The mortality of Yorkshire Canada Geese

Abstract

The annual mortality of Canada Geese Branta canadensis in the Yorkshire population has been determined. Birds staying permanently in the county have a lower mortality than have those which migrate to the Beauly Firth to moult (10.0% versus 24.5%). First year geese are only marginally more vulnerable than adults. The majority of recoveries occur in the shooting season but there is a notable increase in casualties during April and May. This is most marked amongst migrants and is attributed to breeding pressures. Colour ringing has allowed a crude estimate (2-3% per annum) to be made of the rate of meal ring loss: this has to be subtracted from the calculated mortality rates. The use of retrap data is shown to result in an overestimate of mortality since retrapping is selective towards younger geese. The reporting rates ignoring ring loss, are high, at 0.395 for geese ringed in Yorkshire and 0.286 for those on the Beauly Firth. An estimate is made of the gosling output necessary to sustain the present rate of increase in the population. Comparison of this figure with the number of potential breeders suggests that a sizeable proportion of mature geese do not attempt to breed but continue to migrate to the Beauly Firth in summer. It is suggested that these are the less successful members of the community and that density-dependent factors may prevent an over-rapid increase in numbers.