Pied-wagtails in England

Exploit renewable food source on riverbank (tides bring food). Maintain territory size all winter, don’t modify territory size in response to abundance of resources, instead they modify their behavior:

• If food is low, they get it from other place but return to defend territory.
• If food is abundant, they allow one subordinate bird (female or juvenile) to stay to help defend the territory because they can be kicked out later.
• If food is very abundant, defense is abandoned.