Solitude at periphery: Lack of partners limits reproduction of the black stork (ciconia nigra) at themargin of the distribution range

Abstract

Understanding the mechanisms forming species ranges is a central ecological question, which could be answered by analysing factors limiting peripheral populations. In threatened species, such studies are essential for establishing effective conservation measures across the range.We analysed factors potentially influencing breeding in a declining peripheral population of a long-lived bird, the Black Stork (Ciconia nigra).We assessed reproductive success and the effects of intra- and interspecific competition, as well as predation by recording events at nests by remote cameras (camera traps and a webcam). Productivity of storks was low (1.1 fledglings per occupied nest) compared to the other parts of the range and resultedmainly from the lower proportion of successful nests (37%of occupied nests).Themain reason for lowbreeding successwas the occupancy ofmany nests (35%) by single non-reproductive birds. Breeders were often visited by non-local conspecifics, which harassed local birds but only seldomly caused direct damage. Impact of predators and interspecific nest-competitors on reproductive success was low. We suggest thatmany individuals have disappeared from the breeding population and shortage of mates is currently the most important factor lowering reproductive success of the Black Stork at its northern range margin. This mechanism could also limit the peripheral abundance and distribution in other long-lived birds.