The spring migration is almost done. Nelson’s sparrows, cuckoo cuckoos and black-headed warblers are the last migratory birds of spring and are now arriving. Birds set out to raise families.
The eggs that a female lays are, of course, critical to successful reproduction. Hatching the chicks and feeding the young are important steps in many species to introduce new birds into the population of each species. Aside from the efforts of the parents, nothing is more important to breeding success than the nest. A nest is an elaborately designed, multifunctional structure.
A nest is more than a cup for storing eggs. The nest allows the eggs to be efficiently heated by an incubating parent, usually the female. The curvature of the nest ensures that the eggs nestle against each other to take up the smallest possible area. The shape of the eggs and the nearest curvature result in a tight packing of the eggs.
Due to the dense arrangement, the hatching parents can supply each egg with heat from its body. The tightly packed eggs also help store heat. The parents make the nest just big enough for the parents to sit in to ensure a comfortable environment for the eggs and nestlings.
Birds are endothermic or warm-blooded animals. Even in the egg stage, the temperature of the embryos must be kept close to the body temperature of the adult. Especially when the brooding parent leaves the nest to forage for food or to avoid a predator, the eggs are so close that they essentially act like one large egg, rather than about five small eggs.
Without incubation, the eggs release heat into the air through their eggshells. But in a tightly packed configuration, eggs give off some of their heat to neighboring eggs instead of losing it to the air. The same effect occurs when your dogs, cats, or you and your sweetheart huddle together when the temperature drops.
A nest usually has several different layers, but the inner one is usually made up of blades of grass 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 keep the eggs warm.
The predation of eggs and nestlings by snakes, as well as various birds and mammals, is a serious threat. Nests are often built in thickly overgrown parts of a tree or shrub. In addition, the nests are cryptic. The outer layer is usually made up of twigs that blend well with the branch on which the nest is located.
The nest of a ruby-throated hummingbird is a cryptic wonder. The nest is only built by the female and has an outer layer of lichen sewn together by spider webs.
Birds and mammals have good eyesight and can sometimes spot a nest by watching their parents. Parents need to be clandestine while flying to and from the nest.
However, mammals have an excellent sense of smell, so a smelly nest can attract a mammal, resulting in the loss of eggs or nestlings. To minimize the odor of nestling urine and feces for songbirds and some other birds that the nestlings must hatch, the young birds bind their digestive and urine excretions in a membrane called a fecal sac. When a poop sack passes out of the intestine, one of the parents takes the sack, flies some distance from the nest, and drops the poop sack. Watch out for this behavior in a few weeks.
Unknowingly, humans can pose a threat to nesting success. More than one ornithological nesting study has been haunted by a predator watching an ornithologist go from nest to nest to monitor reproductive success. Even more insidiously, some predators learn to follow the researcher’s scent and eat the eggs or nestlings at the end of each trail.
In the next column we will look at the variety of Maine breeding bird nests.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions below [email protected]
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