10 VERTEBRATE REPRODUCTIVE CYCLES 



This becomes clearer when groups of related birds are 

 examined individually. For all groups it is the general 

 rule that as one goes from temperate latitudes towards the 

 poles the egg season becomes later and later at a rate of 

 between 20 and 30 days per io° of latitude. However, 

 as one goes towards the equator differences become 

 apparent between the different groups. One extreme is 

 set by the Accipitriformes (hawks) and the Coracii- 

 formes. (swifts, nightjars, etc.) which lay their eggs 

 earlier and earlier until the equator is reached. The 

 Passeriformes (passerine birds) also do this, though to 

 a lesser extent. The other extreme is set by the typically 

 aquatic birds, the Charadriiformes (waders), Lariformes 

 (gulls), and Anseriformes (ducks), which tend to breed 

 later and later as the equator is approached just as they 

 also do towards the poles. 



These points are illustrated in Fig. 2 in which the 

 relation of the sun to avian breeding seasons is also 

 made clear. Breeding in temperate and subarctic regions 

 is generally at a time of increasing or of maximum day 

 length. Similarly at the equator there is a tendency for 

 the breeding season to be at its height some weeks before, 

 or at the time when, the sun passes overhead, and since 

 in this region the sun passes overhead twice a y6ar, 

 there tend td be two discrete breeding seasons. However, 

 on the equator most species breed only once, either with 

 the northward or with the southward swing of the sun. 

 The hawks whose ranges extend from the northern 

 hemisphere do so just before the northern swing of the 

 sun, while those of the southern hemisphere do so just 

 before the southern swing. Only rarely is a species of 

 hawk found that breeds at both times. 



With the aquatic birds of the northern hemisphere the 

 egg season becomes later as the equator is approached so 

 that it crosses into the southern hemisphere just before 

 the sun's southward swing. Thus on the equator the 

 northern water birds and the southern hawks breed at 

 about the same time. The reverse is also true when 



