Black-throated Diver (Gavia arctica)

Always breeding at lower density than the Red-throated Diver, the Black-throated is drawn to large, deep lakes with clear water, preferably in undisturbed places. Here a single pair usually holds sway, keeping all others at bay with loud calling. A Black-throated Diver won’t tolerate a stranger within its line of sight, so if one appears it will approach with its neck held in a menacing snake-like curve, exposing the black throat – its version of the Alert Posture. The rivals, including both members of the territorial pair will then swim around in a “Circle Dance”, and after a while will begin dipping their bills into the water. This normally causes the stranger to flee, although fights, with much bill-stabbing and splashing, do sometimes occur.

Although the territorial displays are conspicuous and exciting to watch, the actual courtship between members of a pair is much less theatrical, involving the partners mutually bill-dipping, diving with a splash, and swimming very fast underwater. But a feature of this part of the season is repeated copulation. This is performed on land at several favoured sites, and may cause the vegetation to become worn down with use; such sites are known as “false nests”.

Despite its territorial nature, the Black-throated Diver is more sociable than other members of its family outside the breeding season, with small groups often spending the winter offshore.