Out My Backdoor: Mild Winter Doesn't Mean Early Nesting

By Terry W. Johnson

I must admit I have never experienced a winter quite like this one. It has been so unseasonably warm that the best way to describe it is that we are going through springtime in winter.

Although I can remember winters when we have enjoyed a few days of warm weather sprinkled among the cold days of a typical Georgia winter, for days on end now the temperatures have soared into the 70s and even 80s and dropped only into the balmy 50s at night.

This spring-like weather has prompted many plants to burst into bloom far earlier than normal. For example, in my yard spring favorites such as forsythia, kerra, Japanese magnolia, baby's breath, apple, pear, daffodil, coral honeysuckle and even an azalea are all blooming. Because these plants have shown such reckless urgency their blossoms stand a real chance of being nipped by a frost and consequently will not bear seeds this year.

I am sure that you are asking yourself whether the birds that nest each spring in your yard will also be fooled by Mother Nature's cruel trick and begin nesting well outside their normal nesting season. For most backyard birds, however, the answer is no.

This is because many of the birds that nest in our yards are coaxed to begin courtship and nesting by the amount of sunlight that shines on the Earth and not by temperature. Since late December, the days have been getting progressively longer. As the number of minutes of sunlight increase each day, a gland in a bird’s brain called the hypothalamus sends a chemical signal to the pituitary gland to begin producing the hormone that prompts the bird’s gonads to increase in size and thus prepare them for breeding. In response to longer days, birds’ hormone levels also are gradually rising. However, for those birds that typically nest in spring, these chemical changes will not peak for weeks to come.

This biological clock tells birds when to begin their breeding cycle. In most cases, the clock cannot be reset by odd weather conditions such as we have seen this winter. This phenomenon, called photoperiodism, has been shown to control the breeding cycles of at least 80 species of birds.

A famous experiment involving juncos and American crows demonstrated how the process works. One winter a biologist working in Canada housed the birds in two separate cages outside in the harsh Canadian winter.

The birds in one cage were exposed to normal daylight, which in this case meant the days were gradually getting shorter. The birds in the other cage were exposed to artificial light. Here, the amount of light was increased each day to simulate the approach of spring.

When the researcher checked the birds' hormone levels, he discovered that levels in the birds exposed to natural light decreased during the test, while hormone levels rose in the birds subjected to increasing daylight.

As with most things in nature, often there are exceptions to the rule. A case in point is the American robin. While robins normally begin nesting in spring, in a few instances, abnormally warm weather is believed to have contributed to robins nesting in the dead of winter.

Unnatural lighting conditions can also trigger birds to nest early. In one instance, an extremely large display of Christmas lights was thought to play a role in a robin nesting during the holiday season.

In practically all cases, extremely early nesting attempts are doomed to failure. Over thousands of generations, each species of bird has developed a certain nesting behavior that is synchronized with such things as the appearance of leaves on trees and shrubs used in nesting and food abundance.

For example, should a summer tanager return to breed now it would find few, if any, leaves on the oak tree where it prefers to conceal its nest and clutch of three to four greenish-blue eggs. This would increase the chances that a predator would find it. By the same token, since the bird eats lots of insects, it would be nearly impossible for the bird to find enough food for itself, let alone a nest full of hungry young.

Consequently, do not expect to flush a nesting gray catbird from a shrub beside your house any time soon. And if you have not erected the nesting box you wanted to add to your backyard, you still have plenty of time. It is still February, regardless of what the thermometer would like us to believe.

Terry W. Johnson is a former Nongame program manager with the Wildlife Resources Division and executive director of The Environmental Resources Network, or TERN, friends group of the division’s Nongame Conservation Section. (Permission is required to reprint this column.) Learn more about TERN, see previous “Out My Backdoor” columns, read Terry’s Backyard Wildlife Connection blog and check out his latest book, A Journey of Discovery: Monroe County Outdoors.”

I must admit I have never experienced a winter quite like this one.