Archive for the 'Bird Anatomy' Category

Apr 26 2013

Eggs’ Potential

Anatomy of an egg (illustration from Wikimedia Commons)

We’ve had eggs on our minds this week while we’re watching them hatch at the Cathedral of Learning peregrine nest.

Eggs start as the familiar objects we see every day in our refrigerators and miraculously become baby birds.  The process is so amazing that I’m devoting two Tenth Page articles to it.

Shown above is the un-incubated egg we know so well.  If fertilized before it’s laid — and then incubated — it becomes a bird.  Each component plays a part.

  • Blastodisc or germinal disc:  Potential embryo.  If fertilized and incubated this small circular spot on the yolk becomes a chick.
  • Yolk:  Food for the embryo.  The female’s ovary deposits layers on the yolk to increase its size before ovulation.  Yellow layers are laid on during the day, white ones at night, so the yolk has rings like a tree.  It’s housed in a yolk sac which is why you have to “break” the yolk when cooking.  The yolk is ovulated with the germinal disc attached (cradled by the yolk) so the food is next to the potential embryo even before fertilization.   As the embryo develops, the yolk shrinks.
  • Albumen = Egg White:  Food, water, shock absorber, and insulation from sudden temperature changes.  The albumen makes up 50% to 71% of the egg’s total weight.  It’s laid on after fertilization while the yolk-with-germinal-disc rotates gently in the oviduct.  As the embryo develops the albumen shrinks too.
  • Chalazae:  Because the yolk is rotating during albumen deposition, twists form in the albumen.  Chalazae act like springs and stabilizers to keep the yolk and embryo in place inside the egg.  They’re the white twisted bits in the egg white.  (Totally amazing!  Shock absorbers, insulation, springs and stabilizers!)
  • Inner Shell Membrane:  the first of two membranes that hold the embryo-yolk-albumen together
  • Air Space:  Between the inner and outer shell membranes the air space acts as a condenser for moisture exchange.  This is where the baby bird takes its first breath before hatching.
  • Outer Shell Membrane:  The final packaging before the shell is laid on.  It’s attached to the shell when you crack open an egg.
  • Shell: The female’s uterus deposits calcium on the outer shell membrane to make the hard enclosure for the egg.  The shell has microscopic pores to allow air exchange for the developing embryo.
  • Cuticle:  A thin layer on the shell that adds protection.  The cuticle has caps on top of the pores that close when necessary to protect the embryo.

Eggs have the tools and potential to become baby birds.  Next week I’ll show you how.

(illustration from Wikimedia Commons; click on the image to see the original. Today’s Tenth Page is inspired by page 420 of Ornithology by Frank B. Gill.)

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Apr 19 2013

How Do They Hatch?

Chicken hatching in incubator (photo by grendelkhan on Wikimedia Commons)

In the next few days the peregrine eggs at the University of Pittsburgh are going to hatch, so now’s a good time to explore…

How does a baby bird get out of the egg?   It’s a strenuous one to two day process in very tight quarters.

  1. When a chick is ready to hatch, he pulls himself into the tucking position with his beak sticking out between his body and right wing.  This gives him the leverage he needs to whack at the shell.
  2. The chick then breaks through the membrane at the large end of the egg that isolates the air sac and he breathes for the first time.
  3. Next he starts to bump the shell with the curved ridge of his beak where he has a calcified egg tooth that’s sharp enough to crack the shell.
  4. His strenuous hammering is aided by the hatching muscle on the back of his neck.
  5. While still in the egg he communicates with his parents and siblings by peeping and pecking sounds.  The parents know which eggs are alive because they’re speaking.  The siblings know their brothers and sisters are ready to emerge.  In precocial species, which must all hatch at once, the chicks listen to each others’ tapping to coordinate the hatch.  Elder chicks tap slowly, younger ones tap rapidly so that all of them reach the finish line in a 20-30 minute window.
  6. Finally the chick cracks his shell all the way around.  He pushes with his feet and the egg splits open.  His mother moves the shell away and he lies quietly, waiting for his down to dry.

After hatching the chick’s specialized tools aren’t needed anymore.  The egg tooth falls off (in songbirds it’s absorbed) and the hatching muscle shrinks into just another neck muscle.

Watch the National Aviary falconcam for hatching at Dorothy and E2′s nest.  The streaming cam is blurry but it is broadcasting sound so you’ll be able to hear the chicks peeping inside their shells.  That will be our first sign that hatching is underway.

 

(Credits: photo of a chicken emerging from its egg from Wikimedia Commons.  Click on the image to see the original.  Today’s Tenth Page is inspired by page 460 of Ornithology by Frank B. Gill.)

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Apr 05 2013

How Can They Sit For So Long?

Dorothy asks E2 to get up so she can resume incubation (photo from the National Aviary falconcam at Univ. or Pittsburgh)

During courtship E2 is very active but now Dorothy has to plead with him to get up off the eggs.  Dorothy herself is able to sit for 12 hours in a snow storm.  How do they do it?

How do birds instantly switch gears from the frantic activity of courtship to sitting on eggs all the time?

They’re cued by hormones.  Here’s how:

  1. As day length increases after the winter solstice, a bird’s hypothalamus releases LHRH (luteinizing hormone releasing hormone).
  2. LHRH triggers the pituitary gland to release LH (luteinizing hormone).
  3. LH increases production of testosterone in males and progesterone in females.
  4. Testosterone triggers aggression, territoriality and sexual behavior.  It’s good at the start of breeding but doesn’t help raise a family.
  5. Progesterone is the “pregnancy hormone” that induces egg production.  It’s only needed for a short time since female birds are only ovulating and pregnant until they lay the eggs.
  6. On the day before incubation begins the hormones switch.  Prolactin, the hormone that promotes incubation behavior, rises sharply while the other hormones suddenly decrease.  In females, LH and progesterone drop off.  In males, testosterone has been dropping since egg laying began.  If the male shares incubation he has a sharp rise in prolactin, too.  On a graph this hormone switch looks like a sine curve.  There’s a moment where all these hormones are low, then prolactin takes off.

In peregrines, both parents have to be ready to incubate at the same time.  Their courtship rituals help get the couples’ hormones in synch.

This whole process may sound as if birds are at the mercy of their hormones but in every species reproduction is chemically tuned for success.  In humans for instance, progesterone and prolactin switch after delivery so that the mother’s body produces milk to feed the baby.  Individual animals whose hormones malfunction do not have live offspring.

So how do birds incubate so nicely?  In a word, prolactin.

 

(photo of Dorothy and E2 from the National Aviary falconcam at the University of Pittsburgh.  Today’s Tenth Page is inspired by page 448 of Ornithology by Frank B. Gill.)

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Mar 20 2013

Unnatural Selection

Cliff swallow like a butterfly (photo by Chuck Tague)

Cars and trucks have changed the cliff swallow.

For 30 years Charles Brown and his wife Mary Bomberger Brown have studied cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska.  They’ve meticulously monitored, measured and banded the birds at their nests under bridges and overpasses and they’ve counted and measured the road killed birds.

Their attention to detail has paid off in an unexpected way.

Cliff swallows attach their mud nests to cliffs or bridges.  In Nebraska where there are few cliffs, the swallows use busy highway overpasses.   If the swallows aren’t quick to fly up out of traffic they become road kill.

When the Browns began their study in 1982 they typically found 20 road killed cliff swallows per season, but since 2008 they’ve usually found less than five.  The traffic has remained the same while the swallows’ population has more than doubled, yet the road kill numbers dropped dramatically.

What changed?  The swallows changed!

The Browns’ data reveals that thirty years ago Nebraska’s cliff swallows had longer wingspans.  Today’s shorter wings allow the birds to maneuver more quickly and turn away from oncoming vehicles.  In fact, the few road killed birds they find today have longer wings than the rest of the population.

The shorter-winged birds survive to breed, the long-winged birds do not.  In only 30 years, traffic’s unnatural selection has forced cliff swallows to evolve.

If traffic can do this to cliff swallows, I wonder what it’s done to Pennsylvania’s white-tailed deer.

 

Read more about this study in ScienceNOW.

(photo of a cliff swallow near the Rt. 528 bridge in Moraine State Park by Chuck Tague)

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Feb 05 2013

Occupational Hazard

Barred owl at Crooked Creek (photo by Steve Gosser)

How do owls turn their heads this far without killing themselves?

Trauma experts know that when humans turn their heads too far or too fast the arteries to the head are stretched or torn, cutting off the blood supply or producing blood clots that can kill.

Why doesn’t this happen to owls?  A team at Johns Hopkins decided to find out.

Led by medical illustrator Fabian de Kok-Mercado, they used imaging technology on barred, snowy and great horned owls who had died of natural causes. The researchers found four adaptations that make the owls’ wide range of movement possible:

  1. As in humans, the major arteries that feed the brain go through bony holes in the vertebrae but in owls these holes are 10 times larger than the arteries, allowing them to move within the hole without pinching.
  2. The owls’ vertebral artery enters the neck higher up than in other birds — in the 12th vertebrae rather than the 14th.  This provides more slack.
  3. When an owl turns its head the arteries at the base of the head balloon to take in more blood.  In humans the arteries get smaller and smaller.
  4. Owls also have small vessel connections between the carotid and vertebral arteries so if one path is blocked the other still works.

A simple turn of the head that’s so hazardous to us is all in a day’s work for an owl.

Read more about the study here in Science Daily.

(photo of a barred owl by Steve Gosser)

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Feb 01 2013

Slots Help Me Fly

Turkey vulture (photo by Chuck Tague)

A bird’s lifestyle is written in its wings.

Birds who fly fast and maneuver quickly, such as peregrines and swifts, have narrow pointy wings built for speed and agility.  They need this equipment to capture prey in the air.

Birds who soar slowly in search of food, such as red-tailed hawks and turkey vultures, have broad wings with a lot of surface area.

Broad, blunt wings create a lot of wingtip turbulence (remember those vortices?) so soaring birds have feather slots at their wingtips.  This confers two flight advantages.

First, each feather stands alone like a tiny pointy wing with a high aspect ratio (ratio of length to breadth) that’s more like a peregrine’s wing.   The winglets create less turbulence and therefore less drag.

The second advantage is in the gaps. As air is forced upward between the feather slots, it expands on the upper side creating low air pressure on top and therefore more lift.

Turkey vultures are masters of slow speed flight.  They turn and teeter without flapping — not even once!

The slots help them fly.

 

(Photo by Chuck Tague.  Today’s Tenth Page is inspired by page 120 of Ornithology by Frank B. Gill.)

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Jan 25 2013

Fanciful Eggs

We see chicken eggs every day so we tend to assume all eggs are plain and never shiny.  In reality most eggs are not.

Shown above is an illustration of 50 European bird eggs by Adolphe Millot published 1897-1904(*).

The eggs have many shapes and sizes from the goldcrest’s tiny pink oval (#19) to the large pointed pyriform egg of the now extinct great auk (#47).

Few are a single solid color but even those are amazing — from pink to robin’s-egg blue to a beautiful avocado color.  Tinamous are from South America so their eggs aren’t pictured here, but it’s worth clicking this link to see their glossy eggs in several colors.

The dark patterns on eggs are almost fanciful wreaths, caps, scrawls, dots, streaks and blotches.  They’re made by protoporphyrin which is deposited within or on the shell while in the bird’s uterus.  These dark spots are stronger than the plain calcium shell and tend to be deposited where the eggshell is thinnest.  Some birds lay on extra protoporphyrin when their personal calcium supplies are low.

And, as a final touch some eggs are shiny, some are waterproof.  I have read that duck eggs feel oily and that jacanas, who build floating nests, lay eggs that are lacquered (#29, in the top row).

Explore the eggs in the illustration using the quick key below.  If you click on the image you can zoom the original to read the egg numbers.

(Credits:
Illustration of European bird eggs from “ŒUFS” (Eggs) by Adolphe Millot from Nouveau Larousse Illustré [1897-1904], in the public domain via Wikimedia Commons.  (*) This image has been altered as described in the “p.s.” below.  Click on the image to see the original.

Inspiration for this Tenth Page is from an illustration on page 400 of Ornithology by Frank B. Gill.)

p.s.  Key to the illustration, copied from Wikimedia Commons:
The original French designation may not correspond to the modern French term. Eggs 1-50 are bird eggs, reduced in size by about a third.  Eggs 51-72, (*)which I erased from this illustration, were from turtles, reptiles, moths etc. I erased them to highlight only the bird eggs.  Click on the image above to see the complete original on Wikimedia Commons.

#    French        English
1    De bondree    honey buzzard (?)
2    De faucon    falcon (?)
3    D'epervier    Eurasian sparrow-hawk
4    De merle    blackbird
5    De grive    thrush
6    De freux    rook
7    De bruant proyer    corn bunting
8    De gros-bec    hawfinch (or perhaps another grosbeak?)
9    De moineau    sparrow
10    De pinson    chaffinch (or other finch?)
11    De pitpit    pipit
12    De bruant des roseaux    reed bunting
13    De coucou    cuckoo
14    De petit oiseau-mouche    hummingbird (?)
15    De bec-croise    crossbill
16    De troglodyte    wren
17    De sittelle    nuthatch
18    De rossignol    nightingale
19    De roitelet    Kinglet (Goldcrest?)
20    D'accenteur    accentor
21    De bruant fou    rock bunting
22    D'effarvate    reed warbler
23    De rousserolle    sedge warbler (or other Acrocephalus?)
24    De fauvette    warbler (??)
25    De mesange    tit (?)
26    D'hypolais    tree warbler
27    De jaseur    waxwing
28    De loriot    oriole
29    De jacana    jacana
30    De grouse (?)    grouse (?!)
31    De lagopede    lagopus
32    De faisan    pheasant
33    De perdrix    partridge
34    De caille    quail
35    D'avocette    avocet
36    De chevalier arlequin    spotted redshank
37    De pluvier guignard    dotterel
38    De pluvier de Virginie (??)    plover (??)
39    De vanneau    lapwing
40    De chevalier cul-blanc    green sandpiper
41    De sterne hybride (??)    tern (??)
42    D'hirondelle de mer    common tern
43    De sterne de Ruppell (??)    tern (??)
44    De goeland    seagull
45    De plongeon    loon
46    De guillemot    guillemot
47    De grand pingouin    great auk
48 & 49    De macareux    puffin
50    De grebe    grebe

 

Avian Reproduction reference

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Dec 28 2012

Feather Facts

Feathers are to birds as hairs are to mammals .. but not quite.

Here are some feather facts to ponder.

  • Feathers, like hair, are dead structures that have no nerves and cannot change or heal themselves if damaged.
  • Our hair grows continuously.  Feathers grow to completion and then stop, so they must be completely replaced when worn out.
  • Feather follicles have muscles that grip the feathers so they don’t fall out.  Anyone who’s plucked a chicken knows these muscles are strong.
  • In some birds, such as nightjars, the follicle muscles let go when the bird is frightened suddenly.  (I wonder which feathers they lose.)
  • A new feather literally grows under the old one and pushes it out of the follicle.
  • Contour (body) feathers are symmetrical and so are their follicles.  Flight feathers are lopsided: narrow on one side of the rachis compared to the other.  Flight follicles are lopsided too.
  • The same feather follicle can produce very different feathers at different times of year — for example colorful, long feathers for the breeding season and drab ones for basic plumage.  Imagine if our hair could do that! We could automatically change our hair color in the spring.

(Inspiration for this Tenth Page is from page 90 of Ornithology by Frank B. Gill. Photo from Wikimedia Commons; click on the image to see the original)

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Dec 15 2012

Speaking of Plumage

Published by under Bird Anatomy,Songbirds

Speaking of plumage as I did yesterday

Here’s a bird in juvenile plumage.

If you didn’t know that immature white-crowned sparrows are cream-and-brown colored, you’d have trouble identifying him.

Here’s what his parents look like in basic plumage.

Quite a difference!

 

(photos by Marcy Cunkelman)

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Dec 14 2012

Plumage Basics

Birds molt at least once a year to replace worn out feathers.  This process permits them to wear different plumages.

Some birds, like the American robin, look the same before and after.  Others radically change their appearance by replacing their fancy breeding feathers with plainer plumes.  Male scarlet tanagers are an extreme example:  They’re red while breeding and green while not.

Molt and plumage terminology was standardized in 1959 by Humphrey and Parkes who divided plumage names into three main types. (*)

  • Juvenile plumage is worn by young fledged birds.
  • Basic plumage is what birds acquire during their annual post-breeding molt.  We often call this “non-breeding” or “winter” plumage but these terms are inaccurate.  Adult robins are always in basic plumage even when they’re breeding, and “winter” describes the weather North America is experiencing while the bird is away.  To South American birders, a green scarlet tanager is in summer plumage.
  • Alternate plumage is optional.  Some birds don’t undergo a second molt but those who do put on their finest feathers in time for the breeding season.  This is often called breeding plumage.

In some species it takes several years for the young to mature so they progress through as many plumage cycles as it takes to become adults.  Young ring-billed gulls go through three cycles:  Basic 1, Alternate 1, Basic 2, Alternate 2, Basic 3, Alternate 3. Gulls are complicated.

American avocets aren’t quite so complex.  They molt their wing feathers once a year but change out their head and neck feathers twice a year from basic plumage (white) to alternate plumage (ochre) for the breeding season.

The avocets above are lined up in perfect sequence during their post-breeding molt in August.  The bird standing on the left is closest to basic plumage, the bird on the right is closest to alternate plumage, and the bird in the middle is halfway between.

 

Below, another flock has the lead bird closest to alternate plumage and the trailing bird closest to basic.

Look closely at each bird and you can see that the wings of the 1st, 3rd and 4th birds have ragged trailing edges because they’re molting their wing feathers.  The 2nd and last birds have perfect wings, so my guess is that they’re juveniles.  Juveniles don’t molt their fresh new wing feathers until they’re a year old.

When avocets have completed their molt into basic plumage their heads and necks are gray-white like this bird photographed in September.

 

Experts in molt and plumage can probably tell the age of these birds by their appearance.

Not I.  Aging shorebirds by plumage is my final frontier.

(Inspiration for this Tenth Page is from page 110 of Ornithology by Frank B. Gill.
All photos by Bobby Greene
)

 

(*) If you’re a plumage expert, please feel free to correct me.  I’m still learning!

P.S. TO PEREGRINE FANS:  Molting is a wonderful thing.  Last May the male peregrine at Pitt, E2, chased off an intruder but not before this opponent damaged one of his primary feathers.  This gave him a “hole” in his wing.  Over the summer he completed his annual pre-basic molt and grew all new feathers.  Now his wings are perfect.  No gap.

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