Laura Erickson's For the Birds

(Recording of a Sharp-shinned Hawk)

When I’m up at Hawk Ridge helping people with hawk identification, I field a lot of questions that have no answers– the mysteries of the ornithological world.

One mystery is why some Blue Jays go south for the winter and some don’t. Last year we counted a total of 13,750 jays migrating over the Lakewood Pumping Station, and yet there were quite a few that stuck it out in the Northland all winter. No one knows exactly what causes an individual Blue Jay to decide to light out for the territory and what causes another to endure the winter. It may have to do with food availability, because ornithologists have noted that in seasons when acorns are abundant more jays linger than in years when acorns are sparse. In my yard, jays have stayed around for a full year when I kept peanuts available each day. But food availability is probably not the only variable in the blue jay equation–there is some evidence from banded bird studies that young birds may migrate more than adults. And I suspect that individual choice plays as big a role in a Blue Jay’s migration as it does in the annual flight of humans from the Northland to the sun belt. With either species it’s pretty hard to predict ahead of time who will stay and who will go.

Another mystery people ask about is why male Sharp-shinned Hawks are so much smaller than females. When you see a pair side by side, you can’t help but think of the improbable human counterpart–a woman basketball player married to a jockey.

It’s hard to construct an ornithological experiment that could prove conclusively why this size difference exists, but it’s easy to see that the small male, light and quick, is optimally built for sneak attacks on warblers and sparrows, while the more robust female has an easy time snatching blackbirds, jays, waxwings, and robin-sized birds. This means that a single pair of sharpies can exploit a wider range of prey species than a pair of hawks the same size could. But why is it that the female sis the bigger of the two? My own guess has to do with the fact that the female probably weighs a bit more during the breeding season, when her body is preparing to lay eggs, than during the rest of the year. The larger she is, the smaller the percentage of her body weight the eggs would be, and the less the load would affect her hunting and flying ability during the breeding season. But this is just a hypothesis which would be hard to prove one way or the other.

One question people ask that seems mysterious but really has a straightforward answer is—how do hawk counters possibly manage to count thousands of Broad-wings in a day? These hawks gather on thermal air currents and swirl upward in huge masses called kettles. These swirling groups look impossible to count. And, sure enough, if you ask any counter, he or she would agree–it is impossible to count a kettle with any degree of accuracy. Fortunately, it isn’t necessary. Once the broad-wings reach the top of their thermal, they stream out in a line or band that is much easier to count.

Of course, the biggest mystery of all is why otherwise normal people spend all of their autumn days counting migrating birds. What lures people to witness migration year after year is as elusive as a Peregrine Falcon, as magical as the whisper of a Golden-crowned Kinglet, as haunting as the ghost-like flight of Blue Jays through the morning mist. It’s the very mystery of creation.

(Recording of a Sharp-shinned Hawk)

This is Laura Erickson, and this program has been “For the Birds.”