Effects of density beech crop and winter feeding on survival of juvenile Great Tits an analysis of Kluyver’s removal experiment.

Density dependent processes and the availability of food may play an important role in the determination of population size. To investigate the relationship between density and local survival of juvenile Great Tits Parus m. major Kluyer removed first brood fledglings in two years. An analysis is presented of the effects of this removal experiment, beech crop and additional feeding in winter on local survival. Beach-crop index was positively correlated with local survival of both first- and second-brood fledglings. Additional feeding in winter affected local survival of first, but not of second-brood fledglings. The effect of the removal experiment differed between birds fledging early or late in the season. Removal of first-brood fledglings had a negative effect on local survival of the remaining first-brood fledglings. This effect was in accordance with the (quadratic) relationship between density and survival found in control years, indicating that the relationship is causal. This is a destabilizing density dependent process. Possible underlying mechanisms are discussed. Removal of first-brood fledglings enhanced local survival of second-brood fledglings. The enhancement was in accordance with the relationship found in control years, indicating that this relationship too is causal. Density of second-brood fledglings did not affect local survival of first or second-brood fledglings. This study confirms that food and density-dependent processes play an important role in the determination of population size but the effect of these factors differs between birds fledging early or late in the season.

Verhulst S., ARDEA 80 (2): 285-292.