Category Archives: Bird Anatomy

The Essence of Iridescence

Anna’s hummingbird (photo from Wikimedia Commons)

16 July 2023

What causes iridescence? What makes a hummingbird glow red in one position, then dull green when he moves his head?

video from NDTV on YouTube

Other dazzlers, including beetles, shells, and rocks, have similar physical iridescent characteristics.

Six-spotted tiger beetle in Maryland (photo from Wikimedia Commons)
Nautilus shell sliced in half (photo from Wikimedia Commons)
Nautilus shell cut in half (photo from Wikimedia Commons)
Iridescence on anthracite, a.k.a. peacock coal (photo from Wikimedia Commons)
Sun dog — an iridescent cloud (photo from Wikimedia Commons)

Find out what causes iridescence in this 16 minute video from PBS @BeSmart. If you don’t have much time, watch the first 4+ minutes about hummingbirds.

video embedded from PBS @BeSmart

p.s. This article was inspired by All About Birds: What Is the Essence of Iridescence? Ask a Hummingbird.

(credits are in the captions, click the links to see the originals)

American Kestrels Mysteriously Decline

American kestrel at Madera Canyon, AZ (photo from Wikimedia Commons)

18 June 2023

When I began watching peregrine falcons 22 years ago, peregrines were endangered and our smallest falcon, the American kestrel, was doing just fine, but the tables have turned. Peregrines have fully recovered from extinction in eastern North America while kestrels have lost half their population and face an uncertain future. The New York Times described their plight this week in The Mystery of the Vanishing Kestrels: What’s Happening to This Flashy Falcon? Can we save this beautiful bird before it’s gone?

Pair of American kestrels in Colombia (photo from Wikimedia Commons)

American kestrels (Falco sparverius) range from Alaska to Tierra del Fuego and are the only “kestrel” in the Americas, but they aren’t true kestrels like those found in Europe and Africa. Instead, DNA tests have shown that our kestrel is closely related to the larger falcons of the Americas, including peregrines. Falco sparverius evolved to fill the kestrel niche.

Range map of American kestrel from Wikimedia Commons. purple=Year round, orange=Summer breeding, blue=Winter non-breeding

American kestrels are versatile birds. At home in grasslands, meadows, deserts, cities and suburbs, they eat grasshoppers, crickets, large flying insects, beetles, lizards, small rodents and small birds.

Kestrel eating a bug (photo from Wikimedia Commons)

Kestrels nest in cavities in buildings, trees, cliffs and nestboxes but more than half of their sites are unoccupied now in eastern North America. I’ve seen the decline first hand in Pittsburgh. A decade ago there were two kestrel nests within a few blocks of Downtown’s Third Avenue peregrines. Now there are none.

Dr. John Smallwood, a professor of biology at Montclair State University interviewed in the New York Times article, has monitored 100 kestrel nestboxes in New Jersey for nearly 30 years. The number of occupied nests at his sites peaked at 61 in 2002 and has dropped ever since.

What’s going wrong for kestrels? Are they out-competed for prey? Are they ingesting poison? What’s happening on their wintering grounds? Are insect declines affecting kestrels? Are neonicotinoid pesticides a factor? And what about the bigger questions of habitat and climate change?

Many kestrel experts think it’s a combination of causes. Dr. Smallwood agrees, but he still has a top suspect. “If I’m only allowed one word: grasshoppers.”

The one parameter that seems to be declining over time, researchers say, is survival of young birds in the summer.

… the thinking is that those juveniles may be more dependent on insect prey because it’s easier to catch.

— New York Times: The Mystery of the Vanishing Kestrels: What’s Happening to This Flashy Falcon?
Female American kestrel holding a cricket (photo from Wikimedia Commons)

I would not be surprised to learn that the kestrels’ decline is linked to the rapid insect decline in this century which was probably prompted by neonics. Neonicotinoids were first introduced in the 1990s but didn’t take off as a pesticide until the early 2000s.

Meanwhile a nationwide study funded by the USGS and the U.S. Fish and Wildlife Service is looking into the American kestrel’s mysterious decline. I hope they find the answer soon.

Read more at The Mystery of the Vanishing Kestrels: What’s Happening to This Flashy Falcon?

(photos and map from Wikimedia Commons, click on the captions to see the originals)

Canada Geese Can’t Fly in July

Canada goose molting primaries in late June, Ohio (photo from Wikimedia Commons)

14 June 2023

Once a year, from late June until August, Canada geese spend six weeks molting all their wing feathers. This means they can’t fly in July, nor even in late June.

On a walk at Herr’s Island yesterday I saw many Canada geese swimming in the river and a few of their primary feathers — the “fingertip” feathers — scattered on shore. At first I wondered if a goose had been attacked and then I realized the feathers were a sign of their synchronous molt. Here’s a snapshot from a similar discovery made by Rebecca Johnson in 2020. (Click on the snapshot to see her video on YouTube.)

Molted Canada goose wing feather (snapshot from Rebecca Johnson’s UA Museums video on YouTube)

Even if you don’t see discarded wing feathers you can tell a Canada goose is molting because its white rump is visible above the dark tail. It’s really noticeable from above.

Canada goose seen from above in the midst of wing molt in July (photo from Wikimedia Commons)
Canada goose in the midst of wing molt, late June, (photo from Wikimedia Commons)

Sometimes you can see the pin feathers coming in. This marked up photo highlights the pin feathers and visible white rump.

Closeup of Canada goose molting with markup (photo from Wikimedia Commons)

In late June and July when they cannot fly Canada geese are safe only in water. You’ll see them feeding just a short walk from a large body of water and notably absent from landlocked places.

When they can fly again and their tails will look like this.

Canada goose in May in Chicago (photo from Wikimedia Commons)
Canada goose in March in Illinois (photo from Wikimedia Commons)

Have you noticed Canada geese avoiding people lately? They aren’t as bold when they can’t fly in late June and July.

(photos from Wikimedia Commons, wing feather snapshot from Rebecca Johnson’s UA Museums video on YouTube)

p.s. There’s a theory that this type of wing molt led to flightless birds in locations where threats were low. Read more about it at: Simultaneous wing molt as a catalyst for the evolution of flightlessness in birds.

Recognize Your Own Blue Jays

Blue jay (photo by Cris Hamilton)

10 May 2023

As humans we recognize each other by face, body shape and the way a person walks, but it’s rare that we can recognize individual birds. Birds move too fast to examine their faces and in most cases we don’t know what to look for. However if you can “hold them still” in photographs it’s possible to see patterns. This is especially true of your backyard birds that can be photographed over and over.

Blue jays all look the same … but not really. Their facial markings can be unique enough to tell them apart in photos. Lesley The Bird Nerd in Ontario, Canada has photographed her local blue jays for many years and learned to tell who’s who by face. Check out her 6.5 minute video below.

video embedded from Lesley The Bird Nerd

If you have a camera you can do this, too!

(photo by Cris Hamilton, video embedded from Lesley The Bird Nerd)

Left or Right-Footed?

Feral pigeon walking (photo from Wikimedia Commons)

27 April 2023

On Throw Back Thursday an old topic but a good one …

Humans have a trait called handedness in which we show a preference for using one hand over the other.  Interestingly, dominance in the left hemisphere of our brains results in right-handedness and vice versa. About 90% of us are right-handed.

This hemispheric dominance is called functional lateralism and birds have it too. They show it by the foot they use, the eye they look out of, or the crossing of their bills.

Red crossbills, two males, Deschutes National Forest in Oregon (photo from Wikimedia Commons)

Pigeons show it with their feet. If we could watch closely enough we’d notice that a pigeon leads with one foot when it lands, choosing to land first on its dominant foot. Find out more in this 2016 article:

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

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:

(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)