© Marcel Burkhardt
Behavioural ecology of the great tit
The great tit is one of the best-known bird species. Studies integrating the complete food chain, from the prey of a study organism to its predators, and combining various methodological approaches lead to a broad understanding of the mechanisms underlying the life-history of birds. Here we investigate the food chain in forest habitats. Long-term data covering most of the 20th century show the effects of climate on the reproductive output of the great tit.
Aims
Deciduous trees and their leaves provide the basic food resources for insect larvae such as caterpillars. These, in turn, are the most important prey fed to great tit nestlings. However, caterpillars are available only for a short time of the year. Thus, great tits can profit most of an abundant food supply if they time their broods to the peak mass of caterpillars. Within this framework, we aimed to answer two questions. First, we investigated the factors affecting post-fledging survival of juvenile great tits, including food availability and predation. Second, we studied whether a delayed timing of breeding not only reduced the reproductive output at fledging, but also the survival of juvenile tits after having left the nest.
Approach
To investigate the influence of feeding conditions on reproduction and survival, we needed reliable information on the caterpillar mass in the tree canopy. To record the supply in the top of tall trees, we used a lifting platform to collect samples of branches, to count and weigh the caterpillars and arthropods living on them.
In recent years, the Swiss Ornithological Institute developed novel miniaturized radio-transmitters. This sophisticated technique allows tracking even small birds over large distances. Thanks to the tiny radio-transmitters we were able to estimate range use and post-fledging survival of juvenile great tits.
Results
Great tit parents cared for their chicks according to optimal foraging theory. Early in spring, they fed mainly spiders to their young. As soon as the available caterpillars exceeded the mass of the spiders, parents switched to feed exclusively on the more profitable caterpillars. Foraging parents adjusted their feeding efforts on forest trees according to their food supply as well as to the distance of a food patch from the nest site. This optimal foraging strategy reduced foraging efforts by some 30 % compared to uniform foraging by visiting all trees in the home range equally.
Furthermore, territory size and hence breeding density of great tits depended on the local food supply, i.e. on caterpillar density. In habitats with low prey densities, breeding territories were larger than in habitats with rich resources. Our results demonstrate that investigations of the food chains and the underlying mechanisms provide insights into small-scale variation in population densities and into large-scale distribution patterns of birds.
The post-fledging survival rate of juvenile great tits depended on two main factors. First, fledglings in good body condition had an increased survival probability, most likely because they were in a better position to avoid and escape predators. Post-fledging home ranges of families with fledglings in good condition were about three times larger than those with poorly nourished young. The fledglings' body condition probably resulted in large differences in flight performance and spatial behaviour of family groups. Second, the timing of breeding turned out to be a crucial factor affecting not only growth and survival of nestlings, but also survival of juveniles after fledging. In the course of the breeding season, avian predators increasingly preyed on juvenile titmice. Consequently, fledglings of early broods showed a significantly higher survival probability than fledglings of late broods.
Project management
Partners
Prof. Dr. Lukas F. Keller, Zoologisches Museum der Universität Zürich
Dr. Ruedi G. Nager, University of Glasgow