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.
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