This issue deals with predators:

COWBIRDS

Cowbirds are viewed by songbird lovers as public enemy number one and are often blamed for the songbird decline because they have such a huge impact on the nesting success of the unwitting hosts. A female cowbird never builds a nest nor takes care of her own eggs and babies.

The female cowbird plans her attack carefully. She finds a nest while it is being built, visits the nest regularly so she knows when to add her own eggs. She'll even come back to the nest during the day to remove a host egg. The victim birds use time and energy caring the cowbird baby who is insatiable. The host birds come and go at a frantic pace to provide food for this chick who begs urgently for more food. The host birds are programmed to feed the hungry mouths in their nest and do not even notice that the cowbird nestling looks totally unlike the other nestlings. By the fifth day or so, some of their own nestlings will lie dead from starvation. The parents remove the dead babies without a thought and go back to finding food. After all their hard work, they'll be lucky to have one of their own survive plus one cowbird.

While every songbird is at risk of a cowbird invasion, there is one songbird nest that the cowbird will not find so welcoming. The lucky American Goldfinch just happens to be the only songbird that feeds its young a diet of seeds. Cowbirds hatched from eggs laid in goldfinch nests cannot survive the all-seed diet!

Dr. Stutchbury calls cowbirds "serial killers" who attack dozens of host species. A single female can lay more than twenty eggs in one breeding season! Many of the host birds that she will invade produce only one batch of babies per summer, meaning they will have no second chance to make up for the offspring lost because they unknowingly fostered a baby cowbird.

They were called cowbirds because they were once found only in the open grasslands of the Great Plains where they followed bison herds, feasting on the insects kicked up by the bison hooves. Then the eastern forests were cut down and the cowbirds moved into this newly opened landscape. For more than a hundred years, cowbirds have been attacking species like hooded warblers, scarlet tanagers and Acadian flycatchers that had rarely, if ever, seen cowbirds before. Now cowbirds have spread to the southwest and are moving into the northern forests of Canada and Alaska. When forest is cleared for agriculture, cowbirds are given access to new hosts.

Cowbirds now thrive far outside their historical native range. They are so plentiful that a single host nest often contains not just one but sometimes three or four cowbird eggs. These host birds are especially vulnerable because there has been no long history that would have prepared them to recognize the cowbirds eggs as foreign. There is no programmed behavior that would compel them to toss the foreign egg out of their nest.

Cowbirds, however, are well programmed! They hatch earlier than their "nest mates", grow faster and are louder in their begging for food. Parent birds are programmed to feed the hungriest nestlings first. Some species of birds can keep up and still manage to raise a couple of their own young, but some others, like the blue-headed vireos rarely manage to raise any of their own young if there is even a single cowbird chick in the nest.

In a previous issue the problem of forest fragments was discussed where we learned that small patches of forest do not provide the best habitat for birds. A study of five thousand nests in the Midwest found that songbirds nesting in large continuous forests were rarely parasitized, but the same species were five to thirty times more likely to have a cowbird egg in their nest if they lived in a forest fragment.

There are groups advocating widespread culling of cowbirds in an effort to give songbirds a chance against this sneaky enemy. Cowbird control, where applied, has been critical for saving a number of endangered songbird species who live in specialized habitat and have small breeding ranges. These songbirds had already lost out due to destruction of habitat, but the cowbirds were delivering a knockout punch!

In Camp Pendleton, CA an effort was made in the early 1980's to save the least Bell's vireo from sure extinction. Cowbirds were easily attracted to decoys in traps because they are a flocking bird. About two thousand female cowbirds are now killed annually throughout Camp Pendleton which has reduced cowbird parasitism from 50 percent of nests to less than 1 percent of nests, allowing a spectacular recovery of the vireo population resulting in more than five hundred breeding pairs surviving.

EGG PREDATORS

An acre of forest can contain dozens of nests of songbirds species all filled with eggs. These eggs and the birds that hatch from them are highly sought after food for a long list of predators, including snakes, small mammals, deer and even other birds. A female who has just finished laying her valuable eggs has only a fifty-fifty chance that her nest will not be noticed by a predator before her young are ready to leave the nest. Songbirds are accustomed to losing their nests and the victims simply try again, and again, until they run out of time and have to start packing on weight for their migration.

Night time brings another set of predators - skunks, opossums, chipmunks and raccoons who randomly search for nests by running along branches. It's no wonder that only half the songbirds' nests ever produce young!

DOMESTIC CATS

Cats that are allowed outdoors are a huge problem for birds nesting in suburban and rural agricultural areas. A survey of homeowners in southeastern Michigan found that each home had about two to four cats that went outside for at least part of the day. Cats eat adult birds that nest near the ground, like common yellowthroats, and also birds that land on the ground to gather nesting material or look for food. Nestlings and young songbirds that cannot fly well are also easy prey.

The study in southeastern Michigan showed that the six hundred cats in the study would have killed more than six thousand birds during a typical ten week breeding season alone! Cats are only doing what comes naturally, but their predation on birds is not a natural part of the ecosystem. Domestic cats were introduced to North America when the Europeans arrived and only became popular in the late 1800's. The number of cats living the U.S. alone has doubled since the 1960's and now stands at well over 75 million. Unlike natural predator populations, which decline when their own numbers get too high or their prey declines, pet cats are housed, fed and given vaccinations against disease. Even well fed cats kill birds and declawing or putting bells on the collar does not stop them from catching birds. Also, for every loved and cared for pet cat, there is at least one homeless feral cat that also roams our meadows and woods.

A Wisconsin study in the 1990's estimated that free-ranging cats in rural areas killed anywhere from 8 million to 217 million birds a year in that state alone. In some parts of the state, there are close to one hundred free-ranging cats per square mile, a density that far exceeds any other native predator.

Owners desire a good life for their cats and think that giving them outdoor time provides for that, but indoor cats are less likely to contract deadly diseases and are in no danger of getting hit by a car. Outdoor cats live an average of only three years compared with fifteen years or more for indoor cats. Cats are not to blame for the songbird decline, but keeping them indoors will mean that birds nesting in woodlots near farms and homes will have a better chance of staying alive and producing a healthy number of offspring.

This ends my fifth installment. Watch for our next Newsletter where I will continue sharing what I have learned from Dr. Stutchbury's eye-opening book "Silence of the Songbirds". Her writings have touched me deeply and I just had to share it with all of you.