Occupancy as a measure of territory quality

Abstract

SummaryTerritory quality may affect individual fitness and contribute to density‐dependent reproduction, with repercussions on population regulation. We investigated the probable causes and population consequences of spatio‐temporal variations in territory quality, measured by occupancy, in eight black kiteMilvus migransBoddaert populations, one of them studied for 10 years (Lake Lugano) and the rest for 4–5 years.Over a period of years, the occupation rate of territories varied from a random pattern. Some territories were preferred while others were avoided. On return from migration, males and females settled earlier on high‐occupancy territories.The positive association between territory occupancy and breeding performance held in all years of study at Lake Lugano, and in six of seven tested populations. As a result, high‐occupancy territories contributed most of the young produced by each population.The occupation rate of the overall 225 territories was related positively to food availability and negatively to mortality risk, measured as proximity to the nearest eagle owlBubo buboLinnaeus nest.At the population level, spatial variation in mean occupancy was positively correlated with spatial variation in mean productivity, suggesting that mean occupancy could be used as a measure of overall habitat quality and population performance.In the Lake Lugano area, a higher proportion of low quality territories was occupied in years of higher density and annual productivity was related negatively to its coefficient of variation. However, annual productivity was not related significantly to the proportion of low quality territories occupied, so support for the theory of site‐dependent population regulation was only partial.In a review of 22 studies of territory occupancy in 17 species, occupancy always deviated from a random pattern in species in which it was tested and was always correlated with productivity and/or with some other measure of territory quality. Our results confirm the importance of prioritizing conservation of high quality territories. Occupancy may be a reliable method of quality assessment, especially for populations in which not all territories are always occupied, or for species in which checking occupancy is easier than finding nests.