Breeding persistence of Slavonian Grebe (Podiceps auritus) at long-term monitoring sites: Predictors of a steep decline at the northern European range limit

The Slavonian Grebe (Podiceps auritus) has its European northern range limit in northern Norway, and is a species of national conservation concern due to its small population size and unknown population trend. Long-term monitoring at the range limit suggests breeding site use is in decline. We used annual occupancy data from 104 breeding lakes monitored since 1991 in northern Norway to investigate correlates of change in occupancy. Persistence was 100 % until 1999, but thereafter decreased to 25 % (26 lakes with breeding pairs). A particularly steep decrease occurred between 2010 and 2012. Persistence increased with the number of pairs present in each lake in the initial monitoring year of 1991. The number of grebe pairs also decreased in the lakes that had continuous breeding persistence over the entire 22-year monitoring period, suggesting that a large-scale factor caused the population decline. Over the last year of the monitoring series, lake altitude was negatively related to the probability of persistence, indicative that locally harsh climate played some role in breeding distribution. The temporal pattern of persistence was not related to mean winter temperature at the breeding sites; however, the decrease between 2010 and 2011 coincided with a late ice melt in 2010. Monitoring that includes a larger area of the species’ range is required to conclude whether the observed decline can indicate an overall decline in population size, or range fluctuations at the edge of the species’ range. However, investigating the processes that determine population range borders can give insights into important limiting factors pertinent to the conservation of species in the long term.