The spring migration is nearly done. Nelson’s sparrows, clack-billed cuckoos and blackpoll warblers are the last of the spring migratory river and are arriving now. Birds are getting down to raising a family.

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 be 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 attract a mammal, 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 in a few weeks.

Humans can unwittingly be a 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.

In the next column, we’ll look at the diversity of nests of Maine breeding birds.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu

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