While out walking in the neighborhood the other day, we happened to notice a pair of chimney swifts exhibiting some interesting behavior.
Chimney swifts are small, gray-brown tubular-bodied birds—often described as “flying cigars,” with narrow, pointed wings that typically beat so rapidly that they seem to blur. As the name indicates, most now nest in chimneys, although a small number presumably still nest in old, large hollow trees as all chimney swifts had to do before humans invented the chimney. Chimney swifts have been back from their South American wintering grounds now for some weeks. The summer sky around many Maine towns is filled with their twittering calls as they chase each other around and forage for the flying insects on which they depend for food.
On the evening of our walk, we saw two chimney swifts, flying together, zoom into the top of a tall tree that had many small dead branches at the very top. As they reached the branches, they suddenly fluttered to slow down for a second and then continued on. At first, we were puzzled but quickly remembered that chimney swifts make their nests completely out of small twigs that they glue together with their saliva onto the inside wall of a chimney. These birds were trying to break off twigs for nest building! Reading more about it, we learned that they break off the twigs with their feet and then carry them in their bill to the nest. We noticed that only one of the two birds appeared to be trying to harvest the twigs, leaving us to wonder if only one sex might engage in this activity. But the experts tell us that both sexes are known to break twigs off for nesting.
This led us to wonder something else: how would one determine whether a male or female was the one doing this amazing activity? Chimney swifts, after all, are one of the species in which the males and females look essentially alike to the human eye. There are others of course--the black-capped chickadee, tufted titmouse, blue jay, American crow, song sparrow—to name just a few.
This is in contrast to species in which the male and female plumages are so different that early ornithologists looking at museum specimens sometimes considered them separate species. Bird species like the mallard, house finch, house sparrow, northern cardinal, ruby-throated hummingbird, Baltimore oriole, and many of the warblers are quite readily distinguished as female or male based on plumage, at least for part of the year.
In some species, the distinction is much more subtle. In owls and hawks, the female is appreciably larger in size than the male. For doves and gulls, it’s the opposite. The female osprey has a pronounced band across the breast that is usually lacking in the male.
Certain birds have sharp distinctions in certain behaviors between the sexes. Only male ruffed grouse are known to beat the wings to make the drumming sound used to show territoriality and attract mates. Only male American woodcock are known to engage in the spring courtship display involving the loud buzzy “peent” and high twittering flight at dusk. On the other hand, only the female ruffed grouse and American woodcock incubate the eggs and care for the chicks in those species. Contrast this with the spotted sandpiper in which only the male incubates the eggs and cares for the young.
It has been long assumed that a singing bird is a male but new research is finding that in some species, the female also sings. It may be a slightly different song or a song given more infrequently, or it may just be that the female doing the singing is a bit harder to spot. All of which may explain why female song is not as well documented as male song.
Given the particular importance of female birds to the survival and health of every bird species, ornithologists are realizing that more attention needs to be paid to understanding the elements of their biology that are unique. We need to know more about everything from their behavior to their physiology to their migration routes and timing to their winter habitat needs. In many species, almost nothing is known about how females differ from males in all of these areas and more.
Keep your eyes open to see whether you can spot the female birds around your yard and see what they are up to. Maybe you’ll be make your own discovery about ways the sexes are different, and the same.
Jeffrey V. Wells, Ph.D., is a Fellow of the Cornell Lab of Ornithology and Vice President of Boreal Conservation for the National Audubon Society. Dr. Wells is one of the nation's leading bird experts and conservation biologists and author of “Birder’s Conservation Handbook”. His grandfather, the late John Chase, was a columnist for the Boothbay Register for many years. Allison Childs Wells, formerly of the Cornell Lab of Ornithology, is a senior director at the Natural Resources Council of Maine, a nonprofit membership organization working statewide to protect the nature of Maine. Both are widely published natural history writers and are the authors of the book, “Maine’s Favorite Birds” and “Birds of Aruba, Bonaire, and Curaçao: A Site and Field Guide” from Cornell Press.