In the last column, we explored the many functions of bird nests. We’ll stick to nests today, focusing on ground-nesting species.

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, Maine Audubon 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 enclosure. 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 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.

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