Category Archives: Bird Anatomy

Smaller is Normal

Song sparrow, western Pennsylvania (photo by Steve Gosser)

From their 29% population decline to the continued loss of federal protection the news about birds has not been good in recent months. When a December 2019 study from Chicago’s Field Museum found that North American birds have been shrinking since 1978 you may have wondered, “Is this bad news for birds?” Not exactly.

The study published in Ecology Letters measured 70,000 window-killed birds collected in Chicago since 1978. Analysis showed that the 52 species significantly declined in body size during the 40 year period (1978-2018). This mirrors a 2010 study conducted at Powdermill Nature Reserve in Pennsylvania which used 46 years of banding data (1961-2007) to analyze the body size of nearly 500,000 birds in 102 species. Powdermill also saw a decline in body size.

Both studies correlated the annual mean summer temperature of the species’ breeding range and reached the same conclusion: As the climate heats up, birds are getting smaller.

We should expect this.

There’s a biological rule of thumb called Bergmann’s Rule which states that, within a species, populations living in colder climates have larger body size than those in warmer climates. Bergmann’s explanation is that large animals have a lower surface-area-to-volume ratio so they lose heat more slowly in cold climates while small animals have a higher surface-to-volume ratio and can cool off faster when it’s hot.

Song sparrows (Melospiza melodia) provide a good example of Bergmann’s rule because they range across North America from Alaska to Newfoundland and south to Mexico. I saw their variability up close in the Carnegie Museum of Natural History’s Section of Birds in December 2016. My photo below shows sparrows collected in Alaska in the top row, sparrows from Pennsylvania on the bottom.

Song sparrows in Carnegie Museum of Natural History collection, Alaska on top row, Pennsylvania on bottom row (photo by Kate St. John, Dec 2016)

Here’s a closeup placed side by side (below):

  • On the left, two song sparrows collected in Pennsylvania: Pittsburgh (leftmost) and Geneva Marsh.
  • On the right, song sparrows collected in Alaska’s Aleutian Islands at Unalaska (leftmost) and Sanak.

Alaskan song sparrows are so large that they have to be placed sideways in the tray!

Smaller size is normal where it’s warmer.

It isn’t bad news for birds and it tells us two additional things:

  1. Birds’ bodies have been registering climate change long before we humans noticed or admitted it.
  2. Birds can evolve quickly when they have to.

Read about the Field Museum study at North American birds are shrinking. Read more about the Powdermill study at Birds are getting smaller.

p.s. This article was inspired by Andrew Nikiforuk’s As The Birds Vanish.

(top photo by Steve Gosser, remaining photos by Kate St. John)

How to Identify Feathers

Feathers of a great spotted woodpecker, left by a predator, Germany (photo from Wikimedia Commons)

When we find a feather we often wonder, “What bird dropped this feather? What species is this?” Here are some quick tips for identifying feathers.

Before we begin, keep in mind that without a permit it is illegal to collect and/or keep feathers of any native non-game species. You can touch feathers, flip them over, and take lots of photos but you must leave the feathers behind.

Photos are what you really need anyway. Include an object near the feather to give it a sense of scale (size). Remember the location and habitat where you found it so you know what species are possible. Now you’re ready to figure out whose feather it is.

First determine the feather type so you know where it came from on the bird’s body. At this point you don’t care about color.

Types of feathers — Not To Scale (translated from Spanish via Wikimedia Commons)

In the wild you’re most likely to find tail, wing or contour feathers, the same ones you see on the bird. The descriptions below include parts of a feather vocabulary defined here.

  • Rectrix (tail): Tail feathers (plural:rectrices) have barbs of equal length on both sides of the vane. (red arrows)
  • Remige (wing): Wing feathers have short barbs on one side, long ones on the other. (yellow arrows short and long)
  • Contour feather: covers the body
  • Semiplume: insulation under the contour feathers
  • Down: the warmest insulation near the skin
  • Bristle: sensory vane near beak and eyes (unlikely to find)
  • Fitoplume: sensory vane on wings (unlikely to find)

Next, think of birds with colors and patterns at that location on the body.

For additional help use the U.S. Fish and Wildlife Feather Atlas ID Tool for North American birds. At the Feather Atlas you’ll need to know the feather’s size in centimeters before you begin.

Ready for a quiz?

A. The feathers shown above are from a great spotted woodpecker eaten by a predator in Germany. What body part did they come from?

B. Here are two feathers of North American backyard birds. It’s a little harder to tell what body part they came from. (Length: red=9-10cm, blue=12-14cm) What do you think? Can you identify the species?

Two feathers from North American birds (photo from Wikimedia Commons)

It’s challenging to identify feathers. Here are more resources to help.

(photos from Wikimedia Commons; click on the captions to see the originals)

What Do Diving Ducks Hear Underwater?

  • Long-tailed duck (photo by Steve Gosser)

Last summer a University of Delaware study found out what diving ducks can hear underwater. Why is this important? If we know what ducks can hear, we can save their lives.

Long-tailed ducks, common eiders and surf scoters eat crustaceans and mollusks that they pull from the ocean floor. Their populations are in steep decline, in part because hundreds of thousands of them die as bycatch in gillnets.

The diagram below shows a gillnet used for cod fishing in Newfoundland. Though no one fishes for cod anymore, gillnets are still used for other fish where ducks are diving.

Diagram of cod gillnet in Newfoundland, 1882 (image from Wikimedia Commons)
Drawing in the gillnet near Rakovníka (photo from Wikimedia Commons)

Federal fishing laws solved the bycatch problem for dolphins and whales by requiring pingers to warn the mammals away. Fish can’t hear the pingers but dolphins can. Is there a sound that will work for ducks?

University of Delaware grad student Kate McGrew tested long-tailed ducks, common eiders and surf scoters and found out they can hear 1-3 kHz underwater.

Long-tailed ducks can hear 1 to 3 kHz (screenshot from NYTimes ScienceTake video)

Fish cannot hear above 2 kHz so there’s hope for the ducks.

This New York Times ScienceTake video shows how McGew trained the ducks.

Read more in this University of Delaware article: What do ducks hear?

(photos by Steve Gosser and Cris Hamilton)

Wondering About Feathers

Body feather of a male peacock, Pavo cristatus (photo from Wikimedia Commons)

When we think of birds we take feathers for granted. But what do we really know?

How do feathers grow? What holds them on the bird? How are feathers replaced? Do feathers have nerves in them? How do they become curved?

On Throw Back Thursday, find the answers in this vintage article: Feather Facts.

(photo from Wikimedia Commons; click on the caption to see the original)

A Surprising Look at Robins

American robin in flight (photo from Wikimedia Commons)

We usually see American robins (Turdus migratorius) with their wings closed. They perch in a tree, sit on a nest, or walk with their classic 3-steps-and-stop gait. Even in flight robins close their wings, flapping and gliding in a pattern similar to their walk.

This view of a robin with open wings reveals a surprise. The robin’s armpits, called axillaries, match its belly.

Check out this vintage article on axillaries to see other birds with hidden surprises.

(photo from Wikimedia Commons; click on the caption to see the original)

Black Tips Help Them Soar

American white pelican (photo from Wikimedia Commons)

Why do so many soaring birds and seabirds have white wings with black tips? For starters, black plumes are more durable than white ones, providing an advantage at the tips. New research this summer shows another possibility.

A study at the University of Ghent determined that the black-and-white color combination generates extra lift because of the temperature difference between the colors.

Using taxidermied wings, a heat lamp and a wind tunnel, the researchers measured airflow over the wings in a variety of wind conditions. They found that:

… dark feathers grew hotter than lighter colored feathers and they also gained heat faster than lighter colors. The researchers found temperature differences as great as nine degrees between black and white feathers on the same wings—enough to create a convection current in the air just over the wing, moving from the bird’s body outward along the wing.

— from Study suggests dark-colored wing feathers may help birds fly more efficiently,

The study described a convection current that works like this.

Differential heating creates a convection current just over the wings (photo of American white pelican from Wikimedia Commons, annotation by Kate St. John)

American white pelicans and northern gannets both benefit from this additional lift.

Northern gannet in flight (photo by Andreas Trepte, via Wikimedia Commons)

So does the osprey because he’s black and white beneath his wings.

Osprey at Duquesne, PA (photo by Dana Nesiti)

How ingenious!

Black wingtips help them soar.

(pelican and gannet photos from Wikimedia Commons; osprey photo by Dana Nesiti)

White Barn Owls Stun In Moonlight

Barn owl, Scotland (photo from Wikimedia Commons)

Most owl species have camouflage-colored bellies, but most barn owls (Tyto alba) do not. Though their backs blend into their surroundings, the majority have brilliant white faces, bellies, underwings and legs. The rest are better camouflaged in rusty red, below.

Barn owl with reddish belly (photo from Wikimedia Commons)

The white color stands out in moonlight but is this visibility a disadvantage? Does the white owl’s prey see it coming and escape? Are reddish owls more successful on moonlit nights? Researchers ran tests to find out.

Barn owl watching for prey (photo from Wikimedia Commons)

In a barn owl study in Switzerland, scientists have been tracking plumage, prey availability, moon phases and breeding success for over 20 years. When they correlated moon phase with hunting success, they found that reddish barn owls have lower success on full moon nights than white ones.

This seemed very odd so they set up an experiment with full moon lighting and two taxidermied owls posed in flight — one white, one reddish. When a vole was placed in the “moonlit” room and presented with a flying (stuffed) owl, it froze in place for an extra 5 seconds when it saw the white one. Those 5 seconds were just enough time for the white owl to pounce. The reddish owl was out of luck. Apparently the glowing white plumage has its advantages.

White barn owls are stunning in moonlight.

Barn owl in flight, glowing white (photo from Wikimedia Commons)

Read more at “Moonlight Helps White Barn Owls Stun Their Prey” in Smithsonian Magazine.

(photos from Wikimedia Commons; click on the captions to see the originals)

Beaks Built For Fighting

Lesser violetear (green) confronts a sparkling violetear (photo by Julian Londono via Wikimedia Commons)

For centuries scientists have assumed that hummingbird beaks are always shaped for the flowers they feed on, but a recent study of their nectar-feeding mechanisms produced a surprising result. Some male hummingbirds have beaks that are inefficient for feeding but great for fighting.

Alejandro Rico-Guevara, an evolutionary biologist at UC Berkeley, assembled a team to study the biomechanics of nectar drinking. Using high-speed cameras they watched the entire feeding apparatus including bill shape, tongue shape, fluid trapping and elastic pumping.

Surprisingly, they found that male beak shapes in several South American species make it harder for males to draw in nectar. The females have nectar beaks but the males have straight dagger beaks or backwards facing teeth and hooked tips. You can see some of these features on the male tooth-billed hummingbird (Androdon aequatorialis) below.

Tooth-billed hummingbird (photo by Andres M. Cuervo via Wikimedia Commons)

Among these species the females don’t fight much but the males are extremely belligerent. Examples include the tooth-billed hummingbird (Androdon aequatorialis) above, and the sparkling violetear (Colibri coruscans) and saw-billed hermit (Ramphodon naevius) below.

Sparkling violetear looks threatening (photo from Wikimedia Commons)
Saw-billed hermit (photo from Wikimedia Commons)

This video from the UC Berkeley study shows what those beaks are really used for!

For more details see this article at Gizmodo and the study itself at Oxford Academic.

(photos from Wikimedia Commons; click on the captions to see the originals. video from UC Berkeley Research)

Spiny Tongues and Glowing Beaks

Puffin with a beakful of fish (photo by Steve Garvie, Creative Commons license on Flickr)

There are many cool things about Atlantic puffins (Fratercula arctica). Here are two things that might come as a surprise.

(1) When Atlantic puffins fly back to their nests to feed their hungry chicks they need to carry as many fish as possible. How do they clamp them in their beaks? They press their spiny tongues on the fish to hold them against the roof of the mouth.

(B) We think puffins’ beaks are beautiful but we’re seeing only half of it. Where we see yellowish stripes the puffins see glowing ultraviolet. Read more about their colorful beaks in this article from

Atlantic Puffins engaged in (a) billing behaviour, which is associated with sexual signalling (Photo: T. Finch). We identified photoluminescence on the cere (arrow) and lamellae of Puffins found deceased in (b) UK and (c) Canada.

Atlantic Puffins engaged in (a) billing behavior; (b, c) photoluminescence of the bills in UV light (Photo: T. Finch from “Photoluminescence in the bill of the Atlantic Puffin Fratercula arctica,” scientific figure via ResearchGate).

(photo with fish by Steve Garvie, Creative Commons license on Flickr; puffins ‘billing’ in courtship and UV glowing beaks from Photoluminescence in the bill of the Atlantic Puffin Fratercula arctica via Researchgate)

Reflect Light, Stay Cool

Nankeen kestrel in Australia (photo from Wikimedia Commons)

How do birds stay cool in hot climates, especially when there’s no shade?

A 2018 Australian study in the journal Nature found that some birds can reflect the hottest part of sunlight, the near infrared (NIR) spectrum.

Near infrared is long-wavelength light beyond the red end of the visible spectrum. Though we can’t see this wavelength we can feel its heat. In fact more than half the sunlight that reaches Earth is in the infrared spectrum, as shown in the graph below.

Most of the sunlight that reaches the Earth’s surface is in the infrared range (simplified image from Wikimedia Commons)

Australia is a good place to study cooling techniques in birds because 70% of the continent is hot, dry and very sunny. The Australian study examined museum specimens of 90 species, classifying them by habitat and testing them for their NIR reflectant properties. Two species stood out.

The nankeen kestrel (Falco cenchroides), named for his yellow color (above), reflects near infrared light from the crown of his head. The azure kingfisher (Ceyx azurea) stays cool by reflecting NIR from his chest. Their feathers can reflect NIR because they have rounder barbs and denser barbules.

Azure kingfisher, Queensland, Australia (featured picture on Wikimedia Commons)

Here’s a graph from the study that compares them with two other species.

Graph of four species (image from Reflectivity article in Nature Communications)

The nankeen kestrel and azure kingfisher are at the top of the NIR reflective scale but low reflectors of visible and UV light. The reverse is true of the blueish bird, a male superb fairywren (Malurus cyaneus). He’s great at reflecting UV and visible light, probably because he lives where it’s moist and shady. The great cormorant (Phalacrocorax carbo) doesn’t reflect much light at all.

Interestingly, near infrared reflectivity is more prevalent in small birds because they benefit more for their size. You can’t tell it from the photo but the azure kingfisher is only as big as a sparrow.

Too hot? Reflect near infrared light to stay cool.

(image credits: nankeen kestrel, sunlight graph and azure kingfisher from Wikimedia Commons. Graph from “Reflection of near-infrared light confers thermal protection in birds” at, Creative Commons license. Click on the captions to see the originals)