Abstract
Breeding success of four titmice breeding in forests was investigated during 1962–2001. Three of them, Coal Tit Parus ater, Willow Tit Parus montanus and Marsh Tit Parus palustris have shown declining population trends in the annual monitoring of bird species in Sweden. The fourth species, the Crested Tit Parus cristatus, has shown a stable population size. Two of the declining species, Marsh Tit and Coal Tit, had the largest broods during the most recent decade and significantly positive trends in brood sizes during the investigated period, whereas the Willow Tit and Crested Tit had stable brood sizes. Our study indicates that the declines of the Coal Tit, Willow Tit and Marsh Tit populations in Sweden cannot be explained by changes in brood size, number of dead nestlings or number of un-hatched eggs. The stable population size of Crested Tit might be explained by its habit of breeding early in the season, which has become more pronounced during the most recent decades, probably in order to match breeding to the earlier spring arrival nowadays.
Highlights
The bird populations of Sweden have been monitored annually since 1975 (Lindström & Svensson 2002)
In the annual report for 2006, Lindström & Svensson (2007) stated that the Willow Tit Parus montanus and the Marsh Tit Parus palustris had declined for the last 25 years, whereas the Coal Tit Parus ater started to decrease in the 1990s and the Crested Tit Parus cristatus showed a stable population
The aim was to investigate if changes in brood size and the proportion of unhatched eggs and dead nestlings per brood could play a role in the declining populations of the titmice
Summary
The bird populations of Sweden have been monitored annually since 1975 (Lindström & Svensson 2002). In the annual report for 2006, Lindström & Svensson (2007) stated that the Willow Tit Parus montanus and the Marsh Tit Parus palustris had declined for the last 25 years, whereas the Coal Tit Parus ater started to decrease in the 1990s and the Crested Tit Parus cristatus showed a stable population. There are two mutually non-exclusive causes for a decline of a population – lower production of offspring or decreased survival of nestlings or older birds. The aim was to investigate if changes in brood size and the proportion of unhatched eggs and dead nestlings per brood could play a role in the declining populations of the titmice
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