Annual Variation in the Cost of Polygyny: a ten year study of Great Reed Warblers Acrocephalus arundinaceus

Abstract

An isolated population of Swedish Great Reed Warblers was studied between 1984 and 1993 when almost every breeding adult and nestling were ringed. I examined between year variation in several variables that may affect fintness. In particular I looked at the reproductive success of females that can expect no or little male assistance (secondary females) in relation to females that are assisted by their males (monogamous and primary females). Return rates of females, the frequency of total losses and number of fledlings per female showed significant differences between years. The yearly mean number of fledglings per female was associated with the mean annual temperature during breeding. When an exceptionally warm year was excluded this was significant for both primary and secondary females. The success of secondary females relative to the success of primary females increased with increasing temperature implying that the cost of polygyny is higher in cold than in warm summers. The variation in clutch size and relative reproductive success of secondary and primary females increased over the ten year study period suggesting either that the study period was too short for capturing the variation of the variables or that these were subject to a systematic change over the study period. There were indications that the condition for breeding has deteriorated in the last years; clutch size decreased significantly over the study period and particularly low values of fledging mass and relative reproductive success were found in some of the most recent years. This might reflect that population density had reached a level when effects of intraspecific competition influences breeding results. Alternatively, five extremely mild winters may be responsible for a drastic increase of the lake's population of Crucian carps. Whether a large fish population could have caused a decline in invertebrate food for Great Reed Warblers in recent years is discussed.