318 U. S. NATIONAL MUSEUM BULLETIN 217 



in 13 species the clutch contained 4 eggs. Since the number of eggs 

 in a clutch is characteristic of 63 widely distributed species of the 

 family Scolopacidae, this constant number is a reproductive charac- 

 teristic of the family and presumably is not modifiable for adaptation 

 of any of the species to any climate. 



In other species of birds, like the snowy owl, the reproductive pro- 

 cedure is not limited to a fixed annual production of eggs. The 

 number of eggs laid by snowy owls, however, has been related to the 

 great annual fluctuation in the population of lemmings (Salomonsen, 

 1950) and is not an example of climatic influence on egg production. 



The birds of some species lay fewer eggs in a clutch in tropical 

 climates than in Europe, according to Lack (1947), who reported that 

 in most families the clutches of species nesting in equatorial Africa 

 were smaller than the clutches produced by species of the same families 

 nesting in mid-European latitudes. Over the nesting range of 

 western white-crowned sparrows the largest clutches, averaging 5.5, 

 were laid at Old Crow (see p. 238) . Less extensive observations indi- 

 cated that 5 or 6 eggs were usual at Anaktuvuk. In this species two 

 arctic populations laid more eggs in a clutch than were found in 

 southern localities. The average number of eggs in a clutch of song 

 sparrows (Melospisa melodia) in Baja California was near 3 and 

 in southern and southwestern Alaska it was about 4 (Johnston, 1954) . 

 In some species the difference of about one egg seems to represent 

 the distinction in clutch size between hot and temperate or between 

 temperate and arctic climates. However, it is doubtful that the in- 

 creased size of clutch in a few species can be usefully related with 

 latitude, for latitudinal influences are applied without exception to all 

 widely distributed birds and in only a few species is the size of clutch 

 affected. In a significant relation to a general condition there should 

 be no exceptions. 



Repeated Nesting 



Differences amounting to less than one egg in an average clutch 

 can affect the reproductive rate, but the effect is likely to be con- 

 siderably less than that produced by multiple clutches in areas with 

 longer breeding seasons. In the long summer of temperate climates 

 there is time for twice or even thrice the number to be added to the 

 populations of fast breeding birds in comparison with the number 

 from the single clutch possible from an arctic pair. Thus, I have 

 found only one robin's nest late enough in the season to have allowed 

 for the successful rearing of an earlier brood yet leaving time enough 

 for the young to grow sufficiently to migrate before freezing weather 

 terminated the arctic season for robins (L. Irving and Paneak, 1954) . 

 I have found a few fresh eggs of redpolls late in June (see page 114), 



