Saturday, September 02, 2006

Can voles see hawks? Blakeman answers Lyons

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.

–John Blakeman

Friday, September 01, 2006

Form follows function, in nature as in art

Chris Lyons writes:

Regarding the debate over whether raptors have any controlling effect on rodent populations--this is from "The Mississippi Kite", by Eric G. Bolen and Dan Flores, University of Texas Press, 1993

"Among the earliest attempts at what was then considered 'game' management was a bounty system that often included hawks--and the generic and ill-founded designation 'chicken hawk' for just about any bird of prey certainly didn't help matters. In 1885, Pennsylvania enacted a hawk-and-owl bounty law--known at the time as the 'scalp act'--which provided for a payment of 50 cents per raptor scalp. Within two years, 180,000 hawk and owl scalps were bountied, for which the taxpayers of Pennsyvania parted with $90,000--big money for the day. Yet one estimate of the program indicated that farmers saved just about $1 for each $1,205 paid out in bounties! But the folly of the bounty on raptors was not limited to direct payments. Fields and orchards were overrun with mice and other vermin to the point that farmers requested repeal of the bounty law, which happened two years later, but not before crop damage had added another $2 million to the program's cost. Even so, the idiocy of bounties continued in other areas well into the current century. Maryland paid bounties on nearly 90,000 hawks between 1925 and 1930, and Alaska's territorial government authorized a $2 bounty on Bald Eagles in 1917. By 1952, when the bounty ended in Alaska, more than 128,000 Bald Eagles had been cashiered."

There are clearly many factors involved in maintaining a healthy population of our native rodents, which after all are as valuable a part of their respective ecosystems as birds of prey. I would tend to think that the effect of diurnal and nocturnal raptors on rodents has an effect far beyond the number of individuals that raptors prey upon. 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. Unlimited food means unlimited rodents, no matter how many predators they have. Agriculture opens up new and abundant food sources for them, which can lead to their numbers increasing far beyond their normal limits, which can mean ruin for the farmer. However, the presence of food is not enough--rodents must also be able to access the food, and this generally means venturing from the safety of burrows and tree cavities, out into the open, where they must expose themselves to potential predation. Millions of years of evading a host of hunters has encoded wariness into these small creatures--when they sense the presence of their natural enemies, they conduct themselves with far greater discretion, as any New York apartment dweller who has lived with and without a cat knows full well.

Foxes, coyotes, bobcats, snakes, and many other predators are a check on rodents, and a healthy ecosystem will have all of the above predators, and more--but raptors are unique in their ability to strike from above, with little or no warning. 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. Owls take the night shift. If Red-Tails and other such raptors are eliminated from agricultural areas by shooting, trapping, and poisoning (methods that work far more effectively against hawks than against the legendarily fecund microtus and their numberless relations), then vigilance can be considerably relaxed, and the little mammals can venture more boldly from their redoubts, with fewer of the precautions they normally take. The ultimate result is that they can devote more of their time to food-gathering--and reproducing, in ever greater numbers.

John Blakeman is quite right--you can't just view it as an exercise in simple subtraction--it's more about multiplication. And without hawks and other predators to worry about, it seems probable that rodents and other abundant small mammals, can multiply far faster, and the only limit on their populations will be the supply of available food--unless the farmer resorts to poisons, which have a devastating effect on the environment, and are ultimately ineffectual, because they have no deterrent effect--they merely subtract from an unlimited population, which replaces its losses faster than they are incurred. Mice may die from poison, but they don't fear it. Not even if they learn to recognize and avoid it, in which case they don't even die from it. Though many beneficial animals will.

In many cases, it may be that the presence or absence of Red-Tails and other diurnal raptors makes little difference to the numbers of rodents in a given area. In other cases, there is circumstantial evidence that they do, in fact, serve as an important controlling factor on agricultural pests. A sky devoid of circling buteos is a sky under which the vole and the field mouse can move more securely, and breed more prolifically.

Ornamental our raptors surely are, but form follows function.

Thursday, August 31, 2006

At the Moth Tree

Implicit Arches [Lacinipolia implicata]

Last night [August 30, 2006,] following rain earlier in the day, the Moth Tree was oozing sap in new places. Perhaps that was why several new moths showed up. One, a Yellow-striped Armyworm moth, was too high to photograph. The other, a gem-like little moth with pale green forewings filled with black, brown and brilliant white markings, has a typically enigmatic name -- Implicit Arches. It arrived at about 9 pm and stayed for more than an hour. We could watch its long proboscis eagerly slurping down sap. The photo above does not do it justice, alas. I think that something about the brightness of the white markings disturbed the automatic focus feature of my camera.

Other names in the same subfamily as Implicit Arches -- the Hadeninae--are: The Thinker, The Nutmeg, Intractable Quaker, Hitched Arches, Stormy Arches, Confused Woodgrain, Cloudy Arches, The German Cousin, Disparaged Arches and many more. I can just imagine those 19th century moth namers sipping at their brandies and laughing uproariously as they came up with one unlikely name after another.

Wednesday, August 30, 2006

If you're looking for a great bird walk...

A note from Jack Meyer:

My Fall Birdwalks in Central Park will begin Thursday August 17, and end on Sunday October 29.

Walks will be four days a week, Thursday through Sunday

Walks leave at 7:30 AM from 72 St and Central Park West (NE corner). The walks will last until 10:30 or 11:00, with a brief coffee break midway through. The walks are unstructured, and there is no obligation to stay to the end.

The cost is $6 per person. No reservations are needed

If there are any questions, you can reach me at 212-563-0038 (Not after 8PM please), or email .

Should the weather be bad and you want to confirm that the walk is going, phone me anytime after 5AM. If you get my answering machine, you will know that I have already left for the park and the walk is on.

Blakeman: Vole clarification

A Fifth Ave. offspring with full crop of rodent or bird prey [but no voles in CP]
Photo [long ago] by Charles Kennedy


Ben Cacace points out quite accurately that red-tails aren't the only predators consuming voles. Indeed, foxes, coyotes, weasels, minks, owls, and a few others also thrive on these abundant small rodents.
But there aren't many more of these than red-tails. Foxes and coyotes commonly hunt in many square miles of territory each night, not just in two (the typical size of a Midwestern or Eastern red-tailed hawk territory). Owls probably take as many voles as red-tails, although the smaller owl species tend to hunt in forests and woods, where there are few or no voles.
I didn't make my point clear. Ben contends, accurately, that if red-tails actually consumed a third of the vole population each year, that’s not insignificant. That would seem to lop off a major portion of a local vole population, and in conjunction with all the other vole predators, the cumulative effects of vole predation should keep them in check.
Except: – our cute little voles, as I mentioned, don't just sit there during the year. They breed – like – mice. Let’s add to the static numbers of voles the low-range numbers of new offspring produced each year.
If a red-tail’s territory began a year with about 10,000 voles, and a pair of hawks ate about 3600, there would be only 6400 left at the end of the year, presuming that there was no vole reproduction. But reproduction is the more important factor, the one that tips the scale in favor of the voles, regardless of the cumulative predation pressure by all local predators.
In typical periods, each acre typically has 8 to 10 voles.(At populations “highs,” every four or five years or so, there can be 50 or more voles per acre. But I'll use the low-end numbers throughout, to make the point.) That’s 4 or 5 pairs in normal years. Each pair produces an average of 4 or 5 pups from 5 to 10 times a year. Again, let’s use the low numbers, just 5 broods of 4 pups for each breeding pair. How many voles, then, are produced for all the predators to consume in a year’s time? It’s not the static 10,000 I mentioned.
Four pairs in each acre, reproducing only 5 times in a year with only 4 pups, yields 80 voles available for local predators to consume (4 pairs x 5 litters x 4 pups in each). That’s on just one acre. A typical red-tail’s 2-square mile territory has 1280 acres. 80 x 1280 = 102,400.
So, at any one time only about 10,000 voles are available for a hawk to capture. But over an entire year, the population is in the range of 100,000. If the local pair of hawks ate only voles (and they don't, they prefer to vary their diet), they could consume no more than about 3600. That’s a bit less than 4% of all the voles produced in the territory.
Here’s an interesting quantum. How many pounds of vole flesh are annually available to all the vole predators in a two-square mile red-tail territory? A vole weighs about 2.5 ounces. A hundred thousand of these weigh 250,00 ounces, or 15,625 pounds. That’s about 7.8 tons of vole flesh produced each year. For a 2.5 lb feathered predator, that’s a big pile of vole flesh to eat in any useful numbers.
The ecological fact is the reverse of the “hawks control rodents” dictum. It’s the other way around. Vole (and other rodent numbers) actually control hawk populations. That’s why red-tails in California have much smaller territories, because there are abundant supplies of ground squirrels and other rodents. Where prey numbers are small, territories have to be large, to support a single hawk pair. Lots of prey, small territories. Few prey, large territories, and fewer hawks.
Readers can now understand my initial concern about Central Park harboring two pairs of red-tails. Would there be enough rats and other prey for them to subsist? Today, it’s not a question. And does anyone now think that Pale Male, Lola, and the south end pair of red-tails make any dent in either the rat, pigeon, or squirrel populations of Central Park? They don't. The prey reproduce faster than the hawks can eat them, period. It’s the other way around. The small animals of Central Park support two pairs of red-tails, very nicely.
Sorry, but the old conservation maxim that hawks and other predators control rodent populations just isn't so. They are controlled by internal food and territory factors. But that’s a separate story too big to dwell upon here.
–John Blakeman

Tuesday, August 29, 2006

Response to Blakeman on voles

Pale Male with Carlyle in background - 8/28/06
Photo by Lincoln Karim

Ben Cacace, a Central Park birder and long-time hawkwatcher, writes:


A simple question for Blakeman:

I would think taking out a little over a third of any population would be considered a little more than making "a dent in the always-expansive vole population". Are you saying that voles are only predated on by Red-tailed Hawks? You don't mention any other predators having an effect on the other two thirds of the vole population.

Margie Siegal writes:

Regarding hawk control of vole population:

In a square mile of a healthy ecosystem, there are various mid sized predators. There are hawks. There are owls. There are foxes (and probably feral cats) Each of these are a part of the control of the rodent population. If you kill off the hawks, you eliminate a major element (possibly ¼ to 1/3, roughly estimating Blakeman’s calculations) of the rodent control mechanism. Other predators may (or may not, if they are also under human pressure) fill the gap. An analogous situation occurs with deer in Western states. We have killed most of the predators of deer. The deer population has exploded, hunting is not controlling it, and deer are all over our highways, eating our rosebushes and sometimes starving to death.

Sunday, August 27, 2006

Blakeman on voles


Bill Trankle’s close observations of a vole on his property are interesting, particularly because he got to watch a Cooper’s hawk devour the specimen. Cooper’s hawks prefer to take birds, but will occasionally take a small rodent.
Bill asks how many voles might a red-tail consume each day. A good question, as it relates to both the number of voles in a red-tail’s territory, and also to the number of red-tails that can be supported in a large, regional area.
Microtus pennsylvanicus, the common field vole, ranges across the northern portion of North America, from the southern tier of Canadian provinces to all but the southernmost tier of American states. It is absent from the far western states, but other similar small rodents provide red-tail provender out there.
The little creature averages about 70 grams, about 2 ½ ounces. In the summer, a red-tail needs from two to four of these each day to survive. In winter, three to five are required.
Although the following will contradict a great deal of textbook verbiage – none of which is supported by real field studies – I'll deflate a common conservation dictum related to voles and other small rodents. I'm treading on dangerous turf here, as this will contradict a great deal of conservation gospel. But my data, I believe, will support the deflation of the erroneous theory.
The theory is this. We need hawks flying around the landscape to control rodent populations. If we killed off all the hawks, rats, mice, and voles would explode in unchecked numbers. Hawks and owls are essential rodent predators, holding their populations in check. Hawks are “good” and rodents are “bad.”
A nice thought, one that was used in the passage of hawk protection laws. But the numbers don't add up. Here they are.
Studies have shown that typical vole populations average 8 to 10 per acre. During population highs, there can be as many as 50 to 60 of the little rodents in each acre.
Adult pairs of red-tails in rural areas typically have a territory or home range of about two square miles. In areas with reduced prey populations, territories can be larger. With lots of prey, territories can be as small as a half-square mile or so. Let’s use the two-square mile as a working number.
A square mile is 640 acres. Two square miles is 1280 acres. If each acre had only 8 voles, the territory would have 10,240 voles. If each of the pair of red-tails ate five a day (10 for the pair), the annual vole consumption would be 3650, only a bit more than a third.
Of course, the voles aren't just sitting there waiting to be eaten by a hawk. They are inveterate breeders, producing 5 to 10 litters each year, with 4 or 5 pups in each.
Anyone can see that a pair consuming 3600 voles in a territory each year just doesn't put a dent in the always-expansive vole population. Contrary to the well-meaning contentions of hawk protectionists, red-tails don't and can't “control” mouse or vole populations.
Red-tails should be (and are) protected for better purposes, because they are noble, natural denizens of North American wild areas – and now, even of places like Central Park -- not because they supposedly limit rodent populations.
And one last note. Unlike Bill’s Cooper’s hawk, a red-tail never takes 10 minutes to consume a vole. It’s often more like 20 seconds, usually in two or three powerful, flesh-ripping bites. The bird’s only concern is to get the captured specimen into the crop as fast as possible. Red-tails are never models of culinary decorum.
– John Blakeman