Two shaft-tailed finches take part in allopreening, a courtship activity in which they preen one another. Credit:
By Robert Mulvihill / National Aviary ornithologist
This is one of a series presented by the National Aviary, which works to inspire respect for nature through an appreciation of birds.
My mother always insisted that the birds in our Squirrel Hill backyard started singing on Valentine's Day. That's when she noticed that "her" cardinals, Carolina wrens and house finches were once again full-voiced after a long winter interlude.
It's true there often is a "spring thaw" around mid-February each year, but it's not warmer temperatures that trigger the physiological changes bringing birds into breeding condition. The trigger actually is the steadily lengthening period of daylight (called the photoperiod) that begins on the heels of the winter solstice, the shortest day of the year.
Proof that light trumps temperature is the fact that birds will begin to sing on very cold days in late January and February, even when there hasn't been a hint of spring warmth. The whistled "fee-bee" of Carolina chickadees, the funny "heh-heh-heh-heh-heh" of the white-breasted nuthatch, the energetic "churrrrs" of a red-bellied woodpecker and more remind even shivering bird lovers that spring is just around the corner.
But what about birds that live in tropical regions near the equator, where there is little or no seasonal change in day length? Do they simply breed at random times throughout the year? Actually, most tropical birds also are strongly seasonal in their breeding, not unlike birds of temperate regions. Studies show that species such as the spotted antbird from South America do respond physiologically and behaviorally to photoperiod change -- it's just that they are sensitive to much smaller changes.
Near the equator, where the photoperiod is a fairly constant 12 hours, there is seasonal variation in sunrise time. From the middle of February, when the latest sunrise of the year occurs, until mid-May, the sun rises a little earlier each day. The total difference is about 17 minutes (compared with 130 minutes for the same three months in Pittsburgh), but it is enough to serve as the primary trigger for breeding. In the tropics, rainfall provides a second important trigger, because the rainy season brings the flush of food -- insects, flowers and fruits -- birds need for raising their young.
What about birds like those at the National Aviary, which live in controlled environments, with a regular food supply and year-round warm temperatures? Thanks to our naturally lit exhibits, at this time of the year many Aviary birds also exhibit increased singing and other courtship activities, such as allopreening (preening one another), behaviors triggered by the same earlier sunrise times now inspiring our backyard birds to "think spring."
My mother's may be a romantic notion that birds start singing on Valentine's Day, but for bird lovers who visit the National Aviary on Feb. 14, there will be music in the air. Singles, couples and groups of friends can all flock together for an over-21 Valentine's Day Party from 6 to 10 p.m. Proceeds will help support the Aviary's bird care and conservation programs. Find details at www.aviary.org.
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