Manipulation of intruder pressure in red squirrels (Tamiasciurus hudsonicus): effects on territory size and acquisition

Abstract

The effect of intruder pressure on territory size has been investigated in many species of birds but in few species of mammals. We manipulated intruder pressure in an island population (21 adults) of territorial red squirrels (Tamiasciurus hudsonicus) in summer by removing (5) and adding (22) squirrels. When territorial squirrels were removed, neighbouring individuals expanded their territories. Breeding females that did so allowed their young to remain on part of the expanded territory while young from other females settled off the mother's territory. Adding squirrels did not result in a decrease in territory size. Although the introduced squirrels had previously held territories, they did not challenge residents. Instead they behaved as floaters (squirrels without territories); they never called and they occupied areas devoid of territorial squirrels. We conclude that intruder pressure prevents red squirrels from increasing their territory size but increased intruder pressure in the form of floaters or juvenile recruits does not lead to a decrease in territory size.