2) seasonal, variability of food supply
3) annual, food supply varies
4) population density, more competition for food in high populations
5) latitude, more daytime in north--you have 20 hours of daylight in Hudson Bay where there are few competitors, both avian and other. Tropical rainfall may further reduce time for clutches. The tropics lack seasonal burst of productivity. Recent research indicates north more productive than the tropics.
6) longitude: inland much more seasonal, so has a bigger blip in productivity
7) islands have lower clutches--islands are less seasonal than mainland and have fewer predators. Islands are also more densely populated
8) body size --big birds take bigger prey that's scarcer than smaller prey so they can't bring in food as fast???
Some experimental work suggests Lack is right when he suggested that food availability is the most important variable affecting clutch size.