Ontogeny Recapitulates Ornithology

         Since I got the news that I'm going to be an aunt, I've found myself becoming interested in fetal development. This is not a subject I had previously given much any thought to. As I count down the weeks until I meet my future nephew, I've been keeping track of his size using the customary fruits and vegetables that websites suggest. This is problematic because vegetables are variable in size and also boring. My mind went instead to birds. I started looking up a bird that matched the mass* of the fetus each week, much to the possibly feigned delight of my pregnant sister. I now present a full list for the enjoyment of pregnant folks and the birders who love them.

* Note that most birds are less dense that a fetus, and therefore greater in size at the same mass.

PC: Gregory Smith

Week: 9
Mass: 2 g
Bird: Bumblebee Hummingbird

PC: Eric Carlson

Week: 10
Mass: 4 g
Bird: Anna's Hummingbird

PC: Francesco Veronesi

Week: 11
Mass: 7 g
Bird: Wilson's Warbler

PC: James Diedrick

Week: 12
Mass: 14 g
Bird: Vermillion Flycatcher

PC: Eric Carlson

Week: 13
Mass: 23 g
Bird: Tree Swallow

PC: Eric Carlson

Week: 14
Mass: 43 g
Bird: Snowy Plover

PC: Eric Carlson

Week: 15
Mass: 70 g
Bird: Northern Pygmy-Owl

PC: Andy Morffew

Week: 16
Mass: 100 g
Bird: Green Jay

PC: Eric Carlson

Week: 17
Mass: 140 g
Bird: Burrowing Owl


Week: 18
Mass: 190 g
Bird: Merlin

PC: Judy Gallagher

Week: 19
Mass: 240 g
Bird: Purple Gallinule

PC: Jerry Kirkhart

Week: 20
Mass: 300 g
Bird: Long-tailed Jaeger

PC: Ron Knight

Week: 21
Mass: 360 g
Bird: White-tailed Ptarmigan

PC: Andy Morffew

Week: 22
Mass: 430 g
Bird: Swallow-tailed Kite

PC: Bettina Arrigoni

Week: 23
Mass: 501 g
Bird: Bristle-thighed Curlew


Week: 24
Mass: 600 g
Bird: Spotted Owl

PC: David Alvarez Lopez

Week: 25
Mass: 660 g
Bird: Fulvous Whistling-Duck

PC: Ingrid Taylar

Week: 26
Mass: 760 g
Bird: Tufted Puffin

PC: Great Sand Dunes NP

Week: 27
Mass: 875 g
Bird: Swainson's Hawk

PC: Stephen Lester

 Week: 28
Mass: 1005 g
Bird: Western Gull

PC: Eric Carlson

Week: 29
Mass: 1153 g
Bird: Common Raven

PC: Cuatrok77

Week: 30
Mass: 1319 g
Bird: Anhinga

PC: Eric Carlson

Week: 31
Mass: 1502 g
Bird: Roseate Spoonbill

PC: Ozzy Delaney

Week: 32
Mass: 1702 g
Bird: Barnacle Goose

PC: Bob Peterson

Week: 33
Mass: 1918 g
Bird: Turkey Vulture

PC: Silver Leapers

Week: 34
Mass: 2146 g
Bird: Common Eider

PC: Cuatrok77

Week: 35
Mass: 2383 g
Bird: Wood Stork

PC: Francesco Veronesi

Week: 36
Mass: 2622 g
Bird: Arctic Loon


Week: 37
Mass: 2859 g
Bird: Greater Sage-Grouse (male)

PC: Bettina Arrigoni

Week: 38
Mass: 3083 g
Bird: Laysan Albatross


Week: 39
Mass: 3288 g
Bird: Lesser Sandhill Crane

PC: M. J. Taylor

Week: 40
Mass: 3462 g
Bird: Brown Pelican

The Sibley Guide to Birds, 2nd Ed., by David Sibley, 2014


  1. Oh holy crow. This is so clever and so illuminating!! As a two time mom, I was OK with the bee Hummingbird, but couldn't help wincing at the thought of bringing forth a brown pelican. Actually the Raven gave me pause. And I thought about the turkey vulture and was amazed that it outweighs a male snowy owl. Thank you Sarah! Sibley's weights and wingspans are two of my favorite features of his miraculous work. This is the bomb!

    1. I'm so glad you enjoyed it! Thanks for taking the time to write such a thoughtful comment!

  2. Not only are you a crack birder person and stellar (Steller?) birdathon cap'n, and a great writer, but you're a complete goof. I like that in a person.

    1. Awww, thanks Murr. I also appreciate your dedication to goofiness.

  3. I was rooting for the stork...

  4. Gross.

    But also, mass-wise, 7.17 Burrowing Owls fit in one Western Gull? My mind can't handle this information.

    1. Bird mass is a very weird thing. Those feathers really make comparisons difficult. Apparently owls are mostly made of air.

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  7. Thanks for sharing all these incredible photos of all these gorgeous birds. So many have such beautiful coloring. We hope your are having a wonderful start to your week.
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