Category Archives: Bird Anatomy

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)

Taking Out The Garbage

Western bluebird carrying fecal sac away from the nest (photo from Wikimedia Commons)

Nests look like safe havens for baby birds but if they’re not kept clean they can quickly become infested with pathogens and parasites, or their smell can attract predators. For songbirds it’s especially important to keep their nests clean. Fortunately their bodies have evolved to make this easy.

In the photo above, a western bluebird is taking out the garbage after visiting his nest. In his beak is a fecal sac, a package of white mucous membrane surrounding the feces of one of his nestlings. Every nestling produces a fecal sac shortly after eating. The packaging makes it easy to keep the nest clean.

What the parents do with the fecal sacs depends on the species. Most drop them far away from the nest but some species, such as American robins, consume the fecal sacs while the nestlings are quite young and carry the sacs away when the nestlings are older.

Just before songbirds fledge their bodies switch from producing fecal sacs to defecating wet feces over the edge of the nest. Robins’ nests don’t have whitewash beneath them until you can see the youngsters above the nest rim. By the time they are messy, baby robins are almost out of there.

Birds of prey aren’t as fastidious. If you watch peregrine and bald eagle nestcams, you’ll see two differences in their nest sanitation:

  1. Raptor nestlings don’t produce fecal sacs. Instead they back up to the edge and aim wet feces away from the nest.
  2. As the nestlings age the parents become lazy housekeepers, often leaving food debris at the nest as a self serve snack for the young.

Birds of prey aren’t worried that predators will smell their nests. That’s why they don’t always take out the garbage.

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

Casting A Pellet

(Red-tailed hawk casts a pellet, photos by Chad+Chris Saladin)

Why does this red-tailed hawk throw up a long gray lump? Is he sick? Not at all. He’s casting a pellet.

Birds’ digestive systems are very different from ours, beginning with their beaks. Since birds don’t have teeth they swallow most of their food whole. The rest of their digestive system is geared to deal with this.

Birds have little saliva and few taste buds compared with mammals, which chew and physically process food as the first step and then subject it to chemical processing as the second step. Birds reverse this sequence. They start chemical digestion in the proventriculus [then the food] undergoes physical processing in the gizzard.

Ornithology, 3rd Edition by Frank B. Gill, page 164

We chew with our teeth and spit out the bones. Birds chew with their gizzards which then collect the bones, fur, and other indigestible bits into a lump. The bird spits out the lump when it’s a convenient size.

Owls, eagles, hawks and falcons cast pellets but so do many other birds “including grebes, herons, cormorants, gulls, terns, kingfishers, crows, jays, dippers, shrikes, swallows, and most shorebirds.” (quote from Wikipedia)

Scientists examine pellets to find out what the bird ate. One of the long-eared owl pellets below was dissected to reveal the rodent bones inside.

Pellets cast by a long-eared owl, dissected to show rodent bones (photo from Wikimedia Commons)

For whatever reason, it’s rare to see a bird casting a pellet so consider yourself lucky if you witness it, as Chad+Chris Saladin did in the photos above.

A NOTE ABOUT HANDLING OWL PELLETS from Wikipedia: Some rodent viruses and bacteria can survive the owls’ digestive system so wear gloves and sterilize the pellets in a microwave oven before handling. A 2005 study found outbreaks of salmonellosis at elementary schools associated with dissection of owl pellets: Smith KE, Anderson F, Medus C, Leano F, Adams J, 2005. Vector-Borne and Zoonotic Diseases,5, 133–136.

(red-tailed hawk photos by Chad+Chris Saladin; pellet photo from Wikimedia Commons; click on the captions to see the originals)

Too Hot!

Great blue heron gular fluttering (photo from Wikimedia Commons)

With highs over 90 degrees and dewpoints at 70 it’s just too hot in Pittsburgh! We’re coping by staying indoors with air conditioning but what do birds do?

This great blue heron in Florida is using at least five techniques for staying cool. 

  • He’s gular fluttering which looks like panting.  Herons are one of several kinds of birds that can vibrate the skin, muscles and bones of their throats to increase heat loss. See more here
  • He has wet belly feathers.  Aaahhhhh!
  • He’s exposing the skin on his legs to cool them off.
  • He’s holding his wings slightly open to cool off his “armpits” and
  • He’s standing in the shade.

He could also try soaring where it’s cooler or sleeking his body feathers to squeeze heat out of his downy undercoat.  (Maybe he’s doing the squeeze thing. I can’t tell.)

That’s all that most birds can do to cope with heat, but the ostrich has an additional amazing solution. 

When a body is warmer than the surrounding air it loses heat.  We know this happens in winter but the ostrich makes it work in summer.  He raises his body temperature in a controlled fashion —  4.2o C (7.5o F) during the day — so that his body loses heat to the outdoors. 

Male ostrich, bare legs (photo from Wikimedia Commons)

For us, it would be like having a fever on a hot day. 

No thanks! It’s too hot already!

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

Turning Green

Male scarlet tanager in August 2015 (photo by Tim Lenz via Flickr, Creative Commons license)
Male scarlet tanager on an ash tree, August 2015, Chemung County, NY (photo by Tim Lenz via Flickr, Creative Commons license)

Yesterday in Schenley Park I saw a scarlet tanager with blotches on his belly.  He was starting to turn green.

Scarlet tanagers (Piranga olivacea) molt twice a year.  In January through March they molt into breeding or “alternate” plumage while on their wintering grounds in South America. The females don’t change color but the males turn from green to scarlet. Young males often retain a bit of green (click here to see).

When the breeding season is over, they molt back to basic plumage in July through September. The males look blotchy at first but when they’re done they’re bright olive green with black wings as shown below.  By then they’re on their way to South America.
(photo at top in August, Tim Lenz; photo below in Oct, Andy Reago & Chrissy McClarren)

Scarlet tanager in October 2015 (photo by Andy Reago & Chrissy McClarren via Flickr, Creative Commons license)
Scarlet tanager in October 2015 (photo by Andy Reago & Chrissy McClarren via Flickr, Creative Commons license)

I was lucky to see yesterday’s scarlet tanager because he hardly made a sound.  Tanagers have stopped singing now that breeding is over.  This one was singing very softly.


p.s.  Did you know that female scarlet tanagers sing?  According to All About Birds: “The female Scarlet Tanager sings a song similar to the male’s, but softer, shorter, and less harsh. She sings in answer to the male’s song and while she is gathering nesting material.”

(photo credits: scarlet tanager turning green by Tim Lenz via Flickr, Creative Commons license; yellow-green scarlet tanager by Andy Reago & Chrissy McClarren via Flickr, Creative Commons license)

Where The Color Comes From

Indigo bunting (photo by Marcy Cunkelman, May 2015)
Indigo bunting (photo by Marcy Cunkelman, May 2015)

Why does the indigo bunting (Passerina cyanea) look so blue?  It isn’t from the color of his feathers.  In dull light he looks gray!

Last month when I wrote about blue morpho butterflies I learned that their color comes not from pigments but from the structures of their scales that reflect blue light.

This same color trick is what makes indigo buntings’ feathers so blue.

Watch the Deep Look video below to learn how it works.


(photo of indigo bunting by Marcy Cunkelman, video from Deep Look on YouTube)

The Dance Makes A Difference

How can we tell when similar birds are actually different species?

In the jungles of Indonesia the male superb bird of paradise (Lophorina superba) is famous for his courtship dance.  To attract a mate he calls loudly, unfurls his jet black feathers and iridescent green apron, and starts to dance.  If he’s really good at it, the female accepts him.

The bird’s color and dance are so mesmerizing that ornithologists at first dismissed the differences between the eastern and western birds. Now they’ve looked more closely.

This video from Cornell Lab of Ornithology shows how the western bird’s behavior convinced scientists to split the superb bird-of-paradise (Lophorina superba) into two species.

The dance makes a difference.  The bird with the sidestep gait is now called the Vogelkop superb bird-of-paradise (Lophorina niedda).


p.s. Volgelkop is the name of a peninsula in western New Guinea, Indonesia where this bird lives.  On the map the peninsula is shaped like a bird’s head.  Vogel+kop means “Bird head” in Dutch.

(video from Cornell Lab of Ornithology)