Avian clutch size - the average number of eggs found in a particular (or comparable) bird species nest - often correlates with latitude.
The effect was discovered in the 1940s, and has been extensively researched since then. In general, comparable bird species near the equator lay approximately half as many eggs as those in northern temperate habitats.
There are at least seven theories - including the 'Food Limitation Hypothesis', the 'Nest Predation Hypothesis', 'Skutch’s Hypothesis', Ashmole’s Hypothesis, 'Environmental seasonality', 'Day length' and the 'Egg-viability hypothesis'. See: Wikpedia for details.
None of the theories yet has sufficient supporting field-evidence to be accepted as a definitive explanation.