Ultimate factor
One of the factors of the environment which govern the year-cycles of birds in the ultimate analysis. For example, food supply is an ultimate factor controlling the timing of the breeding season in any given species. The actual 'triggering' factor, however, may be one such as daylength or temperature, which has a direct effect on the birds' activities. These 'immediate' influences are called 'proximate factors'.From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver