Category Archives: Bird Anatomy

On Finding Pellets

Red-tailed hawk casting a pellet, 2018 (photo by Chad+Chris Saladin)

22 January 2023

This red-tailed hawk is not consuming the lump near his mouth. He’s casting a pellet of indigestible bones, fur and feathers that came up from his gizzard. Pellets are a normal by-product of digestion in birds of prey. If you find one, it can tell you what the bird was eating.

We always find pellets during annual maintenance at the Pitt peregrine nestbox including these three found during our 9 January visit (paperclip for scale). The pellets can be many months old.

Peregrine pellets from Cathedral of Learning nestbox, 9 Jan 2023 (photo by Kate St. John)

A closeup shows feathers and bones (no fur*) but is not very enlightening due to the pellet’s age. Fortunately I stored the pellets in a ziploc bag. After they thawed a small fly appeared inside the bag, hatched from eggs laid on the pellet in much warmer weather. Ewww!

Closeup of peregrine pellet (photo by Kate St. John)

I imagine the pellets came from Morela since the green perch is one of her favorite places to rest and digest.

Morela casting a pellet, 17 Dec 2021 (photo from the National Aviary snapshot camera at Univ of Pittsburgh)

Peregrine pellets are slightly longer than a paperclip. Some birds make much larger pellets.

On a hike at Audubon Greenway Conservation Area last Wednesday we found a surprisingly large pellet containing fur, bones and a big tooth. It was so large that we wondered if a bird could produce it. I didn’t pick it up but it looked as though it could span my palm.

Pellet found at Audubon Greenway, 18 Jan 2023 (photo by Kate St. John)
Alternate view of pellet found at Audubon Greenway, 18 Jan 2023 (photo by Kate St. John)

A Google search revealed that great horned owl pellets are 3 to 4 inches long, usually cylindrical and tightly compacted. This one may have opened up because it was soaked by heavy rain.

Great-horned owl clutching a feather (photo from Wikimedia Commons)

So what did the owl eat? Whose big tooth was that?

Learn more about owl pellets at The Owl Pages: Digestion in Owls.

* p.s. There is no fur in peregrine pellets because they don’t eat mammals, only birds.

(photos from Chad+Chris Saladin, Kate St. John, the National Aviary snapshot camera at Univ of Pittsburgh and Wikimedia Commons)

White-throated Sparrows Have Four Sexes

White-throated sparrow colors and sexes — green arrows show the only combinations that can mate successfully (photos from Wikimedia Commons)

8 January 2023

White-throated sparrows (Zonotrichia albicollis) come in two color morphs with either white-striped or tan-striped heads. The color tells us nothing about the sex of the bird because both morphs contain males and females. Last week a new article about a 2016 study reiterated the white-throated sparrow’s affinity for mating with the opposite color morph. It’s deeper than just a preference. These birds cannot reproduce with their own color.

Thirty years of research by Elaina Tuttle and Rusty Gonser into the genetics and behavior of white-throated sparrows revealed a mutation in chromosome 2 that makes it impossible for same-color-morph birds to reproduce. The birds seem to know this and only look for mates among birds of the opposite color. Instead of half the population as possible mates, fellow researcher Christopher Balakrishnan points out that “One individual can only mate with one-quarter of the population. This bird acts like it has four sexes.”

  • White-stripe Male
  • Tan-stripe Male
  • White-stripe Female
  • Tan-stripe Female

A system of four sexes is quite rare and there’s a reason. As Balakrishnan says, “it is evolutionarily unstable and one of these alleles will ultimately go extinct.”

White-throated sparrows have declined 69% in the U.S. over the past 50 years and overall (including Canada) by 33%. Are they declining because of habitat loss? window kills? Is their four-sex system also taking a toll? If so they’re probably the only species with that challenge.

Read more in these two articles where I obtained the quotes above: IFLScience: Meet The Sparrow With Four Sexes and NATURE: The sparrow with four sexes.

(four photos above are from Wikimedia Commons at these links: top left, top right, bottom left, bottom right)

Walking On Ice

Canada geese walking on ice (photo from Wikimedia Commons)

26 December 2022

Three days ago the temperature in Pittsburgh fell from 40oF to -1oF in just 25 hours. Standing water froze rock hard. Everyone was walking on ice.

We humans have to wear insulated boots when the temperature is below freezing but birds walk on ice with their bare feet. They don’t they get frostbite because …

Birds are specially adapted to stay comfortable when it’s cold. They have fewer nerves and blood vessels in their feet and the veins and arteries in their legs are intertwined so that cold blood leaving their feet is warmed by incoming arterial blood. 

Dr. Heather Hinam (@SecondNatureMB) of Manitoba, Canada explains in this tweet.

Mourning doves are a slight exception to the rule and occasionally get frostbite. Read about their cold feet in this vintage article.

For more information on animals’ adaptations for cold weather see Dr. Heather Hinam’s tweet: https://twitter.com/SecondNatureMB/status/1606305716288905216

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

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)