Pale Male, Lola and intruder
Photo by Lincoln Karim
Chris Lyons proposes that “the effect of diurnal and nocturnal raptors on rodents has an effect far beyond the number of individuals that raptors prey upon [them]. Rodents need to spend a considerable part of their day foraging, and as is well known, the only real check on their population growth is the availability of food.”
On the first point, he is contending that it’s not only direct raptor predation on rodents, but also their presence in the sky that deters, or somehow, reduces rodent populations. He states that, “Rodents foraging in the open during daylight hours must maintain a constant look-out for hawks, particularly buteos, whether circling overhead, or perched in a tree.”
If small rodents such as the common vole, Microtus pennsylvanicus, had good eyes that could see at a distant, hawk spotting by the prey animals might be factor. But I can assure readers that it’s not a factor. Voles are exceptionally nearsighted, with tiny eyes that are focused on small seeds and grass blades within the narrow tunnel runways they create in grassy or herbaceous environments. I have seen this time after time with my hunting red-tail, Savanna, perched on my fist.
We will be walking through a field mutually searching for our prey of falconry preference, the cottontail rabbit. Now these big animals do see approaching predator hawks (or humans), and when they feel that they've been discovered in their sitting positions they will bolt away at rabbit speed. Rabbits have large eyes on the sides of their heads, allowing them to easily look back over their shoulders when fleeing a pursuing raptor. With this visual ability, the rabbit is able (often) to jump away from the hawk’s stoop or plunge, avoiding at the last instant the sinking of eight needle-sharp talons into the rabbit’s flesh.
But the common little voles respond entirely differently. They don't respond at all, in fact. Personally, up close I have seen things that very few raptor (or rodent) researchers (who aren't falconers) have ever seen. Many times I've watched my red-tail drop down on a foraging vole just six feet away. I've watched how red-tails capture voles – and how voles try to avoid being captured.
Most researchers have to try to discern this by watching the hunting of a distant red-tail perched in a tree. Every 20 minutes or so (often much longer), the hawk will drop out of the tree into the grass below trying to grab a vole that was naturally scurrying through the vole runways. Because the vole scoots between narrow grass clumps, the hawk can only pounce while the vole is in the open, for a second or two. More often than not, the vole is missed by the hawk. Every third or fourth pounce, however, is successful, the hawk has a thoroughly delightful meal (albeit short, in two or three quick bites).
Believe me, the voles never see or know what hit (or missed) them. They spend absolutely no time scanning the skies for soaring hawks, for that would require them to sit vulnerably out in the open. They try to always stay hidden down in the grass runways, keeping a cover of grass overhead. The hawk looks for the minute motions of the grass as the vole runs through the narrow runway. I've seen my hawk pounce on a mere clump of grass, in which a vole had tried to take refuge. The hawk saw the moving grass, not the vole. The vole never had a chance to see the hawk.
Voles don't, and can't try to avoid hawks by visually searching for them. Their eyes just aren't capable.
Secondly, red-tails most commonly hunt from a sitting in a perch, just as Pale Male and the other Central Park hawks do. There is no way a vole can look up into several hundred yards of forest edge and discover a sitting red-tail. Even I, with 10-power binoculars and very fine human vision (and much hawk-spotting experience) often fail to see sitting hawks.
In summary, vole vision is short. Voles do not and can not respond to any nearby hawk. They just can't see them. And even if they could, what would they do, go hide in a burrow until the hawk went somewhere else? If so, how does might that in any way reduce vole numbers or reproduction?
If predation by hawks and all of the other predators fails to limit vole populations, what, then, does? Chris suggests that it’s only a limitation of food. That can be a factor, but because voles are herbivores that can subsist on plain old green leaves (along with preferred grass weed seeds), a lack of food is not the only (or primary) population control. It’s more complicated, related to rodent social influences.
It’s a territory thing. When vole populations get high, as they do every three to five years (on average), the packed-in social structure begins to deteriorate. Mothers fail to properly care for their newly-born pups, some are eaten or tossed out of the nests. Young voles fight each other. All social hell breaks loose, and the social structure of the population collapses. Diseases become prevalent, with consequent deaths.
Most importantly, stress hormones begin to dominate and voles concentrate on surviving, not on maintaining family structure or reproduction. The population crashes, leaving a few isolated survivors who then start the cycle over.This happens, too, with lemmings, prompting the mass migrations to nowhere.
As I said in my earlier posting, the factors that control vole populations are complicated and multi-factored. But they don't involve the presence or predation of hawks.
Chris cites the proliferation of voles that putatively resulted from Pennsylvania’s institution of proliferative hawk bounties. No doubt, thousands of hawks were slaughtered with these killing subsidies. But that’s not what caused the voles to overtake orchards and farm fields. Like lemmings and other small meadow rodents, vole populations cannot remain stable or even. By the nature of the beast, they naturally go up and down. When there are few voles, at the start of a population cycle, everything social works well. Vole females produce lots of pups and because there is no social interference from nearby voles, the pups grow well, making in a few weeks newer nests, with more new young voles. The population grows like this for a few years.
But eventually, there are too many, and they start fighting and social conflict does its hormonal magic, stopping reproduction. That’s what was seen in the orchards. In winter, in vole population highs, when there are 50 to 80 voles per acre (instead of the “normal” 5 to 8 or so), the vole hoards soon learn that apple tree bark is just good eating. That’s why modern orchards control vole populations by merely mowing the grass to low turf height, making the habitat unfavorable for voles. Voles require their constructed runways, and they can't make runway tunnels in low turf.
The facts remain, red-tails and other hawks, even owls, have no effects in limiting the growth of vole populations. They are controlled by internal social interactions that cause stress and disrupt socializing and reproductive hormones.
It’s no longer the 19th (or 20th) century. Field biology must now be based upon actual field evidence, not on presumed, wished-for, or fantasized actions or influences. Old myths die hard, especially ones that involve creatures as noble and entrancing as the red-tailed hawks we all love. But the facts are the facts, and modern biology has revealed them in regard to how vole populations actually get controlled. It’s the internal social and stress workings of the species, not any external removal by predators.
And here’s a final point to ponder. If red-tails and other predators were so successful in controlling vole populations, why aren't they then always driven to the edge of extinction? Since red-tails love to eat voles so often, why don't’ they just eat every one they see and just wipe them out? It’s because they simply don't have the ability to do so, either to limit vole populations when there are many of them, or alternatively, to wipe them out when there are just a few. Under all conditions, red-tails are vole population non-factors.