Population development and breeding success of Grey Heron Ardea cinerea in South-eastern Norway

Abstract

The Grey Heron Ardea cinerea was first found breeding in Østfold county, SE Norway, in the early 1970s, most probably as a result of population growth in Sweden. The population of the county increased until the late 1980s, when approx. 40 pairs bred. A count in 2005 showed that at least 166 pairs were breeding in six colonies. In Follo, Akershus there were found two colonies with 13 pairs. Three more colonies were found in Østfold in 2006. The number breeding in the colonies was lower in 2006 than in the previous year (a 38 % reduction), probably due to the cold winter of 2005/2006. The Grey Heron still breeds in several colonies established in the 1980s. This study of nesting sites in Østfold shows that colonies are mostly placed on south-facing slopes, and the colonies have a buffer of protective trees. They are often placed close to roads and habitation. Human disturbance does not seem to be a problem, with one main exception: logging. Breeding success was studied in four colonies, two of them newly established and two with a known history back to the 1980s. The numbers of chicks were counted at a specific period in the breeding season. The Grey Heron begins incubating after the first egg is laid, and among young chicks there are weight differences. Siblicide is widespread, and is reckoned as an indicator of low nutritional conditions. A large weight difference among the chicks in the brood could indicate a high probability for siblicide. Sixty-four chicks were weighed in 19 nests, and a comparison was made between pairs in colonies of different age and size. The results show that the number of chicks, or breeding success, measured at a specific period in the season, was higher in small and new colonies compared to the larger and well-established ones. But the average weight of chicks was greater in the oldest and largest of the two colonies. This could indicate a lower tendency to siblicide, but is more probably a result of an earlier breeding start in the oldest colony. The chicks were on average fewer and larger, and the pairs were probably in a later stage in the breeding process. The results based upon counts in the nesting period do not support the hypothesis that the Grey Heron gains a higher breeding success in larger and well-established colonies than in smaller and newer ones. But an early start and fledging could increase juvenile survival to the next breeding season. The importance of an early breeding start could be a major explanation for colony fidelity.