November 2009

Recent Ornithological Literature

By in Recent Ornithological Literature

Today’s column will be a review of recently published articles in ornithology that I hope will be of interest.

Foot waggling in Common Loons

If you have spent much time watching Common Loons, you surely have seen a loon on the water raise one foot into the air and either hold it there briefly or shake it, often several times in succession. This behavior is called a foot waggle. After waggling the foot, a loon may put the foot under the wing, a behavior called shipping.

The reasons for foot waggling have been discussed for years but not adequately tested. Some ornithologists suggest foot waggling occurs most frequently in birds that are about to sleep or rest and interpret foot waggling as a comfort movement. Other scientists think that the large foot of the loon with its extensive system of capillaries may be used to regulate body temperature. A foot out of the water might lose heat quickly by evaporation of water into the air. Other ornithologists have suggested that food waggling may function as a social signal.

A recent paper in the Condor by James Paruk describes his work to investigate the function of the loon foot waggle at several sites in the Upper Peninsula of Michigan. He found that 81% of the foot waggles observed occurred while a loon was either preening or resting. Foot waggling was therefore judged to be primarily a comfort movement. Occasionally, a loon would give a single foot waggle in a stressful situation such as after encounters with boaters or a Bald Eagle or after loud thunder. Because only some loons used the foot waggle in such situations, Paruk claims that the behaviors are idiosyncratic – some birds use the behavior and others do not.

The loons did waggle their feet more on sunny days than on cloudy days. The dark bodies of loons heat up more in direct sunlight. This relationship suggests that regulating body temperature may be a secondary function of foot waggling.

By the way, foot waggling is seen in other aquatic birds as well including grebes, Ruddy Ducks, scaups and their relatives, and mergansers.

Tail pumping in Eastern Phoebes

The tail-pumping behavior of a perched phoebe is a good feature to use in identifying this flycatcher. The function of this behavior has been the subject of much conjecture. Michelle Carder and Gary Ritchison recently published an article in the Journal of Field Ornithology that presents a test of possible explanations for the behavior.

They tested four hypotheses by extensive field observations. The first is that phoebes use tail pumping to maintain their balance on a perch. The authors measured tail-pumping rates on perches of different stability (thin versus thick branches) and under different wind speeds. They found no correlations and reject this explanation. A second hypothesis was that phoebes use tail pumping as a territorial display to other males. Through playback experiments, this explanation was also rejected.

Thirdly, tail pumping might be use to aid foraging by flushing insects. The authors found no support for this hypothesis.

The predator-deterrent hypothesis was supported by the author’s data. A phoebe should increase its tail wagging in the presence of a predator to let the predator know “I see you”.  Lower rates of tail wagging when predators are not seen may discourage those predators from attacking a phoebe.

Altitudinal ranges of birds and climate change

As global warming proceeds, marked changes occur in the climate of mountains. The warming means that species restricted by temperature can move higher up a mountain.

For mobile species like birds, scientists expect that the distribution along an altitudinal gradient will occur. Birds must move higher up a mountain to find the temperature regime that best suits them.

Recently, Morgan Tingley and colleagues published a paper that explores this phenomenon. They resurveyed bird abundance in the Sierra Nevada mountains of California along four elevational transects, first established nearly a century ago. Climate information indicates that any site along the transects is warmer now than it was a century ago. The workers focused on 53 bird species and found that 48 of them had shifted their range higher up the mountains over the past century.

[Originally published on October 18, 2009]

Bird Sleep

By in Physiology

With the arrival of cool weather, I expect activity at your feeders has increased. I like to monitor when the chickadees, goldfinches and other feeder birds leave the feeder in the afternoon. The birds are going off to their roosts and the timing of their departure for their roost, relative to sundown, is remarkably constant from day to day.

At their night-time roosts, most birds spend most of the night sleeping. Birds spend about half of their lives sleeping but the amount of sleep they get depends greatly on the latitude and the season of the year. During the early part of the Maine summer, birds may sleep less than nine hours when the night is so short. On the other hand, Maine birds in the winter sleep more than 15 hours a night.

Birds living in the vicinity of the equator experience near equal days and nights each day so spend twelve hours a day sleeping throughout the year.

Some birds spend the night in communal roosts. European Starlings and American Crows are good local examples. In more southern parts of North America, huge roosting flocks of Common Grackles and Red-winged Blackbirds are common.

Migration poses a real challenge for birds in terms of getting adequate sleep. Most songbird migration occurs at night and then the birds must forage during the following day to restock their fat reserves. Not a lot of time for sleeping.

A recent study of migrating Swainson’s Thrushes revealed a behavior that helps these birds fight sleep deprivation. The birds take minute-long naps through the day. During these short siestas, a thrush will puff its feathers out and squint their eyes. A migrating Swainson’s Thrush will spend up 15% of its day sleeping in these very short bouts.

Many birds choose solitary roosts. Red-tailed Hawks, Bald Eagles, Black-capped Chickadees and Purple Finches are good examples.

Particularly during the winter, the choice of roost site can mean the difference between life and death. Many of our northern finches (Common Redpolls, Pine Siskins, Purple Finches) roost in evergreen trees. Such trees offer protection from the chilling effect of winter winds. The birds can also absorb a little of the infrared radiation emitted by the trees. In the brutal economy of a Maine winter, the little bit of heat absorbed from the environment can be critical for a wintering bird.

Woodpeckers, nuthatches and chickadees will spend the night in cavities, often the same cavities used for nesting during the breeding season. House Finches will seek out the eaves of houses for a relatively warm roost site.

Some birds sleep on the ground. Members of the grouse family are typical of this sleep behavior. Northern Bobwhite arrange themselves in tight circles on the ground with the heads facing outward. If a predator disturbs the roost, the birds explode in all directions, certainly giving the predator a dramatic shock.

The posture of a sleeping bird can important in conserving heat during a long winter night. The unfeathered bill is a potential source of heat loss. Many sleeping birds will turn their head under their scapular feathers while they sleep. Mourning Doves turn their head down, nestling it in their dense breast feathers. Some birds sleep while standing on one leg, reducing by half the heat loss across the unfeathered legs.

Aquatic birds may sleep while floating on the water. The Common Eider, abundant along the Maine coast in the winter, snoozes while bobbing on the ocean surface at night. The down feathers of the eiders, renowned for their warmth, keep the eiders warm.

Some birds are able to sleep while on the wing. Some species of swift spend months in the air without landing. Albatrosses often spend months at sea. Sleeping on the surface of the ocean is possible exposes the birds to fish and whale predators. Similarly, Sooty Terns are reported to stay airborne from the end of one nesting season to the beginning of the next breeding season. Ornithologists believe these birds spend time sleeping in flight.

Have you ever wondered why birds do not fall off their perches while they are sleeping? Songbirds have a flexor tendon in the lower part of each leg. When they perch, the flexor tendon tightens, causing the toes to curl around the perch. The tendon essentially locks, keeping the toes firmly around the perch during sleep. The same type of tendon is useful in raptors when they are carrying prey. Once a fish is in the talons of an Osprey, that fish has no chance.

[Originally published on November 1, 2009]