a) AGE OF FEMALE--1st year females have slightly smaller clutches than all others. For example, European Starlings lay on the average 4.45 eggs the first year and 5.60 eggs in subsequent years. Most birds don't live much longer than two years, but they don't usually lay more and more. Black-legged Kittiwakes lay 1.8 the first year, 1.9 in years 2-3 and 2.4 in years beyond 3.
b) SEASONAL--clutch size decreases through season. Field Sparrows lay in May 3.77 eggs, in June 3.69, in July 3.14, and in August 3.00. These numbers, of course, are averages. 0.14 of an egg is kind of messy.
c) ANNUAL--clutch size varies from year to year, depending on severity of season and food supplies. Some examples, followed by minimum and maximum per year are: European Starling 6.4 -7.5; Red Grouse 6.1- 8.1; Common Eider 4.72- 5.44; The European Swift lays 2 or 3 eggs, smaller selected for during lean years, the larger selected for in good years.
d) POPULATION DENSITY-- The Great Tit lays 2 eggs in times of high density, 2 more in times of low density, so there can be a 4 egg difference.
e) FURTHER INLAND PROPORTIONAL TO CLUTCH SIZE--Song Sparrows have the largest clutches in the center of the country (3-6-3). WHY? Lower life expectancy less rich environment harsher climate must be compensated for.
f) LONGITUDE--Away from equator, clutches increase. In House Wrens in: Amazon = 2, Central America = 3, Mexico = 4, southern USA = 5, Canada = 6.
h) ISLANDS PROPORTIONAL TO CLUTCH SIZE-- passerines on islands = 2-3, on mainland = 4-5. WHY?