Food availability limits avian reproduction in the city: an experimental study on great tits (Parus major)

1. The altered ecological and environmental conditions in towns and cities
strongly affect demographic traits of urban animal populations, for
example avian reproductive success is often reduced. Previous work
suggests that this is partly driven by low insect availability during the
breeding season, but robust experimental evidence that supports this food
limitation hypothesis is not yet available. 2. We tested core predictions
of the food limitation hypothesis using a controlled experiment that
provided supplementary insect food (nutritionally enhanced mealworms
supplied daily to meet 40-50% of each supplemented brood’s food
requirements) to great tit nestlings in urban and forest habitats. 3. We
measured parental provisioning rates and estimated the amount of
supplementary food consumed by control and experimental nestlings, and
assessed their body size and survival rates. 4. Provisioning rates were
similar across habitats and control and supplemented broods, but
supplemented (and not control) broods consumed large quantities of
supplementary food. As predicted by the food limitation hypothesis we
found that nestlings in (1) urban control broods had smaller body size and
nestling survival rates than those in forest control broods, (2) forest
supplemented and control broods had similar body size and survival rates,
(3) urban supplemented nestlings had larger body size and survival rates
than those in urban control broods, and crucially (4) urban supplemented
broods had similar body size and survival rates to nestlings in forest
control broods. 5. Our results provide rare experimental support for the
strong negative effects of food limitation during the nestling rearing
period on urban birds’ breeding success. Furthermore, the fact that
supplementary food almost completely eliminated habitat differences in
survival rate and nestling body size suggest that urban stressors other
than food shortage contributed relatively little to the reduced avian
breeding success. Finally, given the impacts of the amount of
supplementary food that we provided and taking clutch size differences
into account, our results suggest that urban insect populations in our
study system would need to be increased by a factor of at least 2.5 for
urban and forest great tits to have similar reproductive success.