Bird Calendars
NOTES FROM NATURE
By Jerry Toll
The Breeding Season
The first warm days of spring are finally upon us. Robins, Red-winged Blackbirds, and waterfowl have returned in profusion. Come June, most of the birds in our area will be nesting. Not all species follow this pattern, though. The owls will have completed their breeding cycle by the time the neotropical birds begin theirs. There are overriding concepts that clarify why different species time their breeding cycles in different ways.
Energy Demands
From an ecological point of view, on both the individual and species level, energy demands largely determine the ability to survive and reproduce. The ability to obtain food, retain the energy, and pass energy on to the next generation is the foundation of survival.
The following quoted passages are taken from The Birder's Handbook by Erlich, Dobkin and Wheye.
"Evolution generally adjusts the timing of avian breeding seasons to produce the most young. Nebraska is in the temperate region. From this region to the subarctic and arctic regions, the main factor is availability of food.
Abundant nourishment is needed, not only by nestlings and juveniles, but by the increased energy demands on breeding adults. For females it's the energetic demand of producing eggs. For males it's the increased energy demands of vigorous displays and defending territories. One or both adults participate in building a nest, foraging for more than one individual (mate and/or chicks), and in some cases territorial defense or guarding young from predators. "sense that it is time to start breeding by the lengthening of the days as spring approaches."
For the neotropical species who spend the majority of their lives in the tropics, migration is a necessary part of the breeding cycle. For some it's the lengthening days; for others it's the internal calendars that tell them conditions will be good for breeding when they arrive.
Even many tropical birds time breeding to environmental changes. Some may time breeding to the prevalent wet or dry seasons. Others may breed irrespective of environmental changes because some influences like food supply are less limiting in the tropics.
"Evolution seems to have latched on to the length of daylight as a timer of activities, since it is a signal that can be used to forecast future events." For instance, the Snow Geese who migrated through Omaha on March 14 could have waited another couple of weeks until the lakes opened up and the snow melted. This would have made their migration easier by making available more food and safe roosting, but it wouldn't have allowed them enough time on the breeding ground to maximize breeding success. The length of daylight told them it was necessary to press on, irrespective of the hardship.
Factors like food supply and weather may fine tune day length. If Red-winged Blackbirds are experimentally provided with abundant food, they will begin laying eggs three weeks earlier than birds without an abundant supply.
Some species become so specialized in their breeding requirements that extinction looms near. Others, though more general in breeding strategy, are threatened by the rapid changes brought on by human influences on the environment.
This article describes broad concepts about ways in which birds time breeding. At every level, birds have found unique and subtle strategies to accomplish successful breeding.
Previous Notes from Nature:
| January 2001 | February 2001 | March 2001 |
| April 2001 | May 2001 | June 2001 |
01/24/08