We see great variety in the shape, location and size of bird nests in Maine. The simplest nests are scrapes on the forest floor, fields or beaches.  The nest of a Killdeer 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. 

Piping Plovers make a similar sort of nest just above the high tide level on sandy beaches. Their nests are at particular risk because the eggs are so cryptic. The density of humans at the beach is high and beach walkers may unwittingly step on eggs or hatchlings.

For the past 35 years, the Maine Audubon Society has been conducting a project to protect Piping Plover nests while still accommodating the use of beaches by humans. Nests are located and surrounded by a wire exclosure. The openings are big enough to allow the plovers to move in and out but small enough to exclude mammal predators like raccoons, skunks, foxes, dogs and cats as well as gulls.  This year, 97 nests have been located.

The cryptic nature of the eggs provides some protection from avian predators. Mammals are a different story. Unlike birds, mammals have a well-developed sense of smell and can locate camouflaged nests with ease. Hence, enclosing a Piping Plover nest with a cage can reduce destruction of that nest by mammals.

Thanks to a link from Cliff Otto, I have learned of a clever new approach to reducing mammal predation on shorebird nests in New Zealand. With the exception of three species of bats, there are no native terrestrial mammals in New Zealand. However, there are large populations of introduced ferrets, hedgehogs and cats, all of which use their sense of smell to find shorebird nests.

Some New Zealand biologists decided to use trickery to reduce nest destruction by these introduced mammals. The biologists prepared a soup using bird feathers and the secretions from the preen glands of several types of birds. The soup had the unmistakable smell of an aviary or chicken farm to humans.

The biologists then mixed the concoction with Vaseline and painted thousands of rocks along a stretch of coastline where many species of shorebirds nest. The painting of the rocks was begun a few weeks before the shorebirds arrived to begin nesting. The biologists refreshed the smell by repainting rocks every three days for three months.

Sure enough, the mammals were attracted to the smells but soon quit following the scents when they realized the scents did not lead to food.

The biologists monitored the nesting success of several shorebird species in the experimental area as well as in a control area where no fake scents were administered.

The results were quite striking. The number of nests destroyed was nearly halved by putting out the fake nest smells. There were 1.7 times as many fledgling chicks in the experimental area compared to the control area.

To show the differences were caused by the fake smells rather than other differences between the experimental and control area, the researchers switched the treatments in the following year. The results were the same: the former control area now had higher nesting success.

Mathematical modeling of the results indicate that this conservation measure should produce a 75% increase in the population number of shorebirds in the next 25 years. Populations are expected to decline by 40% over 25 years without this intervention.

Mammals, particularly introduced mammals, can have devastating impacts on birds. This innovative approach in New Zealand provides a new conservation tool to reduce destruction of nests by mammals.

Most birds construct a bowl-shaped nest above the ground just large enough to fit an adult’s body.  We can consider the bowl-shaped nest of an American Robin as a typical 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.

Other species use specific materials for the inner lining of their nests.  Palm Warblers, a ground-nesting species in bogs, often place ERuffed Grouse feathers in their nests.  Tree Swallows line their nests with feathers, particularly white ones.  In the early breeding season, you can see aerial fights where tree swallows attempt to take white feathers from another Tree Swallow.

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. 

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.

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.

Bald Eagle nests are larger yet.  Some nests may be eight feet in diameter and 12 feet high, weighting over a ton!  Like Ospreys, Bald Eagles use the same nests year after year, adding material to the nest each spring. 

Finally, some of our nesting bird build their nests in cavities. Woodpeckers excavate their own cavities while others rely on natural cavities or abandoned woodpecker cavities.

The list of native cavity-nesting birds is diverse, including Wood Ducks, Hooded Mergansers, Great Crested Flycatchers, Tree Swallows, Black-capped Chickadees, Tufted Titmice, House Wrens, Eastern Bluebirds. These birds make a nest inside the cavity. For chickadees, the outer part of the nest is made of moss and the inner part of spider webs, soft grasses and plant down.

Two introduced species, European Starling and House Sparrows nest in cavities as well. These two species compete with native birds for available cavities. House Sparrows will kill nestlings of other species and take over the nest cavity. I am certainly not alone in having a clutch of Eastern Bluebirds lost to House Sparrows.

A good source for identifying nests based on their structure and location is https://nestwatch.org/learn/focal-species/