A Question For The Lone Ranger

[The Lone Ranger]

Weasels and other small mammals can indeed walk on soft powder snow while making only minimal indentations on the surface. This is a combination of their light weight (a function of the square-cube law) and the relatively large surface area of their feet. Quite a lot of small mammals that are active in Winter grow extra fur to further increase the surface area of their feet and further reduce their tendency to sink into the snow.

Snowshoe hares and lynx are famous for this, of course, but I've seen weasels bounding across powder snow while barely making an indentation. And I've seen snowshoe hares bounding across soft powder at a flat-out run while barely making an impression in the snow, and certainly without being encumbered by the snow to any noticeable extent.

Larger animals like wolves or deer can't manage it, of course, but small mammals (especially like weasels or snowshoe hares, which are specifically adapted for doing so) can easily walk or run atop powder snow without sinking into it very far at all.

Of course, they can get down into the snow if they want. Voles often tunnel within snow, presumably to avoid being spotted by predators, and weasels are famous for burrowing into and through the snow, almost like furry snakes, in pursuit of them.

[thedoc]

Getting back to the original question, the birds are not lingering at the bird feeder to avoid excessive contact with predators. So even if a weasel can maneuver on the surface of soft snow the birds would not linger long. Just curious, what is the weasels preferred prey.

[The Lone Ranger]

During Winter, mice and voles are their major prey items. Weasels usually capture small rodents either at the surface or by burrowing into the snow after them. They've been known to catch and kill animals as large as rabbits and hares, though. They do catch birds sometimes, but that's probably more pure luck in most cases than anything else; occasionally, they'll climb trees in search of prey, and have been known to catch the occasional roosting bird.

[thedoc]

So weasels don't prefer birds because of the difficulty in capturing them as prey. It is interesting to note that the weasels in the photos both were holding small mammals and not birds, so the introduction of weasels as predators of birds at a feeder was a red herring.

[The Lone Ranger]

To clarify a bit regarding foot size and body weight, as a rule, the greater the surface area of your feet compared to your weight, the better you can cope with snow. The ratio of weight to foot surface area is known as an animal's foot load.


For a typical adult human, foot load is typically about 145 grams/centimeter2, which is why a human can't walk atop snow unless it's very densely-packed and/or iced over.

A wolf, especially an Arctic wolf, has proportionately larger feet (and 4 of them, instead of only 2), and typically has a foot load of about 100 g/cm2. So, while it does better in snow than a human does, a wolf typically has to bound through the snow, rather than walking atop it, like smaller mammals can manage.

A lynx has very large feet, specifically adapted for walking in snow, and has a foot load of about 30 g/cm2, making it capable of walking atop packed snow without sinking into it.

A weasel does even better, being smaller. A long-tailed weasel has a foot load of about 10 g/cm2, and can walk atop all but the lightest of powder.

Of course, snowshoe hares take the cake. A typical snowshoe hare has a foot load of about 1 g/cm2, and so can move about atop even very light powder.


ETA: I wouldn't call introduction of weasels a "red herring," per se. I simply mentioned that they (and other small mammals) are actually quite capable of walking atop snow, and so deep snow would make it easier to get to a bird feeder, not harder. Nonetheless, weasels are indeed known to raid bird feeders, especially if they contain suet. Additionally, birders sometimes have problems with weasels, fishers, and other potential predators hanging about their feeders and attempting to capture birds, squirrels, and mice that are attracted to the feeder. Since many birds (chickadees, especially, in my experience) tend to scatter seeds onto the ground as they feed, and many birds (especially mourning doves) preferentially feed on the ground, rather than at the feeder itself, there are typically lots of birds on the ground or snow beneath an active feeder, and often squirrels and/or mice as well. Certainly, no self-respecting weasel (or fisher, or fox) would pass on an opportunity to catch an unwary bird at a feeder if it merely required a quick jump off the snow's surface to snatch it, so it's certainly in the best interest of a bird to grab its meal and quickly retreat to a less-accessible site, rather than lingering at a feeder.

[BrotherMan]

[JoeP]

That was stoatally uncalled for Bort.

[Vivisectus]

yeah, put on that you must a lid.

[JoeP]

[thedoc]

Originally Posted by The Lone Ranger

ETA: I wouldn't call introduction of weasels a "red herring," per se. I simply mentioned that they (and other small mammals) are actually quite capable of walking atop snow, and so deep snow would make it easier to get to a bird feeder, not harder. Nonetheless, weasels are indeed known to raid bird feeders, especially if they contain suet. Additionally, birders sometimes have problems with weasels, fishers, and other potential predators hanging about their feeders and attempting to capture birds, squirrels, and mice that are attracted to the feeder. Since many birds (chickadees, especially, in my experience) tend to scatter seeds onto the ground as they feed, and many birds (especially mourning doves) preferentially feed on the ground, rather than at the feeder itself, there are typically lots of birds on the ground or snow beneath an active feeder, and often squirrels and/or mice as well. Certainly, no self-respecting weasel (or fisher, or fox) would pass on an opportunity to catch an unwary bird at a feeder if it merely required a quick jump off the snow's surface to snatch it, so it's certainly in the best interest of a bird to grab its meal and quickly retreat to a less-accessible site, rather than lingering at a feeder.

I agree that other animals can be a problem at most feeders, but if you examine the photo the perches are mounted on a shell that is lightly spring loaded so the weight of a Squirrel will bring it down and close off the feed. I know this doesn't prevent the other animals from getting the feed that is spilled on the ground, but it does limit it somewhat. My wife was concerned that the weight of some of the larger birds might be enough to close off the feeder to them if they tried to perch. Also the feeder is hung on a Black Locust tree that has thorns, and I understand that this doesn't make it impossible for the squirrel, but it does make it more difficult, and the birds don't seem to be bothered at all.

[thedoc]

We are keeping our daughters dog because of a conflict with her fiancees dog. We have Bamboo floors and I notice it is covered with dogs footprints. I understand that dogs sweat through their feet, and I was wondering if this was a product of evolution from wild dogs and wolves. Sweating through the feet would leave the scent of the animal that has passed and another member of the pack could follow the scent to rejoin the pack. Or an animal from another pack could recognize the scent as not one of their own and know that they are in the wrong territory, and leave, or know that they were going to encounter an animal from another pack.

[mickthinks]

I was thinking about sea birds. And comparing them to sea mammals. The point being that the ancestors of both classes began the evolutionary process out of water and thus the aquatic species of both birds and mammals that we know today are descended from land-dwelling stock, but where there are limits (imposed by the laws of physics) to the size that phenotypes can attain and still remain viable on land or in the air, such bounds are greatly extended in water. Hence many species of whale are far bigger than the largest elephants.

But the largest water-bird, the emperor penguin, is a fraction of the size of the average flightless ostrich. Are there any ideas why mammalian evolution seems successfully to have explored and exploited the capacity for water to support larger body forms and avian evolution hasn't?

Why are there no sea birds the size of blue whales?

[thedoc]

Originally Posted by mickthinks

I was thinking about sea birds. And comparing them to sea mammals. The point being that the ancestors of both classes began the evolutionary process out of water and thus the aquatic species of both birds and mammals that we know today are descended from land-dwelling stock, but where there are limits (imposed by the laws of physics) to the size that phenotypes can attain and still remain viable on land or in the air, such bounds are greatly extended in water. Hence many species of whale are far bigger than the largest elephants.

But the largest water-bird, the emperor penguin, is a fraction of the size of the average flightless ostrich. Are there any ideas why mammalian evolution seems successfully to have explored and exploited the capacity for water to support larger body forms and avian evolution hasn't?

Why are there no sea birds the size of blue whales?

Of the top of my head I would guess that sea birds tend to "fly" underwater and sea mammals tend to "swim", I think it is also a factor of the speed which they achieve. Sea birds seem to be a bit faster than most mammals.

[Angakuk]

My guess would be that it might be due, in part, to there not being any sea birds that live exclusively in the water, the way whales do. Penquins spend a considerable part of their lives on land. I don't know of any whales that do that.

[Crumb]

But how many whales do you really know?

[Angakuk]

In the Biblical sense, none.

[ceptimus]

Birds have to return to land to lay their eggs. For birds to grow as big as whales, they would have to evolve to either give birth to live young or have eggs that would hatch underwater (and the chicks not drown when they hatch). Evolution isn't generally capable of making leaps like that - it has to proceed from generation to generation by tiny changes.

[ceptimus]

Thinking about it a bit more, mammals evolved from reptiles. There are some modern day reptiles that give birth to live young and a couple of species of mammals that still lay eggs. I believe there are some species of reptile that have, over evolutionary time, swapped back and forth between egg laying and live birth.

I suppose an egg-laying species evolves into a non-egg-laying one by finding it an evolutionary benefit to lay eggs with a shorter and shorter incubation period until they reach the point where the eggs hatch instantly as they are laid...

So I suppose there is no reason why birds couldn't follow the same evolutionary path. Presumably it would begin with some flightless birds (the extra weight of carrying young to live birth would probably stop flying birds evolving that way). Eventually you could get some flightless birds that gave birth to live young, and then those birds could gradually become more and more aquatic to the point where they could mate and give birth in the water - and then there would be no reason why they shouldn't start to become bigger and bigger...

[The Lone Ranger]

It is indeed generally thought that one major reason why no birds have fully adapted to an aquatic lifestyle is the constraint of having to return to land in order to breed.

A related problem is that in almost all bird lineages, the male's penis is greatly reduced or entirely absent. Practically everything about bird anatomy is about losing anything that might contribute extra weight. Thus, a female bird typically has only one ovary, a male typically has no penis, etc. (There are a few very noteworthy exceptions, by the way.)

In most birds, mating occurs through a "cloacal kiss." The male and female press their cloacae together and the male secretes sperm -- some of which, at least, will make it into the female's reproductive tract. It's a rather inefficient method of sperm transfer compared to direct injection of sperm through a penis, and most birds compensate by producing very large volumes of semen.



Anyway, while it is indeed true that many reptiles have evolved the ability to produce live young, this has never occurred in birds. The reason, in all likelihood, is because the egg-production mechanism for birds is considerably more complex and specialized than it is in nonavian reptiles. That is, birds appear to be "locked into" their particular way of producing offspring, and so bearing of live young is what's known as an "unevolvable trait." (Or more precisely, a number of major mutations would have to occur more or less simultaneously -- and in several different members of the same species that just happen to be living in close proximity -- in order to reorganize a bird's reproductive mechanisms to the point that bearing of live young would be possible. And the chance of that happening is extremely remote.)


It's probably not coincidental that virtually the only birds in which males retain their penises are waterfowl. The "cloacal kiss" is inefficient enough on land, and would probably be all but useless as a sperm-transfer mechanism in water. So, if truly aquatic birds were to evolve, it would probably be from ducks or other waterfowl that still retain their penises. That having been said, it would almost certainly be a great deal easier to re-evolve a penis than it would be to radically redesign a bird's reproductive machinery in order to allow for live births.

[erimir]

It also seems to me that live birth is not enough, they would also need newborns to be much more developed, so that they could swim from birth. Both of these changes would have to precede the transition to a fully aquatic lifestyle.

But it's not clear what the pressure would be for these changes given the way penguins breed currently, with many of them lacking any land based predators. I suppose if only eggs and immobile young were threatened by predators, that could apply pressure in that direction... But it would also apply pressure towards parental nest guarding behavior, which already exists.

It is more conceivable that those changes could happen to another line of birds, but again they'd probably need to be flightless, which would probably require them to lack land predators, which would obviate the need for those changes. A ratite could conceivably benefit... But would be much less likely to subsequently become aquatic.

[thedoc]

Originally Posted by JoeP

not denting the snow

I've been shoveling the snow off the driveway and the top surface is starting to look like the photo. A good cold night and the snow will be crusted over with ice just like the photograph. Thanks.

BTW, on Sun. I measured the snow on the drive at 28" today I measured it at 18", so the snow has melted and compacted 10".

[mickthinks]

Thanks all of you for pitching in with your thoughts, but the questions remain, though I accept responsibility for not articulating them as clearly as I might have.

There are three related questions in descending order of surprise:

1. Why are there no marine birds as big as the ostrich (the largest bird)?
2. Why are there no marine birds as big as the sea elephant (the largest semi-aquatic mammal)?
3. Why are there no marine birds as big as the blue whale (the largest mammal)?

I see that I posed only the third question explicitly, and I'm happy with the answer that the need to spend time on dry land puts an upper limit on body mass that the whales with their permanently aquatic existence are not constrained by.

I am also happy with the connection between egg-laying and the need to function out of the water: all birds lay eggs so no bird can live like a whale.

And yet ...

That still leaves the first and second questions unanswered. Penguins can live like sea-elephants, yet they are nowhere near that size. They aren't even as big as the biggest bird. I find that astounding.

There's also this: while the constraint of having to lay eggs clearly prevents adaption to a fully aquatic niche, this raises the question why flightless birds such as the penguins have not evolved viviparity. TLR touches on this question, and I've found this interesting discussion of it here: Why Don’t Birds Get Pregnant? | The New York Times which seems to be from a sound and reliable source (your opinion most welcome TLR).

[thedoc]

Originally Posted by mickthinks

Thanks all of you for pitching in with your thoughts, but the questions remain, though I accept responsibility for not articulating them as clearly as I might have.

There are three related questions in descending order of surprise:

2. Why are there no marine birds as big as the ostrich (the largest bird)?
3. Why are there no marine birds as big as the sea elephant (the largest semi-aquatic mammal)?
4. Why are there no marine birds as big as the blue whale (the largest mammal)?

I see that I posed only the third question explicitly, and I'm happy with the answer that the need to spend time on dry land puts an upper limit on body mass that the whales with their permanently aquatic existence are not constrained by.

I am also happy with the connection between egg-laying and the need to function out of the water: all birds lay eggs so no bird can live like a whale.

And yet ...

That still leaves the first and second questions unanswered. Penguins can live like sea-elephants, yet they are nowhere near that size. They aren't even as big as the biggest bird. I find that astounding.

There's also this: while the constraint of having to lay eggs clearly prevents adaption to a fully aquatic niche, this raises the question why flightless birds such as the penguins have not evolved viviparity. TLR touches on this question, and I've found this interesting discussion of it here: Why Don’t Birds Get Pregnant? | The New York Times which seems to be from a sound and reliable source (your opinion most welcome TLR).

Just to clarify the first question, how big is the biggest penguin, (the Emperor I believe?) and how big is the ostrich, an answer in lbs. would be better in my estimation, and the data would indicate just how much difference there is, and would eliminate longer legs and other body parts as a factor.

[JoeP]

Originally Posted by thedoc

Just to clarify the first question, how big is the biggest penguin, (the Emperor I believe?) and how big is the ostrich, an answer in lbs. would be better in my estimation, and the data would indicate just how much difference there is, and would eliminate longer legs and other body parts as a factor.

Those are not questions for The Lone Ranger, they are question for The Google.

Ostrich - Wikipedia, the free encyclopedia

Ostriches usually weigh from 63 to 145 kilograms (139–320 lb)

Penguin - Wikipedia, the free encyclopedia

The largest living species is the emperor penguin (Aptenodytes forsteri): on average adults are about 1.1 m (3 ft 7 in) tall and weigh 35 kg (77 lb) or more.

The New Zealand giant penguin was probably the heaviest, weighing 80 kg or more.

- more than 170lb

This Prehistoric Penguin Was the Biggest Penguin Ever

It would have been 4'2" tall, and weighed over 130lbs.

- around 60kg

So the heaviest-ever penguin species were not far off the average weight of ostriches.

[ceptimus]

The (extinct) Elephant bird, native to Madagascar, was reckoned to weigh 350 to 500 kg (770 to 1,100 lb). One theory why it became extinct is that humans used to steal its eggs. Too late now, but that is an example of how a ratite evolving live-birth would have enhanced its chances of survival.