The Relation between Latitude and Breeding Seasons in Birds

SUMMARY.

• 1
   As one goes north from the temperate latitudes one finds a general tendency for the egg-laying seasons of birds of all kinds to start later and later at the rate of some 20 or 30 days per 10^o of latitude.
• 2
   As one goes south from the temperate latitudes into the northern tropical and equatorial zones one finds a general tendency for the Accipitres, Coraciiformes, and, to a less extent, the Passeres, to start their egg-laying earlier and earlier.
• 3
   The Charadriiformes, Gralise, Herodiones, and Anseres behave differently. In the northern hemisphere they tend to breed later in the tropical and equatorial zones than in the subtropical and temperate.
• 4
   There is a general tendency for birds in the tropics to reach the height of their main breeding seasons somewhat before the sun passes overhead. Two breeding seasons in the year are therefore quite common, but birds which breed only once select either the northward (Accipitres, etc.) or southward (Grallae, etc.) swing of the sun.
• 5
   The main proximate causes of the breeding seasons of birds in nature are thought to be temperature and length of day in the boreal and temperate zones, and rain and/or intensity of insolation near the equator. The time of arrival from migration is often an important factor.
• 6
   Much egg-laying occurs when days are getting shorter, and indeed it often proceeds rapidly while they are decreasing in length and only between 11 and 12 hours long.
• 7
   There is, however, little egg-laying when the day is shorter than 11 hours, and almost none when it is less than 10.
• 8
   Under natural conditions birds exhibit no tendency to start breeding everywhere when the days reach a certain length nor when, they are becoming longer particularly quickly.