Naurulokit taantuvat – kuinka käy vesilintujen? Black-headed gulls are declining – how does this affect other waterbirds?

Hannu Pöysä, Esa Lammi, Silvo Pöysä, Veli-Matti Väänänen

Research output: Contribution to journalArticleScientificpeer-review


Summary: Black-headed gulls are declining
– how does this affect other waterbirds?
Interactions and dependence between species can transmit the effects of species declines and extinctions within and between trophic levels, resulting in secondary endangerments and extinctions. Many mixed-species avian breeding aggregations often include an aggressive protector species whose nest defense is used by other species to defend their nests.Collapse of the protector species may have population demographic consequences to the dependent species.

Here, we summarise results of a study (Pöysä 2019a) in which we investigated long-term changes and dynamics of lake-specific breeding numbers of waterbirds as a response to long-term changes and dynamics of black-headed gull Chroicocephalus ridibundus colonies (see Fig. 1 for examples of variation in black-headed gull colony size). Aggressive nest defense behaviour of small colonial gulls, such as the black-headed gull, is used by many waterbird species
to gain protection against predators. Our data for 15 local waterbird communities are from five regions in Central and southern Finland from the period of 1977–2017; the length of the time series of breeding numbers varied from 22 to 41 years. Our study lakes represent typical lakes in Finland, ranging from oligotrophic to eutrophic. As the black-headed gull prefers eutrophic lakes, most of the study lakes are eutrophic. The hooded crow Corvus corone cornix,
marsh harrier Circus aeruginosus, raccoon dog Nyctereutes procyonoides and American mink Neovison vison are common nest and chick predators in Finnish eutrophic wetlands.

We addressed three study questions. First, do other waterbird species show contemporary changes in breeding numbers along with the black-headed gull, after controlling for common long-term trends between them and the
black-headed gull? Our second question focused on the role of nesting ecology in the black-headed gull–other species
interaction. Specifically, we hypothesised that the trend-controlled temporal association of the other species with the black-headed gull is stronger in species that nest in similar habitats as the black-headed gull than in species that nest far away from gull colonies (see Table 1 for nest sites). Finally, we studied whether the temporal association of the other waterbird species with the black-headed gull is correlated with the recent conservation status of the species (see Table 1 for conservation status). In addition, we discuss the role of predation in explaining common population trends of waterbird and black-headed gull.

We found that breeding numbers of many waterbird species tracked long-term changes in the size of black-headed gull colonies (Table 2). This was true even after controlling for a common trend in the size of the black-headed gull
colony and the breeding numbers of the other species. Furthermore, the trend-controlled positive temporal association with black-headed gull was relatively stronger in species that nest in similar habitats within a lake as the black-headed gull, and in species that have a more critical conservation status due to drastic population decline (Fig. 2). Our results suggest that the overall decline of black-headed gull colonies has resulted in secondary endangerment of many species in waterbird communities. Evidence has accumulated in recent years that the raccoon dog in particular has a more important role as a nest predator than previously thought. Therefore, the removal of alien predators in bird-rich habitats, such as eutrophic wetlands, is an important measure in management programs aiming to halt waterbird declines.
Original languageFinnish
JournalSuomen Riista
Pages (from-to)49–60
Number of pages12
Publication statusPublished - Jan 2021
MoE publication typeA1 Journal article-refereed

Fields of Science

  • 1181 Ecology, evolutionary biology

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