Christine Scheckenhofer


Habitat preferences of Little Bitterns Ixobrychus minutus breeding in wetlands embedded in an urban habitat matrix: a case study from Vienna, Austria

The Little Bittern Ixobrychus minutus is an endangered species with priority conservation status at a European level. The causative factors threatening the species are predominantly of anthropogenic nature, including habitat fragmentation and destruction, as well as disturbance through human recreational activities. Therefore, the Little Bittern population in and around the city of Vienna is remarkable in two respects: (1) many breeding pairs occupy territories located in areas of relatively high anthropogenic disturbance; (2) many breeding sites are situated at isolated and very small water bodies. To shed light on this apparent contradiction, the current study investigated the habitat requirements of Little Bitterns in the state of Vienna. To measure habitat suitability, the study first compares the characteristics of occupied vs. unoccupied water bodies in 2006. Secondly, the study compares 2006 data with previously published records in order to analyse continuity of occupancy as a measure of habitat quality. Results show that even water bodies with small reed bed areas (0.07 ha) are occasionally used by the Little Bittern as a breeding habitat in Vienna. However, the probability of colonisation and the number of territories rise with increasing reed bed area: For a reed bed area in Vienna to have a 50% chance of attracting Little Bitterns to breed, it must be no smaller than 0.65 ha. The degree of isolation of water bodies from water bodies with Little Bittern occurrence did not significantly influence their likelihood of colonization. Similarly, human disturbance was not found to have any detectable effect on the occurrence of Little Bitterns. The city of Vienna shows the largest known breeding population of Little Bittern in Austria. Therefore it is still essential to protect even small existing reed beds.