t post those radar images for days when I collect negative data. All that said, it’s no wonder we had casualties last night – check out the flight!
This morning I had the good fortune to collect data on the misfortune of these two, female Indigo Buntings. Beginning birders take note of these small, drab, brown “sparrows” you might be seeing all summer long, because they rarely sport enough indigo to rely on color for their identification.
I aged both to after second year based on primary covert shape, and both were carrying some fat but not a lot. I coded them = 1. Neither had brood patches. It can be tempting to picture them traveling together last night, perhaps in a small flock of Indigo Buntings, but they ended up on different sides of the building. Also, the top bird in the photos was pretty stiff – as if she had hit several hours before the bird in the bottom, who was limp and seemed just barely lifeless in the hand. Here is where they ended up:
Like so many others, they were oddly placed for spring migrants: Why on the north and west sides of a building when presumably they had come from the south, and likely southeast? I can only picture some kind of local movements that are multidirectional as birds are finding safe spots to put down for the day, and the funnel-like shapes presented by the Noble Research Center draw birds in and, unfortunately more often than not, to their doom.