Fat Energy

Yellow-rumped Warbler

At the Herrick’s Cove Wildlife Festival two weeks ago, biologists from the Vermont Center for Ecostudies (VCE) gave visitors an opportunity to see close-up one of the ways in which scientists study birds. Bird banding uses light weight mist nets that are stretched across likely songbird routes. A bird fliesinto the net and becomes entangled. The biologist carefully removes the bird and takes it to the banding station. There the bird is weighed, measured, and examined to determine its general state of health. By blowing lightly on its feathers, the biologist can see if the bird has remaining fat reserves and whether it is male or female. The bird is then fitted with a tiny, numbered leg band. If it is ever recaught, the band will tell something of its travels, habits, and age.

For example, VCE researchers study Bicknell’s Thrush. On Stratton Mountain, they banded a male, and renetted the same bird over two dozen times for the next ten years. Birds banded on Stratton have also been recaught in the Domincan Republic where they winter.

At the Wildlife Festival, they netted and banded a male Yellow-rumped Warbler. The bird was healthy. It had enough visible fat reserves, the onlookers were told, to make the rest of its journey, possibly to the Canadian boreal forest. Once it reaches its breeding territory and begins singing for a mate and defending a territory, it will use up those remaining reserves.

I have heard or read that sort of information before, but this time it woke me up. In the last two weeks, I have been watching the returning birds with a new appreciation and a new perspective.

Yellow Warbler

Early this week, I walked the causeway at the Hinsdale setbacks. The bushes which line the causeway are favorites for Yellow Warblers, and the boys were busy claiming their territory. Male Yellow Warblers have reddish streaks on the breast. They also sing. I only saw males. They foraged. They sang. They foraged. They chased other males. They sang. They foraged. They chased.

On several occasions three males contested the same alder bush. They dashed about with lightning speed. When the claimant to the alder bush had successfully driven off intruders, he sang his full throated, “Sweet, sweet, sweet, I’m so sweet” - which is the mnemonic used by many birders to describe the song, even though there is nothing at all sweet about the serious conflicts taking place.

These birds had just completed a long and energy-expensive journey from their wintering grounds. Now they engaged in energy-expensive territorial battles in the quest to obtain breeding rights to a female who would critically evaluate the quality of the territory each claimed plus the genetic quality exhibited in his song and his demeanor.

Not to short-change the female: when she arrives and begins her evaluation, she will also have completed a long and energy-expensive journey. She will then expend energy reserves in nest building and in energy-expensive production of eggs. She needs a mate who knows how to find and keep a territory with abundant food to replenish her energy needs and to feed her young.

As a bird watcher watching all of this activity, I am a rather self-indulgent, dilettante. I am there for the fun of it. But the birds pursue a very serious goal - extending their genetic life.

Golden-winged Sunbird

How energy expensive is this breeding season for the bird? We get an idea of how expensive it is for our Yellow Warblers from a study done on an African bird, the Golden-winged Sunbird, a species somewhat like a hummingbird. (“Economics of Feeding Territoriality in the Golden-Winged Sunbird.”) When the bird was at rest, it burned “x” number of calories to maintain its bodily temperature and bodily functions. When the bird foraged for food, it used 2.5 times the number of calories as it did when resting. When it was defending its feeding territory, it used 7.5 times the number of calories as when resting, or 3 times the number of calories as when it was foraging. That spike in energy consumption means that it needs good and abundant food sources to stoke its energy needs when it is defending a territory.

The Yellow Warblers were clearly expending a great deal of energy as they sought to establish and defend a breeding territory.

Blackpoll Warbler

For all of the tremendous energy that birds expend, they are unbelievably efficient energy consumers. Ornithologists studied the Blackpoll Warbler which flies over the Atlantic Ocean nonstop from New England to South America in about 90 hours. They suggest that the flight “is equivalent to a human marathon runner competing in 50 consecutive 26-mile (42km) races without consuming any food or water en route and without losing speed from the first to the last leg ... if this tiny bird were burning gasoline instead of body fat, it could boast a fuel consumption rating of about 720,000 miles per gallon.” (Prius, eat your heart out.)

Birders often try to bring a bird into sight by pishing - “Pisshh, pisshh, pisshh.” Increasingly, modern technology allows birders to carry all bird songs in the field in a small, fit-in-the-pocket electronic gadget. The bird song can be played in the field. A singing Yellow Warbler does not know that it is hearing an electronic device; it thinks another male is intruding on its territory and quickly comes to investigate.

I confess that I pish, and that I sometimes play a recording to attract a bird. I do so less and less. As I come to understand the energy-expensive activity in which the birds are engaged, I realize that I am not helping the bird one bit by arousing or agitating him. It amounts to harassment; it results in a bird spending energy unnecessarily. Reluctantly I come to the conclusion that the use of electronics to attract birds, and probably pishing as well, is in a very gray ethical realm, and may be outright indefensible.

Yellow-rumped Warbler

So, why do so many birds make the expensive, and dangerous, journey from their tropical homes to the temperate forests of Vermont. To oversimplify, they make the journey because for two or three months out of the year, our climate is virtually tropical, we have under utilized habitats, and the food resources are abundant. We like to joke that the reason there are black flies in Vermont (nasty, biting pests) is to remind us that we are not in heaven yet. For the birds, those black flies are heaven - along with the gnats, mosquitos, web worms, and every creepy crawly on the underside of leaves, in bark crevices, and wherever.

The neotropical migrants - the warblers, vireos, tanagers, thrushes, swallows, flycatchers - can make the expensive journey because they can fill empty habitat niches rich with protein to raise young and quickly rebuild their own reserves.

Yellow Warbler

I know those Yellow Warblers were burning energy in their fast and furious chases. I know the Yellow-rumped Warblers now singing in the treetops are burning more energy than they are taking in as they leaf-glean and fly-catch. Since the ice sheets retreated and the forests came, these northern forest have provided abundant resources to offset the costs of getting here and back. By summer’s end, the Yellow Warbler and the Yellow-rumped Warbler will have raised young and replenished fat reserves. They will then return home to the tropics.

It will continue to be so, if those bipeds on the ground don’t screw it up.