What I witnessed on Wind Puddling lake yesterday is an occurrence we have seen many times now. Instead of the usual two-egg or one-egg clutch, a pair has three or even four eggs. Why the supernumerary egg(s)? There are two likely explanations. First, the female’s cycle of egg production might have been thrown off, causing her to add a third egg, when she would normally have shut down after two. Second, the female laid two eggs, then the pair abandoned them, but they have reused the first nest site and simply added two fresh eggs to the two, old, addled ones from their first nesting attempt.
We know that both 3-egg clutches and reuse of a nest containing abandoned eggs can explain extra eggs in loon nests. In 1997, the Washburn Lake female laid three viable eggs within a single nesting attempt. All three eggs hatched, in this case, producing three small chicks. But the task proved too much for the pair. They lost two of the chicks quickly and settled for raising of a single chick that year. That is the only case we know of where a female deposited three eggs in quick succession into a nest and had them all hatch. All other cases of three or four eggs in a nest have resulted from adding of two new eggs to a nest already containing two old, dead eggs from a first abandoned clutch. This is quite common.
Why would loons behave this way? First, let me point out that safe nest sites are sacred to loons. That is, they search — sometimes for years — to locate a site safe enough that the eggs hatch. Once a male has found a successful nesting site, he is loathe to leave it. So we can understand why a pair might wish to reuse a good site. The question is: why do loons not remove spoiled eggs from a nest before laying fresh eggs there? While it would seem highly inefficient to sit on four large eggs, when you are used to sitting on only two, loons have many times hatched chicks from four-egg nests. Thus, loons are capable of keeping four eggs warm. It may even be that the extra eggs provide a bit of extra warmth buffer, since having warm neighboring eggs helps each egg remain warm itself. So perhaps the answer to why loons do not discard the dead eggs is that leaving them in the nest has no cost and might even bring a slight benefit.
Nevertheless, I do not enjoy seeing a sight such as I saw at Wind Pudding. Supernumerary eggs almost always mean that black flies have been savaging the loons again. While not as severe as 2014, 2017 has been another year of black fly abundance. Many pairs have stuck doggedly to their nests, enduring the hateful pests. But about an equal number have already given up on their first nesting attempt or appear on the brink of it. If that attempt occurred at a favored nest location that produced hatched eggs in the past, we can expect to see more sights like this in the coming weeks.