• 1
    Average clutch-size is characteristic for each genus and each family of birds in the same region.
  • 2
    In almost all families, clutch-size is much smaller in equatorial Africa than in mid-Europe.
  • 3
    In mid-European small passerines, those with comparatively safe nesting-sites tend to have larger average clutches and longer nestling periods than those building open nests.
  • 4
    Swifts and petrels have safe nesting-sites and long nestling periods, but only small clutches, correlated with a scarce or uncertain food supply for the young.
  • 5
    In general, the length of the incubation period is correlated with the length of the nestling period in birds, probably because it is difficult to modify the rate of development except as a whole.
  • 6
    In nidifugous species, the specific differences in clutch-size have not been accounted for, though food for the brood is thought to be the basic factor involved in most cases. The number of eggs that the hen Can cover is not a primary factor in most cases, but may sometimes provide a secondary limit.