Chlorinated Hydrocarbons and Reproduction of a South Swedish Population of Whitethroat Sylvia communis

Breeding Biology of the Sylvia Warblers

“…The greatest percentage decrease occurred in June. The decline in clutch size in the Whitethroat is contrary to the observations of Persson (1971b) who found no seasonal decrease.…”

“…Comparative figures of mean clutch size from more southerly populations would be interesting. The mean clutch size of Whitethroats was similar to that of a population in Skane, southern Sweden, studied by Persson (1971b), whereas populations near Stockholm and in Finland had higher mean clutch sizes (Persson 1971b, Haartman 1969 The seasonal change in the clutch size of Sylvia warblers is illustrated in Figure 5. May and June were each divided into two equal halves and the mean was calculated from clutches started in each period.…”

“…The hatching success of Swedish Whitethroats was 81% (Persson 1971b) compared with 69% from the nest record cards. Those species which tended to nest close to the ground had a lower hatching success, suggesting that ground predators may be the most important cause of egg loss in warblers.…”

“…The nestlings have an improved survival. The differences in success rate are significant (P<0.01 using a chi-squared test) and the Whitethroat had the highest rate, 85%, which can be compared with 90% for Swedish Whitethroats (Persson 1971b). Ground predators may become less important as the spring advances and alternative food sources, such as young rodents, become available.…”

Whitethroats, Organochlorines and Arbo Viruses

“…The levels found are roughly comparable with those reported in Sedge Warblers Acrocephalus schoenobaenus, another declining species, arriving in North Wales in the spring of 1963, well before the decline started (Prestt and Ratcliffe 1972), and Whitethroats arriving in Sweden in the spring of 1971, where these levels appeared insufficient to have a detectable effect on breeding success (Persson 1971(Persson , 1972, who expresses her analytical results in terms of lipid levels, which tend to be much higher). It seems likely that the decline in the proportion of pulli ringed in Britain from 9%, 8% and 6% in the years 1967, 1968 and 1969 to 3% in 1970 and 1971, evident in (Ernek et al 1973(Ernek et al , 1974; and West Nile virus, whose antibodies are widely distributed in the local human population and domestic animals, was also isolated from a mosquito there the following summer .…”