The Secret Life of Cavity Nesting Birds

Once a bluebird or flicker or barn owl enters it’s nesting cavity, we can only imagine what is taking place. However, now nest box videocameras have answered many questions, some of which I didn’t even know to ask. Why does a chickadee feeding visit sometimes take 5 seconds and another time 1 minute? Among other possibilities, the adult may quickly stuff a bagworm in one end of a nestling and then move to the other end and patiently wait to collect a fecal pellet. Of the several nestlings, which ones get fed and why? Half grown tree swallow nestlings compete to the extent that the weakest one sometimes starves, even when food is plentiful. Kestrel and barn owl nestlings seem not to compete at all until they are old enough to vie for the coveted entrance hole position. And, what happens at night? Nestbox behavior had been studied decades ago by a handful of dedicated observers who built blinds with windows or peep holes and spent long hours observing flickers, chimney swifts, and several other species. But, infrared lighting has finally revealed what happens during the long periods of darkness inside the nest box. The combination of a monochrome videocamera and infrared led’s reveals nestbox behavior that is otherwise invisible to humans and birds alike.

Seeing the invisible
Tree swallows, as well as mockingbirds in their open nest, were videotaped at night last spring (2003), revealing the adults reaction to mosquitoes. At dusk, when the mosquitoes arrived at the nest, the adult female caught and ate many of them. But, when darkness fell, she no longer attempted to catch them, as they repeatedly bit her and the nestlings all night long. While the infrared video camera recorded this activity in great detail, the birds could see nothing and were completely defenseless against the mosquitoes, flinching at each bite, but not otherwise reacting. Another behavior observed in several species, including chickadees, bluebirds, and especially tree swallows, is the female rapidly and vigorously thrusting her beak deep into the nesting material, apparently in pursuit of blowfly larvae or other insects in the nest. This is done both during the day and night. Perhaps she hears the insect moving in the dry grass nest. And, even in the relative safety of her nest box, she is ever vigilant, jerking her head around to stare at the entrance hole in response to some faint sound outside. I can only imagine the nightime stress endured by open nesting species.

There is nothing special about our yard, other than that we don’t maintain a short grass lawn and have allowed a great variety of plants to return. We are in an overdeveloped part of Maryland where dairy farms are rapidly giving way to suburban grass farms. In such an unexciting environment, devoid of rarities, we have to take pleasure in the little things. When I have the time, I crawl around on my belly a lot, discovering a multitude of tiny creatures. I am, by no means, an amateur entomologist, and have a rather poor memory. This works to my advantage in that on any summer day I can soon find an insect species that I have never seen before (or so I believe). The thrill of this discovery is as great as if I had spotted a new warbler for my life list. But, by far, the most exciting observations in the past three years have been the nest box videos. At the moment, few people on the planet have ever witnessed the secret life of cavity nesters. That will, of course, quickly change, but, for now, I am fascinated with the hidden side of these otherwise common birds. The unifying theme in observing all these nest cams is the realization of just how much effort and time these birds invest in rearing young. Even during incubation, I often observed the female barn owl just standing beside her clutch and staring at the eggs for the longest time. She later did the same thing with her nestlings, picking out one and quietly staring at it for several minutes. And, after the barn swallows and bluebirds survive a nesting, they go on to have a second and sometimes a third brood. Life is definitely not as slow paced for these birds as it appears when I watch them sitting on a perch, preening and singing.