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The question about a diet of dust brought up a few creatures that survive on dust, however they were all insect sized or smaller. This got me thinking about the Blue Whale which survive on a diet of Krill.

I'm looking to create something that survives on nutrients and minerals in the air that is roughly like a bird; i.e. has wings, similar size, hollow bones. I believe that it is impossible for such a creature to survive in an earth like environment simply by feeding on nutrients and other dust-like particles (such as pollen or minerals dust) that either float through the air or settle on the land. (Please do tell me if I'm wrong)

I'm looking to find what environmental conditions are required for such a creature to survive. Answers should ideally consider:

  • diet breakdown
  • energy requirements of the creature
  • where the particles come from
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  • $\begingroup$ Must it feed by flying or can it perch in areas with concentrated air flow/concentration such as a moutain pass or at the base of a waterfall? I am assuming we can create fictional microscopic gnats or something that are invisible so count as air. $\endgroup$ – kaine Oct 29 '14 at 14:16
  • $\begingroup$ It can perch. And yes, feel free to invent invisible air krill. But if you do, please explain their existence. $\endgroup$ – Mourdos Oct 29 '14 at 14:17
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I don't believe such a creature is possible.

There are several types of creature which each much smaller organisms than themselves, the two which spring to mind are whales and jellyfish.

The main reason I don't believe this would translate to birds is because of the energy involved. A blue whale doesn't have to exert a lot of effort to catch it's prey... it's swims, mouth wide open and scoops them up as it goes. It uses relatively little energy (bear in mind that the sea holds most of it's weight). A jellyfish is much the same, it just floats there and food comes to it.

If you consider a bird they are much more active and flap their wings (in some cases at an incredible rate to stay airborn). They expend a lot of energy to do this... in order to catch enough food to supply the required energy the air would need to be thick with bugs (I doubt even Scottish midge season would cut it!). If you watch swallows/house martins hunting flies you'll see it's a very energy intensive job!

So what's the solution?

The closest I can come up with is some sort of floating jellyfish, a natural hot air balloon with long tendrils which could catch and then absorb whatever hit them. I suspect such a creature would be a fair size and would drift with the wind (and so doesn't require a lot of energy to sustain it). Whether such a creature is feasible from other perspectives though I'm not sure!

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    $\begingroup$ soaring doesn't take much energy and a air feeding bird would need to land less $\endgroup$ – ratchet freak Oct 29 '14 at 15:41
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    $\begingroup$ I like the idea of a floating upper atmosphere jellyfish creature. Don't think that answers the question well, but nice idea. $\endgroup$ – Twelfth Oct 29 '14 at 19:19
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Essentially the question is one of balancing energy input and energy output for the organism. I will list different ways on increasing input and decreasing output.

Increasing Input:

While Earth’s atmosphere doesn’t typically contain a high density of organic matter there are times when it becomes substantially more dense. Flying insects frequently swarm and reach extremely high densities. I think feeding off of larger insects like locusts would not qualify as passive feeding, but if you make the insects smaller I presume eventually they would qualify. Even with very small gnats a dense enough swarm should allow for efficient feeding.

Another source of air-borne food is pollen. On earth pollen densities can get pretty high depending on the density of pollen producing plants and the season. An environment in which pollen densities became high enough could allow a bird to feed off of it efficiently. Potentially you could explain such a phenomenon as symbiotic. Many insect species on earth subsist entirely off of energy voluntarily provided to them by plants in order to facilitate pollination. You could imagine the same symbiosis could extend to your bird. The energy costs for feeding the bird would be quite high for the plant, but your bird could potentially carry the pollen vast distances.

For either of these strategies to work you’d want the bird to have a large input surface area, analogous to the mouth of a whale.

Decreasing Output:

The simplest way to decrease output is to have the bird dormant whenever possible. It flies only to feed and otherwise does not expend any unnecessary energy. When it does fly it would need to rely on updrafts and do mostly soaring and gliding.

The math is too fuzzy to really say whether all these strategies would work for sure, but I think they are well within the realm of possibility. There are a lot so other possibilities (like making it cold-blooded), but they would all involve moving further away from what we think of as a bird.

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A creature like this is barely possible.

A bird usually needs between 1/4 and 1/2 of it's body weight in food to live each day. As we will be talking chitinous insects and an active lifestyle, lets use the upper end of that spectrum.

Lets say that allows the bird to fly 1/8th of each day which means the birds is small and its flight is supported by environmental conditions so the flying is easy. This seems high as I am envisioning a house sparrow as the control. With flying creatures smaller=easier.

Using a very rough average sparrow: it is about 30g and has a cross sectional area of about 17 square cm. As it needs to catch its food, lets optimistically say that its feathers collect all food it touches and it picks the food from its feathers while perched. For fun lets use the average airspeed velocity of an unladen European swallow which seems to be about 11 meters per second.

Using these assumptions which for actual science are insufficient, the bird would need to fly through air with an average food concentration of .075g/m^3 of air. If the bird only ate gnats, that would be on average 75 gnats for every cube meter or it would have to catch an eat a gnat every 1/8th of meter it flies. That is 88 gnats a second.

Why do I think that is still possible? I am going to assume the bird eats pollen, spores, or floating insects which cannot avoid it; while resting/picking its feathers it is still able to catch food by positioning itself in a windy area; and it opens its wings to maximize its catching ability. As we are describing an ideal environment for such a creature and it needs a lot less food as it would fly a lot less, it is possible that such a creature could evolve to take advantage of this described niche.

Please note, however, that any such creature is highly dependent on its environment. Most seem like they would be willing to catch larger insects or pick up food on the ground if available. It would also be high susceptible to hazards such a predators or environment contamination. This works well in water as its buoyancy allow it to hold a higher food concentration that air can. It is fun to dream of a giant bird which flies efficiently all the time by gliding everywhere. At the elevations where gliding is possible, however, it would not make sense to me for there to be enough food.

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The problem is not the bird, the problem is the food. It must also stay airborne and be present in sufficient amounts. However even a dust size plant would eventually sink down in the Earth atmosphere.

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I expect that bats in a sufficiently insect rich diet wouldn't have to aim for them, just fly with their mouth open.

Riffing off the jellyfish answer, if the concentration of helium in the atmosphere were a little higher, then an animal may be able to collect helium over its lifetime, reducing the energy expense of maintaining "flight". At that point it could be a very large creature with very little mass, requiring very little energy. However, storms, wind, and such would cause great injury.

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  • $\begingroup$ He can survive with just helium? $\endgroup$ – Vincent Nov 2 '14 at 15:37
  • $\begingroup$ @vincent no, it would eat other things. But as it breathed it would have a membrane or something that would collect helium and trap it, while rejecting other gases. $\endgroup$ – Adam Davis Nov 2 '14 at 20:31
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I'm going to go a slightly different direction here and give you a 'yes' answer.

First we have to imagine a densely abundant food source. There are animals that have algae growing inside their cells so that they can get energy directly through photosynthesis. Imagine a sort of midge or even slightly larger insect that had done so. Its life pretty much consists of collecting sunlight, mating, and going where the wind sends them so that the eggs they lay have less competition when they hatch. All quite easily conceivable.

Now for your bird. As has been pointed out, flying fast enough to catch enough to feed a bird requires an unlikely density to sustain a population of such birds. Soaring would relieve a lot of the energy requirement, but the controlling factor that causes huge swarms is usually either fast wind currents, or brief mating periods, so I'm going to rule these out. You need a largely stationary bird.

There are plenty of examples of sea creatures that do what we are talking about, just google 'filter-feeder', but these rely on an organ within the respiration pathway to collect biological matter for eating. Evolution has a large stretch to go before that will happen with birds. One could imagine a pelican like bird with a baleen like structure in a very large mouth that just sits in the wind all day smiling. This really isn't too far off from how flamingos eat. I'd give that one a plausible on the Mythbusters scale.

I think it is rather more likely that a bird would use its already existing structure though: its wings. A bird with slightly evolved feathers designed to catch small insects could sit out in the wind with its wings outstretched (clinging tightly to a treetop), then pick the bugs out by combing its beaks through its feathers. This is very similar to what sea fans and anemones do. Even without an evolutionary change to the feathers, birds could wet their feathers with sticky saliva to accomplish the same thing. Expending very little energy for a very good payoff.

I think such a bird would likely eventually lose the power of flight as its feathers became more and more specialized, but you've got a few millennia of what you want.

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