In New England, fruit-eating birds are particularly vulnerable in the winter, because they depend so heavily on a food source that ferments, and to get enough protein they need to eat a lot of it. Toxic levels of ethanol can be produced as the natural sugars ferment, causing some consumers to become inebriated. Robins, waxwings and starlings have been found dead in large flocks after eating toxic berries and diving into the ground or colliding with solid structures. In addition, when they are really drunk, they lose mobility, making them helpless in the presence of predators. To the surprise of many observers, birds that appear lifeless on the ground have been known to eventually sober up and fly away. While birds don’t intentionally wish to ingest a lot of alcohol, there are other animals, such as elephants and apes, that will wander for miles to seek the pleasure of fermented fruits.
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In the fall and winter, fruit makes up about 90% of an American robin’s diet. In the spring, only 10% of a robin’s diet consists of fruit; invertebrates make up the remaining 90%. (Summer is a fairly even mixture of both.) At this time of year, earthworms are a popular food item with robins. Watching a robin foraging for a worm can make you wonder whether the robin is using its ears or its eyes to locate the worm. It turns out that most worms are seen, not heard, by robins. Because the sound of worms burrowing in the soil is of low intensity, they usually cannot be heard by robins because of background noise. Using sight, not sound, the robin first aims one eye toward a spot on the ground in front of it, and after holding this position for a few seconds, rotates its head and draws a bead with its other eye on the same spot– an earthworm in its burrow. The robin then quickly thrusts its bill into the burrow in an attempt to get its next meal.
They may not be sweet, plump and juicy, but the fruits of staghorn sumac play a crucial role in the lives of many birds that overwinter in New England. True, they’re not a preferred food for these birds, but because they persist through the winter, these fuzzy fruits are an important source of food in late winter and early spring, when very little else is available. Ruffed grouse and wild turkeys rely on sumac fruit as a source of food throughout the winter, and bluebirds, robins, cardinals, mockingbirds and starlings are frequent visitors to staghorn sumac shrubs this time of year.
While some American robins usually overwinter in northern New England, we have a larger number of them this winter than usual. A lack of snow cover and an excellent crop of fruits and berries are contributing to this phenomenon. You often see them in flocks of a hundred or more, especially on south facing fields, and where there are fruit trees. Migration is a hazardous undertaking for birds, and in general they go only as far south as they need to in order to survive. If there is an ample supply of food, and a lack of snow, robins are more likely to remain in northern New England during the winter, or stop here on their way south from Canada.