A model incorporating a reduction in carrying capacity translates brood size trends into a population decline: the case of Finnish starlings, 1951–2005

Abstract

Finnish starlings Sturnus vulgaris declined by 80% during the 1970s and 1980s, but during this time mean brood size increased. On the basis of previous results, we assumed that the carrying capacity K of breeding habitats declined, especially in southern Finland, the phenomenon being due to regional diversification in agriculture and a corresponding decline in dairy farming. We further assumed that following the changes in agriculture, poorly productive starling habitats (sinks) increased at the expense of highly productive ones (sources). We used nestling ringing data from 1951 to 2005 to estimate trends in brood size and population change. The results showed that mean brood size was greater during stable population stages (at the beginning and end of the study period) than during the years of collapse. The coefficient of variation displayed a contrary pattern to the mean brood size, being at its highest during the decline. We modelled the relation of population dynamics and environmental change with a density‐dependent matrix model that assumed a 80% decline in K. The modelling results obtained were similar to the population dynamics actually observed. The model showed a decrease in per‐capita reproduction rate as a response to habitat deterioration. This was due to a time lag in response to the declining K. On the basis of the dynamics observed and the population model, we concluded that during the decline, a larger proportion of starlings bred in sub‐optimal sink habitats than during the stable population phases, and that this caused the low in the mean and the high in the coefficient of variation of brood size. Survival analyses on nest‐card data (using the Mayfield method) supported our conclusion that the population decline was due to a decreased brood size caused by decreased K.