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 for why this might be - including :

• The 'Food Limitation Hypothesis'
• The 'Nest Predation Hypothesis'
• 'Skutch’s Hypothesis'
• 'Ashmole’s Hypothesis'
• 'Environmental seasonality'
• 'Day length'
• The 'Egg-viability hypothesis'.

See: Wikipedia for details.

To date, none of the theories has sufficient supporting field-evidence to be accepted as a definitive explanation.

Also see :Tropical colourfulnessplugin-autotooltip__plain plugin-autotooltip_bigTropical colourfulness

As far back as the 19th century, biologists pointed out an apparent anomaly in the extra 'colourfulness' of birds and insects in tropical regions.

The reasons for the differences remain unproven, but Alfred Wallace and others suggested that they might be due to the “luxuriant vegetation of the tropics” which acted as a natural camouflage all year round, whereas organisms in the North and South had to adapt their plumage to match the bare trees in winter.