Thursday, August 06, 2009

A Front Porch Mystery

This article appeared in the Glade Sun, a weekly newspaper on July 5, 2009

Enjoying Nature

by Don Hazel

A couple of weeks ago my friend Gary called. Something was just not right on his front porch. About a week earlier, a house finch had built a nest in a hanging fern basket 3 feet from his front door. You probably have house finches around your yard. They are sparrow looking birds with the males having red on their heads and upper breasts. They are well named because they like to nest around houses, often in hanging baskets like Gary’s. I had a pair try to nest in a flower wreath right on my front door. We scared each other enough times that the finch decided to find a quieter place to build a nest.

Gary had noticed a few days earlier that there were 5 light blue eggs with some dark streaks in the nest. But on the day that he called me he noticed a broken egg on the porch floor, but still 5 eggs in the nest…however one of the 5 eggs looked different from the rest. I couldn’t wait to get to Gary’s house to see the eggs and solve his front porch mystery.

The culprit was a brown-headed cowbird. I am sure that you have seen them at your bird feeders or around your yard. The female is just a plain looking brown bird, but the male has a distinctive dark brown head and a shiny black body.

The cowbird is a very interesting bird; it is what is called a “brood parasite”. This refers to the fact that the cowbird doesn’t build a nest…it just lays its eggs in other birds’ nests for the other bird to incubate the egg and raise the cowbird baby. Cowbirds can zip into a nest and lay their egg in a matter of seconds. It normally removes one egg from the host nest and then lays just one egg in its place. This explained the broken finch egg on the porch and the larger and different egg in Gary’s finch nest. Once they hatch, the larger cowbird baby gets the majority of the food brought by his foster parents to the detriment of his step brothers and sisters. But, at least the cowbird isn’t as assertive as the European cuckoo. That brood parasite actually kicks the host’s real babies right out of the nest to starve on the forest floor while the cuckoo baby is raised as a spoiled only kid by foster parents. However, for some rare songbirds, cowbird parasitism does cause a survival problem and for this reason birders often search out and remove cowbird eggs or trap and kill cowbirds.

But cowbirds aren’t bad guys…they are just another example of a species doing what it takes to survive. Before Europeans came to North America cowbirds were confined mostly to the short grass prairies eating insects kicked up by the buffalo herds. If the herd moved 10 miles a day the cowbirds would find themselves many miles from their nests. They survived by laying their eggs in other birds’ nests so the adult cowbirds could move on with the herds. When the white men killed off the buffalo herds and cleared pasture land for cattle the cowbirds expanded their range and prospered in human environments.

Gary elected to leave the cowbird egg in the nest and watch how things played out. At some point only 4 babies hatched and the cowbird wasn’t one of them. We suspect that the mother finch noticed the difference and removed the cowbird egg or maybe removed the baby when it hatched. In any case, we solved the mystery on Gary’s front porch. Check the bird nests around your yard and see if you can spot a cowbird egg in one of them. If you see an egg that is different from the rest, your mystery will already be solved.