POPULATION DYNAMICS OF DUCKS AND GEESE 91 



to variations in breeding success. The mallard and the green-winged teal 

 Anas crecca have a similarly large turnover, but show less striking numerical 

 fluctuations because their breeding success is more consistent. (No convenient 

 summary of the varied data on success could be devised for inclusion here.) 

 A study of the survival of marked teal (Boyd, igSJa) in Europe suggested 

 that about three-fifths of male and one-half of female losses are due to 

 shooting, mostly in winter, and that the kill directly affected annual survival 

 rates. But it also indicated that heavy winter losses due to shooting are offset 

 by reduced losses in subsequent months (including a higher proportion of 

 'natural deaths' than occur in winter), with the imphcation that non-shooting 

 mortaUty should not be regarded as constant and independent of the kill, as 

 some investigators have taken it to be. 



No studies of non-shooting mortality among full-grown geese have yet 

 been pubhshed but it appears that in several species the kill must constitute 

 a much higher proportion of the total deaths than in these ducks. Some of 

 the geese wintering in Britain which have similar adult mortality rates (see 

 Table I) show interesting differences in the annual distribution of shooting 

 losses. In the pinkfoot Anser hrachyrhynchus the kill is largely confmed to the 

 period October to February, with peaks in October and January. In the 

 Icelandic greylag A. anser losses are spread more nearly uniformly through 

 the period September to April. In the Siberian-breeding whitefront A. 

 alhifrons albifrons the kill in April to June seems to be as great as that in 

 autumn and winter, while in the Greenland-breeding A. albifrons jlavirostris 

 most losses occur in May, July, and November to February. Despite these 

 differences in distribution the end results appear to be similar. 



Geese, with their low reproductive capacity and delayed maturity, 

 present problems to the prudent exploiter which are particularly acute in 

 cases like that of the brent Branta bernicla in which recruitment is highly 

 erratic. In geese, and in other large aquatic birds, as Austin (1957) has pointed 

 out, 'maintenance of the adult population seems of greater importance to 

 the survival of the species than the success of any particular year's crop of 

 young'. If brent are to maintain themselves it would seem to be highly 

 desirable to regulate shooting pressure on a seasonal basis so that in years of 

 poor breeding success the kill is kept^small. Greylags or Canada geese on the 

 other hand do not greatly need frequent adjustments of the permissible take. 



CONCLUSION 



Understanding of the dynamics of wildfowl populations is so imperfect that 

 students can as yet contribute little to population research on a broader scale, 

 but the comparative treatment of closely-related forms which is possible in 

 the Anatidae should prove of value. 



