Breeding population increase and range expansion of the Whooper Swan <em>Cygnus cygnus</em> in Oslo and Akershus

Abstract

The Whooper Swan Cygnus cygnus has shown a remarkable breeding population increase and range expansion in Northern Europe during recent decades. Here, I summarize the temporal and spatial pattern of the expansion in Oslo and Akershus counties in southeastern Norway, and assess current and potential future competition with Mute Swans Cygnus olor. The first breeding was reported in 1999, and in 2003 breeding was reported at a second site. From 2006 the number of breeding sites increased rapidly, and the species has now been reported breeding from a total of 20 sites. However, at least seven sites have been abandoned after one or a few years of breeding, leading to temporary decreases in population size in some years. Abandoned sites had lower breeding success than sites that are still occupied. Current population size is 11–14 pairs. TRIM-analyses indicated a rate of increase of 7% per year. In about two thirds of the occupied sites, breeding was preceded by one or more years with presence of pairs that did not breed. Non-breeding Whooper Swans have been observed during summer (16 May–July) at a further 24 sites, suggesting that population size is likely to continue to increase. Oslo and Akershus also has an increasing population of Mute Swans, currently estimated at ca. 50 pairs, but Mute Swans have been recorded breeding at only three of the sites (15%) where Whooper Swans have bred, and at one of these sites there has been no temporal overlap. Thus, the two swan species have had limited interaction at breeding sites so far. However, among the 24 sites that have had non-breeding Whooper Swans during summer, Mute Swans have bred at 10 sites (42%, still present at most sites). This suggests that further expansion of Whooper Swans may soon lead to increased competition with Mute Swans, but there are also numerous other potential breeding sites without Mute Swans present.