Black-headed gull (Chroicocephalus ridibundus) colonies are loud and observant. Numerous birds are observing the landscape, making it difficult for predators to creep up unnoticed. Gulls will make alarm calls immediately when they observe something suspicious, and attack the invader – regardless of whether it is a crow or a human. The colony offers gull chicks a safe place to grow.
Black-headed gulls breed in colonies. Gulls will make alarm calls immediately when they observe predators close by.
Certain animal species may depend on each other in nature. For example, certain bird species set up their nesting places near the safety of aggressive gull colonies. Dependence can lead to trouble, if the protector species disappears for some reason.
Many duck species utilise the protection of gull colonies. Nests within colonies survive better than nests outside. For example, the pochard (Aythya ferina) and tufted duck (A. fuligula) suffer from higher nest predation rates outside gull colonies. The tufted duck in particular prefers nesting within gull colonies. On the other hand, dabbling ducks do not usually nest near the colonies, but later during the breeding season they will take their broods close to the colonies.
The pochard has declined dramatically in Finland. Nest predation is one probable reason behind this trend.
A recent study measured the connections between different waterfowl species and gull colonies. The researchers evaluated the trends of several waterfowl species at 15 Finnish lakes and observed that the population patterns of the other species tend to follow the trends of black-headed gulls. Lakes that lost their black-headed gulls also began losing other species. The researchers showed that the species declining the most were nesting in the same environments as the gulls. This was reflected in their conservation status: species connected to gull colonies were in a worse conservational status than generalist species or species nesting somewhere else.
Aythya species benefit from nesting near gull colonies.
Thus, the loss of gull colonies appears to lead to declines in other species. Colonies have traditionally protected nests and broods. Breeding season is a sensitive time for ducks, which may suffer from high predation rates during this time. The loss of gull colonies leads to duck species being exposed to increased predation.
Why then are the gulls disappearing? The exact reasons are unknown, but the species is suspected to also suffer from elevated predation pressure. Even black-headed gulls cannot protect their nests against every predator species. For example, the alien species raccoon dog (Nyctereutes procyonoides) and American mink (Neovison vison) have invaded Finland, and both are suspected to destroy gull colonies. Black-headed gull colonies have disappeared from many traditional nesting places, affecting the breeding success of other waterfowl species. Gull colonies are therefore far more important for breeding ducks than earlier believed.
Read more:
Close Encounters of the Natural Kind 