As the intruder flew in and began to descend onto Townline Lake, I barked at my assistants,”Make sure you have your binoculars trained on the bird when it dips below the trees. That is when you can see the bands.” They hastily complied. “It’s banded!”, Melanie gasped, as the bird wheeled and landed about 150 yards from us and near the non-incubating pair member. We paddled our overloaded canoe as quickly as we could to join them and were tickled to see that the other pair member had decamped from the nest and was also racing alongside us to confront the intruder. Fortunately, the three loons engaged in stereotyped social behavior (circling and diving and bowing heads) and permitted us to approach them closely during this period. So we easily read all four colored leg bands on all three birds. This meant that we had the data we needed to later identify the intruder from our records.

Intrusions are relatively scarce during incubation, so Melanie and Tammy had seen few encounters between territory owners and intruders. Tammy remarked afterwards that the three birds had seemed to interact in a friendly manner. That is, they had exhibited social behavior and excited diving, but no aggression. Indeed the encounter had remained amicable for a full hour, at which point the intruder had finally separated itself from the pair and departed from the lake. “They acted as if they knew each other”, remarked Tammy. While seeking to purge all condescension from my tone, I explained to her that this was unlikely to be the case. “Intruders”, I explained, “are acting selfishly: they visit pairs in order to size them up as potential opponents in battle, often making repeated visits. At a certain point, an intruder learns that the owner of its sex might be vulnerable and then attempts to evict the owner in battle.” Since we had many years of data to support this interpretation of intrusions, I was quite confident in my explanation.

But I was wrong. At least, I was wrong in this case. Since we nailed the band combinations perfectly, it was a simple matter to look up the intruder in the banding database afterwards on my phone to find out that “Wb/Wb, S/Ar” was banded as a chick on – wait for it – Townline Lake in 2013. Since the Townline pair have both been on the territory for 15 years or more, this male intruder was the 4-year-old son of both pair members. In other words, we had just witnessed a family reunion.

Naturally, I was befuddled. My fluent, engaging discourse on the selfish and violent aims of intruders, which had seemed like gospel moments before (at least to me), was dashed upon the rocks. In a desperate attempt to wipe the egg off my face, I offered a Rube Goldberg of a hypothesis. Perhaps, I suggested, young intruders pass through a period during which they do not intrude to take over a territory, but instead seek only to sharpen their social skills. If young adults do need to develop social skills as a means to lay the groundwork for full-blown attempts at territorial eviction, then it might be the case that youngsters are less picky about where they intrude than older intruders. Such age-dependence in territory targeting by intruders might explain why they would even intrude into their own natal lake and interact with their parents, as this 4-year-old male had done.

While rather complex, my thrown-together “social learning” hypothesis is plausible. Equally important, it generates precise predictions. First, young nonbreeders (3-5 years old) should intrude into more different lakes than older nonbreeders (6-9 years). Second, for reasons of incest avoidance, older intruders should never visit their natal lake, which is likely to be owned by their parents still. In other words, loons looking to evict the owner of an established territory should not intrude into their natal lake, while young loons aiming only to learn how to behave socially might be expected to intrude in their natal lake — and everywhere else. I have not yet looked at intrusions systematically to look for this age-based pattern, but I was curious enough, after our recent observations on Townline Lake, to do a quick check. Two other cases of young nonbreeders intruding into their natal lake fit the expected pattern. One of these birds, in fact, was a mere two-year-old when it visited its parents on Bolger Lake.

We must analyze more data to see if we have a statistically significant pattern showing that young nonbreeders are those that tend to visit their natal lakes. But so far, the pattern is holding up. Who knew? In many cases, we scientists blunder upon a finding – as happened here – when we make an observation that does not fit the view of nature that we had constructed. Indeed, this is one of the rewards of our discipline.