To determine the major factor of change in provisioning rates to Varied Tit Parus varius nestlings in relation to increasing elevation, we used video recordings to investigate the number of parent visits and the size of prey carried by parents to their nestlings during the breeding process. Video cameras were installed at the entrances of 35 nests (17 at 300 m a.s.l., 7 at 900 m a.s.l., and 11 at 1,400 m a.s.l.). We also measured air temperature during the breeding season at three study sites. The provisioning rates per nestling showed a significantly negative relationship with prey size provisioned, except during the early nestling stage (days 1 to 4), when small prey were selectively provisioned by the parents. Prey at the higher elevation during days 5 to 15, also, were significantly smaller than at the lower elevation. The mean temperature at each breeding site during the breeding period did not differ significantly due to the delayed onset of breeding at higher elevation. Therefore, we conclude that the increase in the provisioning rate per Varied Tit nestling at higher elevation is associated with a decrease in prey sizes at that elevation, rather than with a difference in the temperature.
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Vol. 15 • No. 1