Category Archives: Bird Anatomy

Time To Molt

Ruby-throated hummingbird molting in August in Illinois (photo from Wikimedia Commons)

12 July 2022

Feathers are vital to a bird’s survival but they wear out and have to be replaced by molting. The best time to do this is when feathers are not urgently needed for migration, courtship or warmth. That makes summer the time to molt. Here are a few examples.

Ruby-throated hummingbirds (Archilochus colubris), above, have to look flashy at the start of the breeding season so they molt their body feathers from June to August. On the wintering grounds they molt flight feathers in preparation for their strenuous spring migration. Look closely at ruby-throats this summer and you’ll see that their body feathers are not as perfect as they were in May.

Northern cardinals (Cardinalis cardinalis) wrap up their last brood of the season in mid summer and begin to molt in mid July. By August they will look very ragged, male and female shown below. Some will be bald.

Male northern cardinal molting in June (photo from Wikimedia Commons)
Female northern cardinal molting in August (photo from Wikimedia Commons)

Male and female peregrine falcons (Falco peregrinus) molt at slightly different times. Females molt their primary wing feathers while they’re incubating eggs (March-May) because their mates are doing all the hard flying to provide food. The males molt their primaries in July after teaching the young to hunt.

Birds molt the same flight feather on each side of the body so that flight remains balanced. Morela’s wings look sleek while she’s sunbathing because she replaced her wing feathers a few months ago.

Morela’s wing feathers are not in molt, 10 July 2022 (photo from the National Aviary falconcam at Univ of Pittsburgh)

However she is molting her two central tail feathers. Click on the photo below for a highlighted version showing the two growing feathers.

Morela is molting her central tail feathers, 10 July 2022 (photo from the National Aviary falconcam at Univ of Pittsburgh)

Meanwhile Ecco is looking very ragged (below). I saw him flying yesterday with a feather obviously growing in on each wing.

Ecco is molting, 9 July 2022 (photo from the National Aviary falconcam at Univ of Pittsburgh)

Have you noticed that Canada geese (Branta canadensis) are not grazing in their usual upland haunts? They are staying near water because they cannot fly while they molt all their primary feathers at once.

Not-molting vs. molting appearance during flightless period in Canada geese (photos from Wikimedia Commons)

Read about their flightless period here.

For adult birds, summer is the time to molt.

(photos from Wikimedia Commons and the National Aviary falconcam at Univ of Pittsburgh)

Female Mallard Becoming Male

Intersex hen mallard, Duck Hollow, 20 March 2022 (photo by Michelle Kienholz)

22 March 2022

On Sunday at Duck Hollow we saw a female mallard with odd plumage. She was paired with a male mallard but she resembled a male in eclipse plumage. Was this duck a hybrid? Or was it something else?

Intersex hen mallard with her mate, Duck Hollow, 20 March 2022 (photo by Michelle Kienholz)

Michelle Kienholz was so intrigued that she took photos and sent them to the Duck ID group where she learned an amazing thing about female ducks. This odd mallard at Duck Hollow is an “intersex hen.” She is becoming male in a process called spontaneous sex reversal (SSR).

Unlike mammals whose sex chromosomes are XX in females and XY in males, female birds have WZ sex chromosomes and males have ZZ. This means that female birds have all the equipment they need to be female but if something suppresses the “W,” they are left with only “Z” and express as male. (Males cannot become female because they have no “W” at all.)

Female ducks are born with two ovaries but only one develops. The left ovary actively pumps out hormones to stifle the male genes, making the bird truly female. If a disease damages the only ovary and it stops producing hormones the female duck spontaneously turns into a male. Experiments have shown that the now-male duck is able to breed and fertilize eggs.

Because most ducks are sexually dimorphic a female with a dead ovary eventually looks male as well. The intersex hen at Duck Hollow is partway through her/his outward transformation, which is why she/he is in eclipse-like plumage.

Notice the clues in her/his feathers that indicate the transition:

  • tail feathers are black and curly white,
  • green feathers interspersed on head
  • breast is darkening (top photo)
  • color line between neck and breast is becoming white
Intersex hen: tail end is black with white feathers, green feathers on head (photo by Michelle Kienholz)
Intersex hen, color line between neck and breast is becoming white (photo by Michele Kienholz)

Read more about Spontaneous Sex Reversal in ducks and see a video in this BBC article: How Does a Duck Change Its Sex?

For more information on bird sex chromosomes see Anatomy: W and Z. For photos of eclipse plumage see Mallards in Eclipse. And here is an article about spontaneous sex reversal in chickens, a problem for chicken farmers.

(photos by Michelle Kienholz)

Turning Redder, Losing Stars

Northern cardinal at the feeder (photo by Chuck Tague)

22 February 2022

Despite recurring winter weather we are more than halfway to spring and the birds know it. As their bodies prepare for the breeding season they develop brighter feathers, skin and beaks. Here are two backyard birds who make this transformation. One turns redder, the other loses stars.

Northern cardinals (Cardinalis cardinalis) molt from July to October, changing out their old feathers for new. At first the male doesn’t look bright red because the very tips of his new feathers are actually gray. You can see the gray feather tips on his back in the photo below.

Northern cardinal: Notice the gray tips on the feathers of his back (photo from Wikimedia Commons)

By mid-winter the gray tips wear off and the male cardinal becomes brilliant red for the breeding season.

Cardinals get their color from what they eat so diet plays a part and there are regional and habitat differences that affect the color. But no matter where they live, male cardinals turn redder in winter.

European starlings (Sturnus vulgaris) have a similar strategy for changing into breeding plumage. When their feathers are new in autumn each one is tipped with white so their bodies appear to be sprinkled with stars — hence the name “starling.” This close up shows that on new feathers the stars are tiny V’s on the feather tips.

European starling with starry winter plumage (photo from Wikimedia Commons)

Over the winter the white tips wear off, especially on their head and breast feathers. By the time it’s breeding season their faces and chests are shiny, sleek and iridescent. Starlings lose their stars in the spring.

European starling in sleek breeding plumage, March 2021 (photo from Wikimedia Commons)

There is one more transformation that starlings make that will tell you which are male and female.

In the winter starlings’ beaks are dark brown but turn bright yellow in spring. You can tell the difference between male and female by looking at the color at the base of their beaks — the part closest to their faces.

Just like the baby colors — girls are pink, boys are blue. You can see it with binoculars.

Here’s a side-by-side comparison. The blue on the male at left is easiest to see. The pink on the female at right is pale and takes more effort to figure out.

Male (left) and female (right) starling beaks in breeding plumage (photos from Wikimedia Commons)

(Cardinal photo by Chuck Tague. Starling photos from Wikimedia Commons; click on the captions to see the originals)

High Gloss Eggs

Elegant-crested tinamou egg about to hatch (screenshot from video by Jan Harteman)

19 January 2022

These high gloss eggs look like ceramic but were actually laid by members of the Tinamou family, native to Central and South America.

  • Great tinamou egg
Great tinamou adult (photo from Wikimedia Commons)

Tinamous are shy, secretive ground-dwelling birds that resemble chickens but are closely related to ostriches and emus. 46 species range in size from the 8’7″ small-billed tinamou (Crypturellus parvirostris) to the 16-19″ grey tinamou (Tinamus tao). No matter where they live, rainforest, savannah or shrubland, all of them lay shiny eggs in a nest on the ground.

Great tinamou eggs in nest (photo from Wikimedia Commons)

One would think that shiny eggs would be easily found by predators, especially after researchers examined the cuticle, the egg’s outside layer, and found:

They quantified its smoothness down to the nanometer scale and measured the shininess of the mirrorlike surface, finding that tinamou eggs are up to 14 times as glossy as the average chicken egg. A spectroscopy test also revealed that the blue eggs were iridescent (the green and brown eggs were too shiny for the spectrometer to accurately measure).

New York Times: Easter Eggs without a Kit

However, the male tinamous who build the nest, incubate, and rear the chicks are generally successful as long as their habitat is not destroyed. 80% of the species are stable.

Watch the eggs’ beauty transform in this video of elegant crested tinamou eggs (Eudromia elegans) in a captive breeding facility.

When the chicks grow up they will look like this.

Elegant crested tinamou (photo from Wikimedia Commons)

Read more in the New York Times: Easter Eggs Without The Kit.

(photos from Wikimedia Commons, embedded from Tumblr and screenshot from embedded YouTube video; click on the captions to see the originals)

The Drab Ones Are Not The Females

White-throated sparrow, white-striped color morph (photo from Wikimedia Commons)

14 December 2021

White-throated sparrows are back for the winter. Here’s something to remember when you see them.

In the world of birds, the bright ones are male and the dull ones are female, right?

Not so for white-throated sparrows (Zonotrichia albicollis). In this species the bright white versus dull tan stripes are color morphs. The bright white-striped bird at top can be either male or female. The tan-striped below is also either sex.

White-throated sparrow, tan-striped color morph (photo from Wikimedia Commons)

The bird with bright yellow lores on the left could be female. The one with dull yellow could be male.

White-throated sparrows: white-striped and tan-striped (photos from Wikimedia Commons)

Amazingly the colors match up to personality traits regardless of sex.

White-striped birds are bold, aggressive, philandering and not particularly caring of their kids. They are not the best parents.

Tan-striped birds are gentle and very caring of their young. They’re the good parents among white-throated sparrows.

Since each bird can tell the other bird’s personality at a glance, you would think the gentle would mate with the gentle and the bold with the bold. But that’s not how they do it. They always mix it up.

White-striped (aggressive) males mate with tan-striped (care-giving) females and tan-striped (gentle) males mate with white-striped (philandering) females. Thus the color morphs and personalities persist.

Learn more about their amazing social behavior in this article by GrrlScientist in The Guardian, May 2011.

You can’t tell a white-throated sparrow’s sex by its color but you pick out the good parents in the flock.

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

A Drone With Bird Legs … So Birds Aren’t Real?

Bird-like legs of the SNAG robot (screenshot from Science Magazine video)

10 December 2021

News this month is that robotics experts have created a drone with legs that can land and grasp like a bird … inspired by peregrines!

Does this bird-like drone indicate that “Birds Aren’t Real” is true? After all, “Birds Aren’t Real” is a …

Gen Z-fueled conspiracy theory which posits that birds don’t exist and are really drone replicas installed by the U.S. government to spy on Americans.

— New York Times: Birds Aren’t Real, or Are They: Inside a Gen Z Conspiracy Theory
Birds Aren’t Real billboard in Memphis, with birds on it (photo from Wikimedia Commons)

Perhaps the leggy drone is the missing evolutionary link between drones and all the “birds.”

Nope. “Birds Aren’t Real is a parody social movement with a purpose” and Pittsburgh is one of its hotspots. Please do read this New York Times article so you can understand those billboards!

Curious about the drone? Read more here: This drone has legs: Watch a flying robot perch on branches, catch a tennis ball in midair.

(screenshot of drone from Science Magazine video, photo of billboard from Wikimedia Commons; click on the captions to see the originals)

Why Don’t Sleeping Birds Fall Off the Branch?

African gray parrot and common grackle, each sleeping on a perch (photos from Wikimedia Commons)

2 December 2021

We take for granted that birds sleep on a perch without falling off, yet we drop whatever we’re holding when we fall asleep. (Many’s the time that my book falls off the bed!)

How do birds continue to hold on after they fall asleep? The answer is in this vintage article.

(photos from Wikimedia Commons; click in the caption to see the originals)

Do They See What We See?

GG looks up from a meal (photo by Chad+Chris Saldin)

30 November 2021

We humans assume that what we see is what everyone else sees, including other species. But this isn’t so.

Peregrines see much finer details at a greater distance that we do. The details don’t blur for them in a 200 mph dive. (Click the link to learn more.)

Tellus in a stoop (photo by Chad+Chris Saldin)

Cats cannot see red-green nor distant details, but they see much better in the dark. Who needs distance vision while looking for a nearby mouse at night? Click here to see photos of our vision versus cats’. Notice the normal vs. red-green-color-blind examples below.

Domestic cat (photo from Wikimedia Commons)
Normal vision vs deuteranopia (red-green color blind) (images from Wikimedia Commons)

White-tailed deer see regular blaze orange as gray but if the orange has fluorescence it stands out for them. Their vision is best in the blue range so that they see well in twilight.

White-tailed deer at Schenley Park (photo by Kate St. John)
Non-fluorescent blaze orange looks gray to deer ( sign from PA Game Commission, Blaze Orange Vest on Amazon)

Birds see ultraviolet light though we cannot. Here’s how we know this and a hint at what birds look like in ultraviolet light.

Do other species see what we see?

No. Birds see more.

(peregrine photos by Chad+Chris Saladin, deer photo by Kate St. John, remaining photos from Wikimedia Commons)

How Birds Chew

Cedar waxwing about to swallow whole fruit (photo by Steve Gosser)

28 October 2021

Have you ever noticed that birds gulp their food? Of course they don’t chew — they don’t have teeth — but much of what they eat still has to be “chewed” before they can digest it.

That’s where grit comes in.

Gravel (photo from Wikimedia Commons)

Birds chew with their gizzards, a specialized stomach with thick muscular walls that grind up food, often aided by particles of stone or grit.

Diagram of bird digestive system, annotated (image from PA Game Commission)

Birds regularly eat grit to aid their digestion, as Ecco is doing in the photo below.

Peregrine falcon, Ecco, ingesting gravel (photo from the National Aviary snaphot camera at Univ of Pittsburgh)

So when you see birds swallowing things whole rest assured they’re chewing inside … in the gizzard.

Red-tailed hawk with prey (photo from Wikimedia Commons)

p.s. Other animals have gizzards too including crocodiles, alligators, mullet (a fish) and earthworms.

(photos from Steve Gosser, Wikimedia Commons, and the National Aviary snapshot camera at Univ of Pittsburgh)

Birds With Teeth?

Ostrich face (photo from Wikimedia Commons)

4 October 2021

Teeth are so important that every toothless animal today is descended from ancestors that had them. This includes anteaters, baleen whales, pangolins, turtles, and birds.

Giant anteater with his snout in an ant hole (photo from Wikimedia Commons)

The ancestors of birds were theropod dinosaurs. They definitely had teeth.

Velociraptor, a theropod ancestor of birds (image from Wikimedia Commons)
Tyrannosaurus rex, skeletal head (photo from Wikimedia Commons)

Then about 116 million years ago a common ancestor of all birds developed genetic mutations that inactivated the genes for tooth formation. Eventually birds’ teeth disappeared, replaced by horn-like beaks.

There are probably several reasons why teeth disappeared but the main one is this: In order to fly well it’s important to reduce excess body weight. Bones and teeth are heavy so over time birds evolved hollow bones and toothless beaks. They compensate for the lack of teeth by chewing food in their gizzards.

American robin taking flight (photo from Wikimedia Commons)

They also compensated by using their beaks as multi-purpose tools to grasp, twist, pry, crack open shells, and sever tendons.

Each species developed a beak for its lifestyle. A few of them evolved beak modifications that resemble teeth.

Among tooth-billed hummingbirds (Androdon aequatorialis) the males have a “straight bill with a prominent hooked tip and backward-pointing tooth-like serrations on the distal half. The modification is absent on the female bill, and thus may be related to sexual selection,” describes Birds of the World. Perhaps the “teeth” are used for fighting.

Toothbilled hummingbird beak (photo from Wikimedia Commons)

The critically endangered tooth-billed pigeon (Didunculus strigirostris), the national bird of Samoa, has a stout curved bill with a specially toothed lower mandible.

[The bill] is adapted to feed on Wild Mahogany Dysoxylum spp. fruits, although first-year birds apparently cannot do so. These fruits have a tough capsule that it is able to open using its strong bill, removing the flesh using a sawing action with the lower mandible.

Birds of the World, Tooth-billed pigeon account 

If you want to stretch the definition of “teeth” peregrines have two of them. The tomial tooth on each side of the upper beak is used “to kill prey quickly by biting their necks and severing the vertebrae.”

Peregrine falcon showing tomial teeth on upper mandible (photo by Chad+Chris Saladin)

Does that make them birds with teeth? Not really. No dentin, no enamel, and they aren’t using them to chew.

(photos from Wikimedia Commons; click on the captions to see the originals. Peregrine photo by Chad+Chris Saladin)