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Serving Northern St. Louis County, Minnesota

Sibling rivalry

The darker side of life in the North Country to play out this month

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One of the more puzzling biological oddities in the North Country should play out in our yard in the next few weeks. As usual, our local family of gray jays have been bringing their fledged young around the house and I’ve been feeding them bits of whole grain bread, which they love.

As is typically the case this time of year, there are two of the slate gray fledglings constantly dogging whichever one of the parents they are following at the time. Gray jays don’t produce large broods like other birds. In my experience, just one or two young leave the nest each year.

From all appearances, the relationship of the young is pretty typical for bird siblings— competitive for food when one of the parents is around, but otherwise tolerant, even friendly, towards each other.

But that will soon change. Sometime in June, the larger of the two birds, which is typically a male, will turn on its sibling and drive it away from the home territory. The unlucky outcast will most likely die— about 85 percent of them will be dead by the end of summer, according to at least one study. The few that survive typically do so by finding another unrelated gray jay pair that lost its offspring that year. In those instances, the unrelated adults may allow the young bird to remain in their territory, essentially adopting it.

From a basic evolutionary standpoint, all of this is highly unusual for two reasons. First, most animal species are more likely to cooperate with siblings, since they share so much common DNA. Effectively killing off a sibling may make sense in an episode of Game of Thrones, but it runs counter to basic biology, which argues that species try to advance the opportunities to spread their genes, even if it’s through the genes of a brother or sister.

Perhaps the strangest element of the gray jay’s strategy is that it would seem to create a gender imbalance. Since males tend to be larger and the larger of the siblings tend to drive off the other, which is statistically more likely to be female, you would expect that males would comprise a substantially larger portion of the gray jay population than the 50-50 ratio that would seem more biologically advantageous.

In fact, there is some evidence that such an imbalance may exist in gray jay populations, although I’ve seen no study that has confirmed it. Yet a significant percentage of adult gray jays do not breed in any given year, which suggests there may be an imbalance of eligible partners.

In the end, it seems likely that the gray jay’s unusual habits are a reflection of the winter food scarcity of the environment in which they live, and the need to limit their population to avoid outstripping the carrying capacity of their habitat.

The survival strategy employed by gray jays is based on maintaining a sufficiently large territory to allow them to cache enough food during the summer and fall to survive the winter. Stability is key to this strategy. Breeding pairs don’t migrate— they stay on their home territories year-round and they know their territories intimately. This strategy is highly effective— studies suggest that gray jay adults on their home territories can live a dozen years or more, which is considerably longer than most small birds. That means that gray jays don’t have the same biological imperative to produce a big brood of young ones every year like most small birds. Indeed, if doing so overtaxes the carrying capacity of the available habitat, all the birds on the territory could perish.

At the same time, restricting reproduction too much within stable territories presents the problem of maintaining sufficient genetic diversity within the gray jay population. The offspring of the adult pair can’t simply inherit the territory when the adults die, or the result would be inbreeding. Forcing one of the young, usually a female, to leave the home territory each year, apparently provides enough mixing within the overall population to prevent that.

So why does the sibling rivalry come to a head in June? Most likely because the remaining juvenile will need to start creating its own caches beginning in midsummer. Forcing the other sibling to leave at that point prevents the other bird from knowing the location of those caches. Gray jays are known as “scatter hoarders,” which means they scatter small pea-sized caches of food (mixed with sticky saliva) in cracks and crevices on tree trunks, usually spruce, and they can create as many as a thousand of these caches and recall where they are located. If the remaining juvenile allowed the other sibling to hang around through the summer, it would reveal the locations of its caches to its rival, who could raid them at a later point.

This is probably one reason that gray jays begin nesting so early. The nesting season, for example, begins in late February or March, at a time when food availability is typically at its lowest in the boreal forest and when snow lies deep in the woods. But gray jays rely on their food caches to make winter nesting possible. The female lays her eggs, usually two or three, and remains on the nest continuously to incubate them and even for the first few days that they hatch. The male feeds her on the nest during incubation and brings food for her and nestlings as well.

The young jays are usually fledged by early April and the parents give all of them a couple of months to learn the ropes of being an independent gray jay before the sibling rivalry kicks in.

Gray jays are loyal to that schedule, and adult birds won’t re-nest if they lose their first brood, even if it happens in March, well before most other birds are even thinking of the nesting season. If they don’t succeed initially, they simply wait another year.

I’m not looking forward to seeing how it all plays out next month, but I already have a good idea which of the two juveniles will soon be gone. It seems harsh, but it’s the way that gray jays have learned to survive in an unforgiving habitat.