How dense are birds? Some new (old) data. Also, hummingbirds and ketchup.

June 1, 2013

I recently reread Dubach (1981), “Quantitative analysis of the respiratory system of the house sparrow, budgerigar and violet-eared hummingbird”, and realized that she reported both body masses and volumes in her Table 1. For each of the three species, here are the sample sizes, mean total body masses, and mean total body volumes, along with mean densities I calculated from those values.

  • House sparrow, Passer domesticus, n = 16, mass = 23.56 g, volume = 34.05 mL, density = 0.692 g/mL
  • Budgerigar, Melopsittacus undulatus, n = 19, mass = 38.16 g, volume = 46.08 mL, density = 0.828 g/mL
  • Sparkling violetear,* Colibri coruscans, n = 12, mass = 7.28 g, volume = 9.29 mL, density = 0.784 g/mL

* This is the species examined by Dubach (1981), although not specified in her title; there are four currently-recognized species of violetears. And apparently ‘violetear’ has overtaken ‘violet-eared hummingbird’ as the preferred common name. And as long as we’re technically on a digression,  I’m almost certain those volumes do not include feathers. Every volumetric thing I’ve seen on bird masses assumes plucked birds (read on).

This is pretty darned interesting to me, partly because I’m always interested in how dense animals are, and partly because of how the results compare to other published data on whole-body densities for birds. The other results I am most familiar with are those of Hazlehurst and Rayner (1992) who had this to say:

There are relatively few values for bird density. Welty (1962) cited 0.9 g/mL for a duck, and Alexander (1983) 0.937 g/mL for a domestic goose, but those values may not take account of the air sacs. Paul (1988) noted 0.8 g/mL for unspecified bird(s). To provide more reliable estimates, the density of 25 birds of 12 species was measured by using the volume displacement method. In a dead, plucked bird the air-sac system was reinflated (Saunder and Manton 1979). The average density was 0.73 g/mL, suggesting that the lungs and air sacs occupy some quarter of the body.

That result has cast a long shadow over discussions of sauropod masses, as in this paper and these posts, so it’s nice to see similar results from an independent analysis.  If you’re curious, the weighted mean of the densities calculated from Duchard’s Dubach’s (1981) data is 0.77. I’d love to see the raw data from Hazlehurst and Rayner (1992) to see how much spread they got in their density measurements.  Unfortunately, they did not say which birds they used or give the raw data in the paper (MYDD!), and I have not asked them for it because doing so only just occurred to me as I was writing this post.

There will be more news about hummingbirds here in the hopefully not-too-distant future. Here’s a teaser:

SkeletonFULL

Yes, those are its hyoids wrapped around the back of its head–they go all the way around to just in front of the eyes, as in woodpeckers and other birds that need hyper-long tongue muscles. There are LOADS of other interesting things to talk about here, but it will be faster and more productive if I just go write the paper like I’m supposed to be doing.

Oh, all right, I’ll say a little more. This is a  young adult female Anna’s hummingbird, Calypte anna, who was found by then-fellow-grad-student Chris Clark at a residential address in Berkeley in 2005. She was unable to fly and died of unknown causes just a few minutes after being found. She is now specimen 182041 in the ornithology collection at the Museum of Vertebrate Zoology at Berkeley. Chris Clark and I had her microCTed back in 2005, and that data will finally see the light of day thanks to my current grad student, Chris Michaels, who generated the above model.

This bird’s skull is a hair over an inch long, and she had a body mass of 3.85 grams at the time of her death. For comparison, those little ketchup packets you get at fast-food burger joints each contain 8-9 grams of ketchup, more than twice the mass of this entire bird when it was alive!

References

  • Dubach, M. 1981. Quantitative analysis of the respiratory system of the house sparrow, budgerigar and violet-eared hummingbird. Respiration Physiology 46(1): 43-60.
  • Hazlehurst, G.A., and Rayner, J.M. 1992. Flight characteristics of Triassic and Jurassic Pterosauria: an appraisal based on wing shape. Paleobiology 18(4): 447-463.
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12 Responses to “How dense are birds? Some new (old) data. Also, hummingbirds and ketchup.”

  1. Vertebrat Says:

    Measuring plucked birds may be fine for sauropod-estimation purposes, but for understanding birds it’s odd to ignore 1/6 of the mass (and much of the volume, if you count the air underneath). It makes sense to ignore mammal hair, but ignoring feathers is almost like ignoring scales – are they not part of the bird?

    Duchard = Dubach, right?

  2. Mike Taylor Says:

    Wait, why would it make sense to ignore mammal hair on this basis?

  3. Mike Taylor Says:

    The meta-point is that, as always, it’s fine to include or exclude feathers — provided only that you’re explicit about what you’ve done. Best is to of course to measure masses and volumes bother with and without integument.

  4. Matt Wedel Says:

    Measuring plucked birds may be fine for sauropod-estimation purposes, but for understanding birds it’s odd to ignore 1/6 of the mass (and much of the volume, if you count the air underneath). It makes sense to ignore mammal hair, but ignoring feathers is almost like ignoring scales – are they not part of the bird?

    Of course. I am pretty sure the reason that Hazlehurst and Rayner and others have plucked their birds before dunking them is practical, not philosophical. First, depending on how fluffed they are, feathers can trap varying volumes of air next to the body, and it is probably simpler to just do away with that as a methodological shortcut (while recognizing that of course this is important to understanding how birds actually live). Second, and probably more pertinent, the shafts of feathers are usually air-filled but can become waterlogged, which would throw off the results. Birds have ways of keeping this from happening, but for researchers it could be a real headache–potentially you’d have to check every shafted feather and make sure it hadn’t gotten filled with water, which would probably take longer than it would take the feathers to dry anyway.

    Duchard = Dubach, right?

    Gah, yes. Thanks for catching that! I wonder where I pulled Duchard from?

  5. Mark Robinson Says:

    Wow, those hyoids are amazing.

    I think it makes sense to exclude hair and feathers when calculating densities since an individual’s volume may vary considerably depending on how cold, frightened, or un-groomed/preened it is. Presumably you would never try to measure the volume of sparsely-haired animals like elephants by trying to define their shape by the extent of their few hairs.

    To be mathematically rigorous when calculating the average densities for each bird species, you should calculate the density of each individual and then take the average of that. The difference prob won’t be large but may become important when used in other formulae as any uncertainties or errors are magnified.


  6. I’d say that many birds are pretty dense – just look at the many stereotypical behaviours they show instead of ever learning ;)

  7. Mike Taylor Says:

    Assuming that hummingbirds are about as dense as average birds — 0.75 — then in order to mass 3.85 g, the bird above would need to have a volume of 5.133 cm3, which is the volume of a 1.73 cm cube.

    If “this bird’s skull is a hair over an inch long”, we can call that 2.54 cm (and shame on you, Wedel, for using Imperial units on a science blog!) That means that the volume of the bird is the same as that of a cube 68% as long as its skull.

    That takes some believing.

    On the other hand, I’ve learned quicky that intuition is a bad guide to anything when it comes to hummingbirds. As will soon become clear in a followup post.

  8. Vertebrat Says:

    Wait, why would it make sense to ignore mammal hair on this basis?

    Because there’s usually not much of it, and it has fewer functions — a shaved mammal is not as crippled as a plucked bird.

    That means that the volume of the bird is the same as that of a cube 68% as long as its skull.

    That’s mostly because the skull length includes a long nectarvore beak. The rest of the skull is only about a 1cm sphere = 0.5 cm3, and the body is something like a 20x14mm cylinder = 3.1 cm3. That leaves 1.5 cm3 for beak and neck and legs.

  9. Mike Taylor Says:

    “A shaved mammal is not as crippled as a plucked bird.”

    Tell that to a frozen mouse.


  10. […] hummingbirds are slightly longer-billed than the Anna’s hummingbird mentioned in this post, but even so the skulls are probably no more than 30mm long. I recently helped London clean up a […]

  11. Charlotte Brassey Says:

    A word of caution. Dubach only estimates volume of the respiratory system using the silicon casting technique. She’s estimated ‘total body volume’ by assuming a specific gravity of 1, and adding together weighed body mass and respiratory system volume. Any density value calculated from this can’t really be compared to other literature values derived experimentally from fluid displacement etc.

  12. Matt Wedel Says:

    Ah, that is an extremely useful thing to know, which I had completely missed. Many thanks for enlightening us!


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