Study area
The island of Aasla is situated on the fringe of the inner archipelago in Rymättylä in the
Archipelago Sea, SW Finland (60◦18’N, 21◦57’E). The study area is divided into 20 different
sites, which includes bays, sounds and sea areas along the shore of the island, as well as lakes
(oligotrophic to eutrophic lakes of the glo-type). Some of these sites are Natura 2000 sites.
The sea habitats range from exposed barren coast to sheltered bays with a lot of reeds. The
characteristics of Aasla and its surroundings offer suitable staging and breeding sites for a
wide range of species of birds. The censused sea area totals 24.4 km2, and the land area of
Aasla is 16 km2 (Rönkä 2008, Rönkä et al. 2011)
Waterbird censuses
The waterbird censuses in Aasla have been conducted by L. Saari from 1975–onwards.
The census method is a combination of point and round counts (Koskimies and Väisänen
1991). The sea areas are censused by walking standard routes along the shore around the
island and stopping at standard sites. The census route was chosen so that all breeding birds
in the area could be counted.
Waterfowl counts are performed nine times per year.
- The census of the breeding populations is repeated three times during the spring:
first at the turn of April–May, second in the middle of May, and third at the turn
of May–June. Each species is counted during the census that best suited its
breeding phenology, as recommended by Linkola (1959) for inland waters at the
same latitude. The annual phenology is taken into account in the precise timing of
the censuses. The pair numbers are based on counted pairs (or equivalents, see
Koskimies & Väisänen 1991).
- The July, August, and September censuses provide breeding success data.
- The winter population are assessed through the censuses carried out in November,
December and January.
The size of the breeding population (pair numbers) are based on the number of adults (or
equivalents, see Koskimies and Väisänen 1991), because searching for nests is impractical for
most of the species due to the habitat structure. The numbers of individuals are converted into
pair numbers for each species according to Linkola (1959), either by dividing the individual
number by two or by using the number of males or females as the pair number.
The breeding success (chick numbers) and the age class of chicks are assessed by brood
counts in July except for the mute swan, for which yearly fledgling counts from September
are used because in Aasla the mute swan chicks mostly fledge in September. The age class of
the chicks of the other species are estimated according to the classification given by Pirkola
and Högmander (1974) for anatids.
References:
Koskimies J. &Väisänen R.A. 1991. Monitoring bird populations. Zoological Museum,
Finnish Museum of Natural History, Helsinki.
Linkola P. 1959. Zur methodik der quantitativen Vogelforschung in den Binnengewässern.
Ornis Fennica. 36 (3-4): 66-78.
Pirkola M.K. & Högmander J. 1974. Sorsanpoikueiden iänmääritys. Suomen Riista 29:
50–55.
Rönkä M. 2008. Assessment of coastal bird populations and habitatson the Finnish coast
of the Baltic Sea: Implications for monitoring and management. Annales Universitatis
Turkuensis, Sarja AII Osa 229. Biologica – Geographica – Geologica. Turun yliopisto,
Turku. 158 s.
Rönkä M., Saari L., Hario M., Hänninen J. & Lehikoinen E. 2011. Breeding success and
population trends of waterfowl – implications for monitoring. Wildlife Biology 17(3): 225–
239.