New Findings: Eviction Breeds Eviction!

As most of you know, most loons acquire their territories by force. That is, a non-breeding adult intrudes into a territory defended by a breeding pair, challenges the pair member of its sex, battles with that pair member and, if lucky, evicts that owner. (An evicted owner is sometimes killed in battle, especially if it a male, but more often simply leaves its former territory after a defeat and looks elsewhere to settle.) By the way, evictions (also called takeovers or usurpations) are about equally frequent in males and females.

We have now observed or inferred 226 territorial takeovers, and we are at a point where we can begin to look for other patterns in these data. I have been analyzing the data to learn whether one eviction leads to another. For example, if the territorial male on a lake gets evicted, does that place his mate at risk for losing her position as breeder as well? Based on 2207 breeder-years of data, the answer is a resounding “Yes!”. When a male is evicted from his lake, his mate is at substantial risk for losing her position. Female eviction also exposes male breeders to an increased threat of eviction, although the pattern is not as strong.

What can we make of this pattern? It is early, and many analyses remain to be completed, but here is an early read on this finding. Breeders are largely stuck with the danger of eviction and with the problem that their mate’s eviction threatens their own position. Females, which are about 20% smaller than males, are really in a bind, as their size probably prevents them from helping their mate avoid eviction — thus protecting themselves indirectly. On the other hand, males might be expected to chip in and help their mate drive off a would-be usurper, if doing so protected their ownership as well. Yet we have never observed a male teaming up with his besieged mate to drive off a potential usurper. If this happens, it must be rare. Indeed, males appear to be dispassionate observers of female battles for territorial ownership, despite the risk it poses to them. Why? Perhaps males are better off with a mate that fights strongly (such as a proven usurper), since having a vigorous mate ensures future years of eviction-free breeding. It is a bit sad and selfish to say it, but if your mate is vulnerable to eviction, it is probably best to let her fend for herself than to intercede and save her.