Not All Intruders Are Equal

As I gaze vacantly out my back window, I wonder, as all of you must, when life will return to normal.

Linda Grenzer reports that our southern study lakes are on the brink of opening. Eager to occupy their territories as soon as possible, fight off pretenders seeking to supplant them, and set about the business of nesting, our breeding birds are bottlenecking on the Wisconsin River and various dammed flowages along the river’s tortuous path, she says. Their vigil will end in the next week or so. Offered even a sliver of open water in their territory, breeding pairs — sometimes the male alone, for a few days — will take possession.

We, on the other hand, are trapped at home. Three of us, Brian, Annie, and myself, are in southern California, which has never seemed so far from Wisconsin. Lyn, another new team member (along with Annie) is homebound in Ontario, Canada. As each day of lockdown passes, and as forecasters assure us the worst is still weeks away, we wait, our uneasiness mounting. Will we be allowed to travel to the study area come May? June?

To pass the time and convince myself that the project has not stalled, I analyze loon data. At present, I am resuming an analysis that a Chapman undergrad from Minnesota began in 2012. Katie and I looked at how the behavior of young nonbreeders changes over time. Nonbreeders, we know, do not begin to evict pair members and settle on territories until they are five years old. Further, we have learned that young adults gain substantial body mass in their first 5 years. Could it be that 2-, 3-, and 4-year old nonbreeders, which are too young to compete for territories, behave differently from older nonbreeders when they intrude into breeding territories?

It seems logical that loons should take some time to develop physically and behaviorally; after all, similar maturation occurs in humans. But what is the larger importance of loon maturation to territorial behavior? That is, how should the fact that young loons are not initially a threat to the ownership of territorial pair members affect behavioral interactions between young loons and pair members?

Maturation should shape nonbreeder behavior strongly. Not being competitive for territories, 2-, 3-, and 4-year old nonbreeders must feed themselves well, learn about the lakes in the small region where they live, and get to know the local territorial loons of their sex — which they must ultimately defeat in battle in order to claim a territory. Since they are in an information-gathering phase, youngsters should keep their heads down during territorial intrusions, signal to territory holders that they pose no threat, and hope to escape attacks from the pair that way.

What about pair members? A territorial male in the process of a breeding attempt with his mate should virtually ignore the intrusions of nonbreeding females and also young male nonbreeders, because neither threatens his ownership of the territory. (It is important to note that pair members fight and defend their breeding positions alone; males never help females stave off eviction, nor do females participate in male battles.) Likewise a territorial female should let her mate worry about nonbreeding males that visit her territory and instead focus her attention on 5- through 8-year-old females, which are strong enough to evict her. The picture becomes more complicated when a pair hatches chicks. In the presence of chicks, which are helpless and easily killed by any intruder, both male and female pair members should quickly confront and drive off intruders of any stripe.

Finally, both pair members and nonbreeders should calibrate their territorial behavior to the quality of the territory. To be specific, both groups should exhibit more intense behavioral interactions and more aggression on territories with a record of producing chicks, because those territories are worth a lot to both groups.

Thus, we have a set of predictions about how nonbreeders and breeders should behave with respect to sex and age of intruders, presence of chicks, and territory quality. I am in the midst of testing these predictions using two decades’ worth of field observations. At this stage, I have analyzed only nonbreeder behaviors. Below is the crude table I just produced that summarizes my findings to date.

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I will spare you the challenge of deciphering my shorthand and tell you what I have found so far. First, we predicted that young nonbreeders would show little aggression towards territorial pair members. That is borne out by the data. The second column and second and third rows in the table show that age is positively correlated with aggressive behavior of intruders and aggressive vocalizations by male intruders (i.e. the territorial yodel). In short, young nonbreeders — 2, 3, and 4-year olds — show less aggression (lunges, chases, and other attacks) than older nonbreeders. Furthermore, the strong tendency for young nonbreeders to initiate short dives (IDs) and join dives (JDs) when circling with territorial pair members (2nd column; 7th row) suggests that these diving behaviors signal youth and lack of threat to pair members, which probably protects youngsters from being attacked by the resident pair.

We can already say a bit about my prediction that the presence of chicks should crank up the heat on territorial aggression. Intruders that visit pairs with either nests or chicks (4th column) show consistently more aggressive behavior and tense social interactions with pair members than do intruders to a territory without nest or chicks. We presume this pattern results from the heightened aggressiveness of pairs themselves, which bleeds over into their interactions with nonbreeders. I expect my ongoing analysis of pair members’ responses to intruders to reveal a similar pattern.

Finally, look at the third column, which shows very consistent results in territories that produced chicks the previous year. The “Strong positive”s here show that nonbreeders exhibit more splash dives, bill dips, circle dances, initiates dives, and joins dives when interacting with territorial pairs that reared chicks the previous year. I am still puzzling over these results (and the lack of an increase in aggression). At present, this pattern suggests that: 1) nonbreeders confront territorial pairs more often when the territory has been shown to be a chick-producing territory, and/or 2) pair members approach and interact with intruders more often when they are on a high-quality territory. In any event, the quality of a territory where pair members and intruders interact strongly affects the nature of their interactions.

We have far more data on pair members themselves than on nonbreeding intruders, so the results of that ongoing analysis are likely to be even more robust than those shown in the table. Needless to say, these findings are a vital salve for my current frustrations with the lockdown. If it extends through May, I will require quite scintillating discoveries indeed!