By Bob Engel
Marlboro College Professor Emeritus of Biology and Environmental Science
We suffered through the first two weeks of May here in New England. The jet stream warped to the south in such a way that anything cold and lousy in Eastern Canada came our way day, after day. Further, because the jet stream bends back north a bit east of us, we had to deal with a late nor’easter that has crept up the coast, pulling even more cold air down from Canada. As a result, we were gifted with an inch and a half of snow the morning of May 14 in Marlboro. Oh, rapture! Both April and the first half of May have been challenging!
On May 13th, I happened to be in Grafton and noted a temperature that was about eight degrees warmer than the Arctic of Marlboro. And the birds were more common — they felt the weather, too. What kinds of migratory birds are hit hard with a late and cold May? It’s really pretty simple: those birds that depend on active insects are hit harder when cold weather depresses bug productivity.
One of our early, insect-dependent birds include tree swallows. There are a couple of nest boxes at our place and the swallows use them every year. Frequently, all of this starts in late April, with the swallows feeding on the wing in our field. This year, they tried to get started on one or two isolated warmer days. They have had to retreat to somewhere, giving up on Marlboro. As I write this, it's the middle of May, it’s late morning, and the temperature is a measly 42 degrees. Not a single insect in view and, therefore, no swallows, either. I expect them back next Wednesday. Warm enough for bugs and productive enough for mobile, feeding swallows. Nesting might be started this Thursday, with a predicted high of eighty degrees! This projected balmy stuff will be a week or two later than usual.
We are warbler deficient, too. On a recent mid-50's day, I heard a Yellow-rumped warbler, and one grumpy Ovenbird. It’s almost a week or two weeks late for Blackthroats, Chesnut-sided, Yellowthroats, Blackburnians, and Magnolias. Why? Emergent bugs have not been productive enough to replace the egg masses, pupae, and overwintering adults that were slowly depleted by the winter avian residents. So several neotropical birds I’m used to spotting in the second, third, or fourth of May are just not here. It was a little better in Grafton yesterday, even though the temperatures there were still stuck in the mid-fifties. Ovenbirds were common, and also present were warblers like Blackthroated blues, a Blackburnian, a couple of Chestnut-sideds, Yellowthroats, etc. These birds were much better represented, though perhaps lower than normal, than they are in Marlboro.!
So let’s ask a basic question. What determines the times that birds — especially males — get here? With all migratory arrivals, the pressure to arrive early is driven by the availability of potential breeding territories. In theory, the earlier one arrives, the better are the pickings of high quality territories. As time passes, males take them, until high quality territories start to become limiting. Later birds, including less experienced birds, can then only acquire lower quality stuff, or nothing at all. So early arrival matters. But what happens with the late persistence of cold, wet, even snowy weather? Pretty limited food. So these migratory male birds are thus caught between a rock and a hard place. The females may drift in a bit later.
Why? They often have been excluded from quality winter habitat by territorial conspecific males and have to feed for a week or more to make the trip north. I’m guessing that next week, say Wednesday (5/17), Thursday (5/18) will signal a significant Marlboro influx of birds spinning their wheels during a couple of weeks of cool spring.!
Author update: Tree swallows and Barn swallows appeared right on time on Wednesday (5/17). Georgia weather!