When loon biologists measure reproduction, they often count chicks older than five or six weeks of age as having fledged. This is reasonable. Even though loon chicks remain dependent upon their parents for 11 weeks or more, almost all chick mortality occurs in the first few weeks of life. If a chick makes it to five weeks, it is almost certain to make it to adulthood.
Not so during this past week. As the unpleasant photo by Dan Pagel shows, one of the the Blue-Southeast chicks died quite violently several days ago after a sudden attack by an adult. Based on the photo and the swiftness of the death, we suspect that severe heart or liver damage caused the death. Since Dan was a few hundred meters away when the vicious attack occurred, he was unable to identify the perpetrator. Regardless of what loon committed this act, it comes as a shock. The departed chick was over twelve weeks of age, healthy, and off by itself feeding when it fell victim to the brutal assault. Twelve weeks is much older — I believe as much as eight weeks older — than we had ever observed a chick to be killed by a foreign adult. In light of the tempestuous territorial scene at Blue-Southeast recently, it is tempting to view this chick as a casualty of the conflict that caused its father to lose his territory. Certainly, the new, unmarked male that had taken over on Blue-Southeast and had already been observed to peck the chicks in his first weeks of ownership is a suspect. A second possibility is that the chick carelessly strayed into the west territory on Blue Lake and was attacked by a member of the West pair for territorial reasons. But territory defense is scant and subdued at this time of year, so that explanation is problematic.
I was still coming to grips with the events at Blue Lake a few days ago when I received a second piece of news, equally discouraging. One of the chicks at Buck Lake, near Rhinelander, had died. The cause of death in this case is less clear; the chick’s leg became injured a few weeks ago — a lake resident reported that a snapping turtle latched onto it! — and the chick was not able to move about effectively afterwards. Like the Blue chick, the Buck chick was far past fledging age, having reached 11 weeks.
What can we conclude from the co-occurrence of two deaths in very large and healthy chicks? I think these two unfortunate events simply remind us that, while chicks that have reached near-adult size are far more likely than 2-3 week old chicks to survive to adulthood, they are still young animals. As we have reported in a scientific paper, loons suffer a higher rate of mortality during their early years. Adult loons have had years to hone their ability to feed themselves and cope with environmental hazards, including conspecific attacks; youngsters are still getting it down.