Thursday, January 16, 2014

First Robins of Spring?

We often hear people talking about the first Robin of Spring - as if they have disappeared for the winter and come back one at a time. Although robins are considered harbingers of spring, many American Robins spend the whole winter in their breeding range, basically the entire continental United States. But because they spend more time roosting in trees and less time in your yard, you're much less likely to see them. The number of robins present in the northern parts of the range varies each year with the local conditions. This morning, I found hundreds of Robins clustered around a single holly tree in a neighbor's yard.
Holly berries, rock hard even a month ago, must "ripen" before birds find them edible. OK, think something closer to "rot" rather than "ripen." Only after weeks of freezing nights followed by above-freezing days will berries soften enough for birds to show interest. Well, we certainly have had weeks of freezing nights lately! Did you ever see a Robin hovering before?
American Holly produces bitter chemicals calls sapon ins that make the leaves and berries unpalatable to most forms of wildlife. Production of unpleasant or poisonous chemicals is a common strategy in plants to discourage animals from eating them. In American Holly, the saponin level in berries declines as they ripen, and then apparently declines further during the course of the winter when the plant is mostly dormant. So by late winter, when food supplies for birds are at their lowest, American Holly berries are available and edible for a wide range of fruit - eating birds. The birds, in turn, disperse holly seeds in their droppings, helping the hollies reproduce and disperse. Of course, the Robins only find berries on the female trees, so a tree without berries might be a male, or simply be already stripped by Robins and other fruit eating birds.
I was amazed at the "politeness" of the Robins. Large numbers foraged for berries on the ground, while others moved from branch to branch in the tree. Large numbers waited their turn on the nearby rooftop and at least five different trees. When a car drove by, all the birds flew off, then more returned to resume the feast.
video

1 comment:

dick dennis said...

Today the robins are in the streets drinking the slushy water. Maybe they are flushing out whatever toxins remained in the berries.