Overall, female Sparrowhawks improved in the mean number of young they
raised per year up to the 5th year of life, and then declined. An att
empt is made to find to what extent this mean trend is caused by simil
ar trends in the breeding of individual females, and to what extent to
the progressive appearance and disappearance of high performing femal
es among breeders of different ages. It is concluded that the mean tre
nd could be attributed entirely to similar age-related trends in indiv
idual birds.