POPULATION DYNAMICS OF DUCKS AND GEESE 87 



significant proportion of mature birds fail to lay. In the case of Arctic nesting 

 species (such as Branta hernicla and Anser caerulescens) years in which very few 

 females breed successfully occur frequently. These non-breeding years seem 

 usually to be due to a delayed thaw in the spring causing nesting sites to be 

 unavailable or to severe weather in the early nesting phase of the breeding 

 cycle inhibiting laying or destroying eggs. In species breeding further south 

 {B. canadensis) catastrophes such as losses of nests due to flooding may affect 

 particular colonies, but non-breeding years rarely, if ever, occur and failure 

 to breed is usually a result of unsuccessful competition for nest sites or other 

 limiting social behaviour. So far as is known at present losses of eggs during 

 incubation and of young between hatching and fledging are relatively small 

 in northern-breeding geese. In Canada geese egg losses may be substantial 

 but gosling deaths are few. Geese do not become mature until at least two 

 years old {A. brachyrhynchus and A. albifrons not until three) so that full- 

 grown young birds are *at risk' for at least twenty months before possible 

 entry to the breeding population. Yet in Anser pre-breeding losses are rela- 

 tively few. Though geese in their first winter suffer substantially heavier 

 losses than do older birds, the first year mortality rates of the Arctic breeding 

 species are well below those found in ducks. There is some evidence that in 

 A. brachyrhynchus and A. c. caerulescens individuals in their second year of life 

 survive better than mature geese. The examples o£ B. canadensis cited here 

 appear to be affected by very heavy juvenile mortahty: in the moffitti 

 population this 'mortality' may well have included substantial emigration. 



The shelduck has a goose-like low mortality-rate after hatching, and 

 delayed sexual maturity (at three or four years) but loses eggs on a duck-like 

 scale. 



The dabbling ducks are prolific, supplementing their first clutches by 

 re-nesting attempts when necessary, which they can do by virtue of nesting 

 in temperate regions and beginning nesting activity early in the spring. They 

 suffer massive losses of eggs before hatching or of ducklings before fledging, 

 though steady populations apparently do not withstand large losses in both 

 these stages. They mature in the summer following their birth, so that the 

 pre-breeding period is short, but nevertheless juvenile losses are usually 

 heavier than in geese, as are those of mature birds. 



The diving ducks of Aythya seem to resemble Anas, with the possible 

 <listinction that losses before fledging seem to be inflicted more on ducklings 

 than on eggs. The pochards, though not Arctic species, nest late in the 

 summer and do not re-nest so freely as Anas, but their output of eggs seems 

 to be as high. This is apparently because, despite the lower frequency of 

 re-nesting, the proportion of complete breeding failures or non-breeding 

 among mature females is low. The situation is rather obscure, because 



