It’s not the squirrels’ fault – but maybe a little, all the same…

All corvids and all species of squirrels present in Quebec are known to attack bird nests. So obviously, all these animals have an impact, to one degree or another, on bird populations. The question is: to what extent?

In the case of crows and ravens, it seems that the effect is rather small. A meta-analysis (i.e. a study which brings together data from several studies on the same question) published in 2014 in theInternational Journal of Avian Science found that corvid abundance had no negative impact on bird populations or their “productivity” (number of young, survival rate, etc.) in 81% of some 325 measurements considered.

When there was an effect, it was observed more often on productivity than on abundance (46% vs. 10%), and crows exerted a greater influence than other corvids (magpies, in particular). But ultimately, the study concluded, “their impact is small overall (…and) these results suggest that in most cases bird populations are unlikely to be limited by corvid predation “.

An important nest predator

The case of squirrels is a bit more complicated because the two main species that we have in southern Quebec, the eastern gray squirrel and the American red squirrel, apparently do not have quite the same behaviors in this regard. Both, we hear, are nest predators who eat eggs and nestlings when they can. But let’s say that they don’t do it with the same appetite.

The red squirrel has long been recognized as an important nest predator. For example, in a study published in 1997 in Ecological Applicationsresearchers placed fake nests with quail eggs in the boreal forest (where the gray squirrel is absent, it must be said) and noted which animal species came to “visit” them. The red squirrel appeared most often, with just over a third (36%) of attacks.

Other work has even observed effects of the presence of red squirrels on birds, at a population scale. One of them appeared last year in Ornithology from the pens of researchers from UQAT and Laval University.

“We placed acoustic recorders in 50 wetlands in the boreal forest and left them there for a few weeks,” explains Marc Mazerolle, co-signer of the study. And according to the songs that were recorded, we were able to establish the number of species present in the sites [ainsi que la présence ou non d’écureuil roux, dont le cri était également enregistré].»

Result: around 30 species of birds were generally detected in each of the sites but, in sites where the red squirrel was present, there were on average almost 4 species fewer.

The effect is still not catastrophic, says Mr. Mazerolle, since “the red squirrel does not specifically or mainly target bird nests. He will do it opportunistically, when he finds it.

“But still,” he continues, “if we have a high density of red squirrels in an area, there is a greater chance that the nests will be attacked, even if it does not reach dramatic levels.”

In addition, other work has observed that the red squirrel harms the reproductive success of birds. For example, a study in the mountains of Vermont noted that in years when fir trees produce many cones, squirrels are more numerous, they move higher into the mountains, and they significantly reduce the survival of Bicknell’s thrush nests. .

Less guilty

The gray squirrel, for its part, is “considered less carnivorous” than other species of tree squirrels, we read on the Adirondack Park website in the United States. It also eats insects, carcasses, eggs, etc., but less than its red “cousins”.

In this regard, a study carried out in Minnesota with false nests, placed in the forest and filmed to see predators, identified eight species that visited them. The eastern gray squirrel was one of them, but they came 7th out of 8 in terms of frequency of predation, a sign that other species weigh much more heavily than it on avian reproduction.

And studies of bird populations show essentially the same thing.

There are, of course, certain cases and more or less anecdotal accounts of gray squirrels having devastated nests locally, particularly in Great Britain where it is an exotic and invasive species. But more systematic data doesn’t really show an impact, even in English studies.

For example, one of them examined the evolution of 29 populations of prey species present in England which would have been likely to be affected by the abundance of gray squirrels, but found no sign of this.

Another, which focused on urban areas, observed only a “small reduction in the abundance of species most susceptible to nest predation by squirrels.” And a review of the scientific literature on the issue found “no strong evidence that the decline of any of the national forest bird populations is caused by the gray squirrel.”

So if there are fewer birds in a given location, it’s unlikely that it’s the gray squirrel’s fault.

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