Data from: Sea buckthorn berries (Hippophae rhamnoides L.) predict size and composition of a great tit population (Parus major L.).

In seasonal environments variation in food abundance in the non-breeding
season is thought to affect songbird population dynamics. In a unique
tit-sea buckthorn berry system we can estimate the berry abundance and
both the tit consumption and population dynamics. Six hundred nest boxes
were available to great and blue tits (Cyanistes caeruleus) for breeding
in spring and roosting in winter. We followed the dynamics including the
recapture histories of individually marked great tits from 2008-2013. In
each year we estimated 1) the winter sea buckthorn berry availability, 2)
an index of berry consumption in December based on the colour of the
faeces of roosting birds, 3) the number of breeding great and blue tits,
4) both recapture probability and the return rate of the great tits and 5)
immigration rates. December berry abundance positively predicted the
number of breeding pairs of both species in the subsequent season and
great tit return rates in the second half of the winter. There was support
for a sex specific berry effect on the adult return rate in the great tit:
female return rate was associated less strongly to berry abundance than
male return rate. This skewed the sex ratio of the local breeders in the
following breeding season. Intriguingly, annual berry consumption in
December was not related to berry abundance, and individuals consuming
more berries tended to have slightly lower return rates. Reproductive rate
was not related to berry abundance. There was hardly support for a
relation between immigration rates of first year breeders and berry
abundance. Taken together these results imply that berry stock not only
affected population size but also the population composition through sex
specific exchange with the surroundings. Since population density covaried
with berry abundance, density dependent effects provide an alternative
explanation for the patterns observed.