I have touched upon this theme before. A peril of longitudinal investigation is that one decides, after a period of time, that one understands the system. So it has been with the Loon Project.
For many years I have thought I had a good handle on territorial behavior. Indeed many aspects of the loon territorial system have become clear during the course of my work and are not in doubt. Both sexes usually fight to claim their territories and face the constant threat of eviction. Males, which establish strong ties to a territory through controlling nest placement and learning where the best nest sites are, fight harder than females, and sometimes die during territorial battles. Early senescence among males sets the stage for them to become very territorial and aggressive as they reach their declining years (their mid-teens in many cases), which seems a means to help them eke out another year or two on a familiar territory.
But I might have been off in my understanding of the role of lake size and body size in territorial behavior. I have always thought of breeding territories on large lakes as much sought-after, because large lakes have ample food for rearing chicks. (Small lakes, you might recall, run low on food for chicks, resulting in lower fledging success.) If large lakes produce more young, I reasoned, large-lake territories must be highly desirable. Competition must be fierce, then, for these territories. A recent analysis of territorial tenure — how long a male or female can hold onto their territory before getting evicted from it — has forced me to rethink the effect of lake size on territorial competition. The figure below is a plot of territorial tenure versus body mass for males on lakes smaller than 20 hectares (50 acres) in size (like Langley, whose current pair is pictured). As you can see, small males — especially those below 4600 grams — have very short stays on small lakes, in most cases, while large males — notably those heavier than 5000 grams — often enjoy very long territorial tenure. This pattern suggests that, contrary to my expectation, territorial competition is fiercer on small lakes than large.
Let’s look at the same pattern on medium-sized lakes (20 to 80 hectares; or 50 to about 200 acres). You can see that the overall pattern is still evident, although it is weaker here, because a number of very large males (5400 to 5800 grams) have anomalously short tenure.
Finally, let’s inspect the data only for males on lakes larger than 80 hectares (200 acres). In contrast to my earlier hypothesis, large males are not holding their territories any longer on large lakes than are small males, as you can see from the plot below. Males of all sizes may enjoy long tenure on large lakes.
How on Earth do small males hold their territories much longer on large lakes — which seem much in demand, get more intrusions, and appear difficult to defend — than on small lakes, which get fewer intrusions and should be more easily held? I don’t know exactly how males hide in plain sight on large lakes, but it might have to do with the difficulty that territorial intruders have in simply finding a nesting pair and identifying nesting habitat on large lakes. Consider the Lake Tomahawk-Little Carr pair. This pair nests in a marsh at a well-hidden location. When one bird is incubating, its mate is usually far off in the wide open portion of Lake Tomahawk, which is many kilometers long and has an area of 1400 hectares (about 3500 acres). A male intruder might well find and socialize with the off-nest pair member on on the big water, but it would have no way of knowing that the mate of this loner was on a nest hidden far away in a marsh. Similarly, when the eggs hatch, the pair quickly leads the chicks to the main bay of the lake, far from the critical nesting area. Pairs with chicks provide an enticing cue to young males seeking territories, because the presence of chicks tells of the availability of nesting habitat. But a male intruder that encounters the Tomahawk-Little Carr pair and their chicks on the main bay of the lake would face the needle in the haystack problem in locating the precious nesting area that yielded the chicks. A dangerous battle might win the territory, but the knowledge of how to use the territory (that is, where to place the nest) would vanish with the old male’s departure. Hence, large lakes appear to be less valuable to males.
A male intruder bent on taking a territory likely to yield chicks in the future would be better-served by evicting a chick-rearing male on a small lake. Such an intruder would have a much smaller set of nesting areas to inspect and would likely find and use the nesting area that produced the chicks. Thus, we might expect stronger competition among males for small, easy-to-learn territories — a pattern that dovetails with the longer tenure that large, competitive males enjoy on small lakes, compared with small, easily-evicted males.
What about females, you might ask? Do large females on small lakes, like large males, have an advantage in holding their territories when compared with large females on large lakes? If my hypothesis is correct, and the value of a territory depends upon knowledge of safe nesting areas, then large female size should not be especially beneficial on small lakes. Indeed, any impact of female body mass on territorial tenure should be equal across all lake sizes. Why? Because females do not control nest placement in this species. An intruding female that evicts a breeding female with chicks and pairs with the breeding male would have access to that male’s knowledge of nesting sites on a lake of any size. As predicted, large size is no more beneficial to small-lake females than large-lake females. (Indeed, size has an overall weaker effect on competitive ability in females.)
So my post hoc hypothesis for the fierce territorial competition on small lakes holds for the time being. The explanation I have given is not the only one consistent with these data, by the way. In fact, the entire complex pattern described above might be explained by a completely different scenario. Large males might hold small territories longer simply because they are in better body condition. This is highly plausible, because mass is a good predictor of health and condition. Thus, only large males might be able to hold onto small lakes for a long time, because they are in good enough condition to survive in a habitat with limited food. Small males, by this logic, are already in sub-prime condition, so they are destined to have short territorial stays on lakes with limited food. In contrast, medium-sized and large lakes do not show the pattern, because food is not limited on them.
As you see, we have a long way to go to figure out exactly what the above graphs are showing us. Distinguishing between the “small-lakes-are-highly-competitive” and the “holding-small-territories-requires-good-body-condition” hypotheses will take some years. The difficulty of using what seem like beautiful, clear data to reach a firm conclusion provides a nice window on what it is to be a scientist.