June 2021

This month is the height of the nesting season for Maine birds. All of our birds have laid eggs and many of those have hatched,

The eggs that a female lays are, of course, vital for successful reproduction. Getting the chicks hatched and, for many species, feeding the young are critical steps in adding new birds into the population of each species. Aside from the efforts of the parents, nothing is more important in breeding success than the nest. A nest is an intricately designed, multi-functional structure.

A nest is more than a cup to hold the eggs. The nest facilitates efficient warming of the eggs by an incubating parent, usually the female. The curvature of the nest insures that the eggs nestle together to occupy the least possible area. The shape of the eggs and the next curvature result in close packing of the eggs.

The close-packed arrangement means the incubating parent can provide each eggs with warmth from its body. The close-packed eggs also help with heat retention. The parents make the nest just large enough for the parent to sit in, ensuring a cozy environment for the eggs and nestlings.

Birds are endotherms or warm-blooded animals. Even in the egg stage, the temperature of the embryos must kept near adult body temperature. Particularly when the incubating parent leaves the nest to forage or avoid a predator, the eggs are so close that they essentially act as one big egg rather than say, five small eggs.

In the absence of incubation, the eggs will lose heat to the air across their egg shells. But in a tightly packed configuration, eggs will give off some of their heat to neighboring eggs rather than losing it to the air. The same effect occurs when your dogs, cats or you with your sweetheart huddle together when the temperature drops.

A nest usually has several distinct layers but the inner one is usually composed of grass stems or other fine vegetation. These materials are good insulators. The eggs are laid in this inner layer so that the blanket-effect of the inner lining can help keep the eggs warm.

Egg and nestling predation by snakes as well as various birds and mammals is a serious threat. Nests are often built in thickly vegetated parts of a tree or shrub. Furthermore, the nests are cryptic. The outer layer is usually made of twigs that blend in well with the branch on which the nest is located.

The nest of a Ruby-throated Hummingbird is a cryptic marvel. Made only by the female, the nest has an outer layer of lichens stitched together by spider webs.

Birds and mammals have good eyesight and can sometimes discover a nest by watching the parents. The parents need to be furtive in flying to and from the nest..

Mammals, however, have a superb sense of smell so a smelly nest can lead a mammal to a nest, resulting in egg or nestling loss. To minimize the smell from the urine and poop of nestlings for songbirds and some other birds that have to incubate the nestlings, the young birds bind their digestive and urinary wastes within a membrane called a fecal sac.  When a fecal sac is passed out of the gut, one parent will take the sac, fly some distance from the nest and drop the fecal sac. Keep an eye out for this behavior.

Humans can unwittingly be an unintentional threat to nest success. More than one ornithological nesting study has been plagued by a predator watching an ornithologist as she goes from nest to nest to monitor reproductive success.  More insidious, some mammal predators learn to follow the smell of the researcher and eat the eggs or nestlings at the end of each trail.

Sound signals provide important ways for birds to communicate. Playing a recording of a bird’s song may attract the attention of a female and usually attracts the attention of males, intent on driving away the intruder. Alarm calls may attract a diverse group of birds, congregating to mob the threat that caused birds to issue the alarm.

The use of playbacks of recordings of those sounds has been a powerful tool for ornithologists for decades. Playing vocalizations at different times of the day or at different times of a season helps us understand the function of different sounds. Playbacks can be used to map the boundary of a bird’s territory. The ornithologist walks toward a singing male. The male will usually ignore vocalizations of a neighboring male until that male (or the playback) crosses into the first bird’s territory. In this fashion, it is possible to map the boundaries of a territory with amazing accuracy.

With the advent of smartphones and bluetooth technology, it is easy for a birder to play bird vocalizations to attract birds for a closer look. There are lots of apps of bird sounds available for smartphone use. A small bluetooth speaker emits a surprisingly loud sound, amplifying the relatively weak smartphone speaker. These playbacks are highly effective.

The use of playbacks has some ethical implications. A male bird’s stress hormones increase immediately after hearing the sound of a putative intruding male. Birds become agitated, increasing their metabolism. They must increase their food intake to replace those calories used in response to a perceived threat. The use of alarm calls has an even stronger effect. Playing a recording of chickadees mobbing an owl or the vocalizations of a Northern Saw-whet Owl attracts numerous individuals of multiple species.

I do not use playbacks when I am birding by myself. When I lead a field trip, I occasionally will judiciously use playback to attract hard-to-see birds. My logic is that the minute or so of stress on a bird from hearing a playback allows everyone in the group to see the bird. Spending ten minutes of more following the bird around to allow everyone to get a look may be more stressful on the bird. I never use playback in a frequently birded area because other birders may be using playbacks at that site.

Other birders use playbacks often. One example is Birding Bob, a New York City birder who plays vocalizations often and at high volume in places like Central Park. The New York Times recently posted a video op-ed of Birding Bob with commentary from several other birders. The video provides a balanced look at the pros and cons of using playbacks. You can see the video by clicking here.