The latest news about Maine lakes and ponds.

Addressing Bird Feeder Fallout

November 19, 2008 - Feeding the birds is an activity that has increased greatly in the past 40 years. In the United States, 43 percent of households maintain bird feeders. In the United Kingdom, bird feeding is even more popular; 75 percent of households there feed the birds.

Homeowners in the U.S. and the U.K. purchase 500,000 tons of birdseed each year. This bounty is enough to support 300 million chickadees living on nothing else. In short, bird feeding represents a major subsidy to many species of birds.

We know that food often limits bird populations, so bird feeding may have positive benefits for birds. However, we know surprisingly little about the effects of bird feeding, particularly on larger geographic scales. Most of the work that has been done, including some of my own, has been concerned with local effects.

Gillian Robb, an Irish ornithologist, and several of her colleagues have recently published a review of avian responses to supplemental feeding in the journal Frontiers in Ecology and the Environment. Today's column is the first of two in which I will summarize the major points of the article.

The energetic costs of reproduction are huge for birds. Favorable times for nesting are often brief in birds, so an early start may be beneficial for breeders. In 34 of 59 studies reviewed by Robb, bird feeding resulted in earlier nesting. In most cases the shift was less than a week, but in some, a shift was as long as a month earlier.

Unfortunately, earlier nesting can sometimes result in negative impacts. Birds given supplemental food may begin nesting before their natural food supply becomes abundant enough to provide enough nutrition for their nestlings. As an example, chickadee adults do well feeding on sunflower seeds, but their nestlings need caterpillars and other sources of animal protein to grow and thrive.

Food supplementation can affect the quantity and quality of eggs laid by female birds. In 44 studies reviewed by Robb, 28 presented evidence that bird feeding increased the number of eggs laid.

As an alternative (or as a second effect), a female bird with access to supplemental food may increase the quality of her eggs by laying larger eggs. Larger eggs cool more slowly than smaller eggs when the adults are off the nest; larger eggs have a greater chance of hatching than smaller eggs.

Florida scrub jays given high-fat, high-protein food laid eggs with more water and protein in them. Some popular bird foods may be a rich source of macronutrients. For instance, peanuts are high in vitamin E. This vitamin E can be passed into the eggs by a female bird. These enhanced nutrient levels result in better immune responses by nestlings to the threats of disease.

Supplemental food may increase the chances that a pair of birds can have two clutches during a single breeding season. For instance, when black-throated blue warbler females were given food after their first clutch, all of the females started a second clutch compared to only 50 percent of the females that were not given supplemental food. This effect even carried on into the next year. Two-thirds of the females given extra food in one year had two broods the following year compared to none for the females not given extra food in the prior year. This striking result shows the dramatic and long-lasting effect that bird feeding can have on bird reproductive success.

In humans, the sex of a baby is determined by the sex chromosomes. Females have two X chromosomes whereas males have a single X chromosome and a much smaller Y chromosome. In birds, sex is determined in the opposite way: males have two similar chromosomes, called Z chromosomes, while females have a Z chromosome and a smaller W chromosome.

Birds differ from mammals in that female birds have the ability to control the sex ratio of their young. The kakapo, a flightless parrot found only in New Zealand, provides an interesting example. The kakapo is an endangered species whose population declined to as few as 70 individuals.

Wildlife biologists decided to try to increase the nesting success by giving the female kakapos supplementary food. Unfortunately, the kakapo females that received the extra nutrition responded by producing more male offspring! So the wildlife biologists had to lower the amount of extra food provided to strike a happy medium: enough to increase the nesting success of the female, but not so high as to cause the females to produce mostly male offspring.

Stay tuned until the next column for more information on the impacts of feeding the birds.

Herb Wilson teaches ornithology and other biology courses at Colby College.
HERB WILSON, Portland Press Herald, November 13, 2008


Lakes:
Regions:


Print this story

Email this story

return to Lake News