88 H. BOYD 



populations o£ Ay thy a typically show greater fluctuation than those oiAnas, 

 while in some of them {A.Juligula, A. madia), and possibly in most, females 

 may not mature until two years old. The large duckling losses seem to be 

 associated with the abandonment of broods by their mothers long before the 

 young birds can fly. In the dabbling ducks, mothers remain with their broods 

 until they are flying strongly and duckling losses are largely confmed to the 

 first few days after hatching. Losses of young Aythya are less easy to measure, 

 because the (motherless) broods tend to aggregate, but seem to continue at a 

 substantial level throughout the pre-flight period. 



ADULT SURVIVAL 

 The black portions of Fig. i, showing recruits to the (steady) breeding 

 population, suggest that there are generic resemblances in adult losses. It is 

 rather easier to obtain information about adult survival than about the 

 production and survival of eggs and young. Table I shows values of the mean 

 annual survival of adults of twenty-three species in ten genera of seven 

 tribes. The examples are confmed to populations not showing marked 

 trends. It has not been possible to use 'adult' here in a wholly consistent way. 

 In most cases the survival rate shown appHes to birds at least one year old, 

 rather than to birds known to be sexually mature. Differences in survival 

 between pre-breeders more than a year old and older birds are slight, 

 normally much less than the fluctuations in adult survival between one year 

 and another. 



This extended sample strengthens the inference that closely related species 

 have similar adult mortahty rates (J = i — 5, where s is survival) and that 

 differences between genera may be substantial. On the view that the death- 

 rate balances the birth-rate, rather than the other way about, this is equivalent 

 to support for the hypothesis that the effective fertility (as defined by Capildeo 

 & Haldane, 1954) of closely related species is similar. 



Another inference from the general observation that geese are bigger than 

 ducks is that large wildfowl survive better, and produce fewer offspring, 

 than small ones. Using average weight as a crude but simple measure of size, 

 the Spearman rank coefficient between body weight and adult survival for 

 the twenty-six species in the sample is +o- 579, P <^ o* 01. 



Males survive appreciably better than females in some species, especially 

 among the diving ducks, in which there seem usually to be an excess of males 

 in the adult population, despite the parity of the sexes at hatching. 



EXPLOITATION 



The statistics so far presented, though inadequate for constructing a detailed 

 working model of any specific population, provide a 'natural history 



