1) FOOD is by far the most important ultimate factor. In the Blue Tit, the male provides up to 40% of the female's total food requirements through COURTSHIP FEEDING. In Arctic Terns, the more food the male brings, the earlier egg-laying occurred and the larger the clutch. American Goldfinches feed young seeds and consequently breed later than seedeaters that feed young insects (most others). Eleonara's Falcon breeding corresponds with peak of fall migration. African raptors breed before grass covers rodents during summer rains. Wood Storks breed in summer when fish are concentrated in pools (although fish are not necessarily most abundant then).
2) COMPETITION. The competition for hole nesters is so severe that they nest when less competition for holes-- early in nonmigrants. Black-capped Chickadees in South Dakota begin singing in January or February. Different populations of seabirds often breed at different times, because of competition for cliffs or burrows. North American migrants to South America may affect resident birds, reducing food availability.
3) NESTING CONDITIONS--riverbank nesters have to wait for dry season when water is down.
4) PREDATION PRESSURE--driver ants in Africa are seasonal and cause seasonality in birds.
5) WEATHER--controls food and nesting conditions. Heavy rains avoided by tropical birds. Cormorants in Lake Victoria avoid windy season.