July 11, 2009

A characteristic feature of all bird species is a nest in which eggs are laid. Nests range from small depressions in the forest floor to massive structures weighing more than a ton. In today’s column, I will provide an overview of the diversity of bird nests.

The simplest nests are scrapes on the forest floor, fields or beaches. The nest of aKilldeer is a good local example. The female lays four eggs in a nest scrape just big enough to contain the eggs. As you would expect, the eggs are well camouflaged. In the woods, American Woodcocks and Whip-poor-wills create nest scrapes for their eggs. For most species that create nest scrapes, little effort is made to line the nests with soft material. Birds that make nest scrapes spend very little energy in making a place to lay eggs and raise their young.

Most birds construct a bowl-shaped nest just large enough to fit an adult’s body. Let’s use an American Robin nest as a typical example of a bowl-shaped nest. Robins are not great architects but still have a remarkably complex nest. The outer part of the nest is formed of twigs, coarse grass and sometimes pieces of cloth, string or other human-made products. This outer layer gives the nest strength. Within this outer layer, robins place a smooth layer of mud. Finally, a layer of fine grasses is laid down to surround the eggs and aid in insulation. Once the outer part of the nest is built, the female sits in the middle of the nest for the rest of the construction. A snug fit is therefore guaranteed for the incubating mother. The nest is usually in a tree between three and 25 feet high.

Other species use specific materials for the inner lining of their nests. Palm Warblers, a ground-nesting species in bogs, often place Ruffed Grouse feathers in their nests. Tree Swallows line their nests with feathers, particularly white ones. Ruby-throated Hummingbirds create tiny nests to hold their two eggs. The nest is made of down and small pieces of plant material bound together with spider webs. The outer part of the bowl is covered with bits of lichens to aid camouflage. Black-capped Chickadees make their nests in a tree cavity. The outer part of the nest is made of moss and the inner part of spider webs, soft grasses and plant down.

Waterbirds typically create bowl-shaped nests on the margins of lakes or ponds or even on floating vegetation. In most cases, the outer layer of the nest is made primarily of vegetation. If water levels rise, waterbirds will quickly add additional vegetation to keep the inner part of the nest dry. The inner lining is made in part of down feathers that the female pulls from her breast. These down feathers create a wonderfully warm place for the eggs. You take advantage of the excellent insulating qualities of duck down if you own a down coat or sleeping bag.

The largest nests in Maine are made by birds of prey. An Osprey nest may be five feet across. The outer portion is made of sticks and miscellaneous debris. The inner lining is made of smaller twigs, grasses and other soft material. Both the male and female participate in nest building. The male tends to bring material to the nest site and the female incorporates the material into the nest.

The largest nests in Maine are made by Bald Eagles. Some nests may be eight feet in diameter and 12 feet high. The weight may exceed one ton! Like Ospreys, Bald Eagles use the same nests year after year, adding material to the nest each spring. Great Horned Owls will readily adopt an abandoned Osprey or eagle nest.

Some of our birds place domes over the tops of their nests. Such nests are characteristic of many of our wrens. The family name for the wrens, the Troglodytidae, comes from the Greek for cave dweller, reflecting the shape of the nest. One of our most common warblers, the Ovenbird, builds a domed nest on the forest floor. The shape of the nest suggests a Dutch oven. By the way, there is an unrelated group of tropical birds called ovenbirds that create clay-shaped, domed nests which look just like a brick oven.

Distinctive nests are made by Baltimore Orioles. Their nests are made of grasses, vines, and hair. The nest is a deep pouch, bound to the forks of a branch at the rim. Two sprites of the Maine woods, the Ruby-crowned Kinglet and Golden-crowned Kinglet also create hanging nests that are much smaller and more difficult to see than oriole nests.

[Originally published on June 27, 2009]

Most of the birds in Maine are in the process of nesting now. Among the different bird species, there is great variation in many facets of reproduction. For instance, whether one or both parents brood, the type and location of the nest, the size of the eggs.

One striking feature that varies widely among different species is clutch size, the number of eggs laid in a single nesting attempt. Some birds (albatrosses, petrels, some penguins and some terns) have a clutch size of only one. Hummingbirds and doves always have clutch sizes of two. At the other extreme, some ducks and geese, pheasants and rails may have clutch sizes as high as 20 eggs. Our songbirds have clutch sizes varying between two and 12, depending on species.

Why is there such striking variation in clutch size among different species? This question has been the basis of a great deal of research. In this column, I will briefly introduce some of the findings of this body of research.

To begin, we must realize that reproduction is a taxing experience for birds. Birds that undertake a particularly heavy reproductive effort in one year may reduce their chances of surviving to reproduce the next year. Birds have to make a trade-off between producing very large clutches in a short life and moderate-sized clutches over a longer life.

David Lack, a British ornithologist, was one of the first scientists to rigorously study clutch size variation in birds. He came to the conclusion that a female bird should lay the number of eggs that will produce the most fledged, independent offspring.

Let’s consider Lack’s conclusion for three females which lay three, five and seven eggs, respectively. The female and her mate with only three eggs will probably succeed in fledging three young. The pair with five eggs will have to work harder but still might lose only one chick, fledging four young. The pair with seven eggs have bitten off more than they can chew. The demands of the seven hungry chicks are too much for the parents and perhaps four of the seven chicks will die from starvation. So, what is the best clutch size to have for this species? A clutch size of five – a trade-off between ambition and caution. The pair with only three eggs is not ambitious enough; the pair with seven eggs is too ambitious.

Ornithologists have tested Lack’s prediction by altering the clutch sizes of birds. One species that has been studied intensively is the Tree Swallow. These birds frequently nest in nestboxes at high densities, making it easy for an ornithologist to monitor large numbers of nests. Tree Swallows usually lay five eggs. Experiments are done by removing one or two eggs from some nests and placing them in other nests. After the egg-switching, some nests have three, four, five, six and seven eggs. The results largely confirm Lack’s prediction. A clutch size of five is the most productive clutch.

In other species, that have been examined, birds lay one less than the most productive clutch. Scientists believe that these birds are cutting back on their reproductive effort in a single year to increase their chances of having a long life.

Clutch size within a species often shows striking variation with latitude. The typical pattern is that tropical members of a species have lower clutch sizes than members of the same species that live in the temperate zone. For instance, female Northern Flickers that live in the tropics only lay clutches of three or four eggs while flickers in the northern U. S. A. and Canada may have ten or more eggs.

One possible explanation for this variation is the number of predators. The tropics have many more predators that are threats to eggs and nestlings, including snakes, other birds and various mammals. The chances of a nest being found and destroyed by a predator in the tropics is very high. In the temperate zone, the number of predators is much reduced. Many temperate nests are never discovered by predators.

Ornithologists therefore argue that tropical birds should not invest heavily in a single clutch because it is likely to be lost. It’s not prudent to make a large investment in a risky environment. It is much better to have a low clutch size, allowing the parents to live a longer life. With luck, one of the many clutches a female will lay over her lifetime will escape the predators. On the other hand, a temperate bird does not have the same high risk from predators so can lay a bigger clutch. It’s OK to make a large investment if there is little chance of losing it all.

[First published June 27, 2009]