86 



H. BOYD 



because a proportion of the mature females fail to lay. There are five genera 

 represented in Fig. i: two 'grey' and two 'snow' geese, all in Anser; three 

 *black' geese, Branta; the common shelduck, Tadorna tadorna; four 'dabbling 

 ducks', Anas', and four pochards. Ay thy a. The sequence of the species is 

 that in the classification by Scott (1957). 



100 200 300 400 500 600 700 800 900 lOOO 



Anstr brachyrhynchus 



A. a . olbif rons 

 A.c. cQtrulescens 

 A.c , otlontlcus 



Branta btrnlcia origntolls 



B . conodeniis interior 

 B. c. moffitli 



Tadorna todorno 



Anas p. platyrhynchos 



Holland 

 A . p. platyrhynchos 



England 



A . rubripes 



Morylond 

 A . rubripes 



Eastern Canada 



A. d. discors 



Ccntrol Canoda 



Aythyo vallisneria 

 A . americana 

 A. colloris 

 A. fuligulo 

 A.m. morilo 



300 400 500 



700 600 900 



Fig. I. — ^The average production of eggs by some wildfowl populations and the survival of 

 offspring to breeding age. The theoretical egg production of 100 mature females is shown by 

 the vertical bar. The average actual output is indicated by the total length of each column. 

 Losses between laying and fledging are shown by diagonal shading ; losses of eggs to the 

 right, of young before flying to the left. Where there is doubt about the relative size of 

 losses before and after hatching the two patterns of shading are combined to show losses 

 between laying and fledging. The unshaded area shows losses between fledging and maturity 

 and the black area is proportional to the number of young surviving to maturity. 



The histograms suggest that the fecundity of very closely related species is 

 similar and that there are much greater differences between genera (and 

 tribes). Geese {Anser, Branta) lay few eggs. Not only are their clutches small, 

 but they are usually unable to produce more than one clutch a year and a 



