Wesołowski T. 1991. Bedetung des Bruterfolgs für die Abname des Feldsperlings Passer montanus in der Schweiz. (Importance of nesting success for the decrease of Tree Sparrow Passer montanus in Switzerland). Der Onithologische Beobachter 88: 253-263.
Using data from 1017nest record cards collected by the Swiss Ornithological Institute in 1913-89, the temporal variation in reproductive output of Tree Sparrows in Switzerland is described. For analysis the data were divided into four periods. Egglaying started one pentade earlier in 1940-61 than in other periods, whereas the number of broods/year was lowest in 1962-79. Clutch size did not change , but the numbers of eggs hatching and of young leaving successful nests were lower, of unhatched eggs higher in 1962-79 than in other periods. Overall, in 1962-79 the reproductive rate was 21% lower than in any other period. Such curtailment in production of young could be responsible for the decrease in numbers occuring in recent decades. It is suggested that the fall in reproductive rates in 1962-79 was due to a widespread use of chlororganic pesticides during this period.