Abstract
The breeding populations of Horned Grebe Podiceps auritus and Red-necked Grebe Podiceps grisegena were surveyed in Sweden 2011. The estimate for Horned Grebe was 2,000 pairs, an increase from about 1,200 pairs in 1996 towards the estimate of 2,200 pairs in 1972. The estimate for Red-necked Grebe was 1,300 pairs and the population size has most certainly increased in the last decades. Habitat use among the breeding birds differed between species and region. In south Sweden, both species were most commonly found in artificial water bodies, and a recent increase in the numbers of such waters may have contributed to the population increases. In north Sweden the species were mainly found in natural inland water bodies or along the Baltic coast. Similar long-term population trends were also seen in a long-term observational dataset of passage birds from Ottenby Bird Observatory in southeast Sweden. There was no obvious effect of the coldness at their wintering grounds on the amount of birds seen at Ottenby the following year, but indirect effects of winter harshness cannot be ruled out.
Highlights
For conservation purposes it is important to track the status and trend of a given population
The first time was in 1972, when the population was estimated at 2200 pairs and again in 1996, when the results suggested a drop in the Swedish population to 1200 pairs (Regnell 1981b, Douhan 1998)
The numbers on Öland had on the other hand decreased over the study period and has probably not been involved in the late positive trend seen at Ottenby
Summary
For conservation purposes it is important to track the status and trend of a given population. Trends can be estimated when the size of the population is regularly estimated (Sutherland 2006). A common approach for bird population monitoring is to take repeatable sample measurements, either in the breeding areas (e.g. line transects or point counts), along the migratory pathway (e.g. standardized ringing or observation schemes), or during winter for birds forming dense flocks (e.g. counting numbers: Bibby et al 2000). Breeding bird censuses are per definition spatially linked to a certain region, which is a great advantage when monitoring a local population. Observation schemes along the migratory pathway are relatively cheap and can be carried out regularly, and a relative measure of the amount of birds passing can be summarized on a yearly basis. The apparent weakness with this method is that the population size and the true distribution of the birds counted rarely are known
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