Extra Food Boosts Jackdaw Colonies
Abstract from Ibis
7th September 1996
Manuel and Juan Jose Soler, writing in the most recent issue of Ibis (138, 377-383), were able to increase the numbers of breeding Jackdaws and their breeding success by providing them with additional food. This may not seem surprising but the mechanisms involved are quite subtle.
For instance normally the Jackdaws select the smallest possible entrance to their preferred nest sites. This reduces the risk of predation. However in the two colonies where they provided extra food many sub-optimal cavities were used. However the predation rates at these were nowhere near as bad as expected because the attendance at the colony by the adult Jackdaws was so mush greater than in the control colonies.
The extra food meant that he adults did not have to spend so much time foraging and so they were on hand to defend the colony against predators like Ravens. The extra food had the adults laying earlier and bigger clutches and increased the fledging success. In other studies this has not always been the case. They suggest this is determined by the nesting strategy of the species concerned, If, like the Jackdaw, the bird's breeding strategy is to reduce the number of nestlings if the food supply will not support the feeding of all the chicks hatched. So when the colony had food provided chicks that would have died were reared successfully - and their fledging periods were longer than in the control sites. This was because the birds had easier food finding but brought in much more to feed their bigger broods.