During incubation there’s not a whole lot of activity at a bird’s nest except for this: Mom (or Dad) periodically stands up, stares at the eggs and draws each one toward her with her beak. She’s not just rearranging the eggs, she’s turning them.
Other than a few notable exceptions, all birds turn their eggs because it’s required for the embryos’ survival. For instance:
The temperature in the middle of a clutch is warmer than the edge. Birds move the outer eggs to the middle to keep them evenly heated.
In the early days of incubation, it’s important that the embryo floats inside the egg while the membranes that support its life are growing and developing. Turning optimizes membrane growth.
Eventually the chorion and allantoic membranes will be pressed to each other and to the shell. If these membranes adhere too soon the chick will not be able to move into the hatching position later and get out of the egg. Turning prevents premature adhesion.
The albumen (the egg white) is the embryo’s fluid cushion and water supply. Turning the egg optimizes the fluid dynamics of the albumen so the chick can absorb it properly.
Egg turning is so important that it’s a wonder some species don’t do it. One notable exception are the megapodes who lay their eggs in compost heaps and let the heat of the decomposing vegetation incubate them. No turning there!
I’d rather watch a peregrines’ nest where things are happening, if only a bit of egg turning.
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:
As day length increases after the winter solstice, a bird’s hypothalamus releases LHRH (luteinizing hormone releasing hormone).
LHRH triggers the pituitary gland to release LH (luteinizing hormone).
LH increases production of testosterone in males and progesterone in females.
Testosterone triggers aggression, territoriality and sexual behavior. It’s good at the start of breeding but doesn’t help raise a family.
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.
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.)
Spring is moving north and so are the robins. This week a big wave arrived after Monday’s snow. Now that they’re here, how soon will they nest?
Robins nest later the further north you go. In 1974 Frances James and Hank Shugart were curious about the conditions that governed their nesting times throughout the U.S. Using climate data and Cornell nest watch information from 8,544 robins’ nests they developed a model that predicted when robins would nest in a particular region.(*)
The model shows that robins cue on weather. Hatching is timed to occur when local humidity is 50% and temperatures are between 45 and 65 degrees Fahrenheit. By April 23, Pittsburgh’s highs and lows are exactly in that range so our birds are getting ready. Here’s what they’re up to:
Robins spend 5-7 days building their first nest of the season.
Egg laying begins 3-4 days after first nest completion.
Eggs are laid one per day for a clutch of 3-4 eggs.
Incubation lasts 12-14 days.
From nest building to hatching, the first nest takes 26 days. (Subsequent nests take less time.)
Our robins should be nest building right now except for one thing: Do they have enough mud to begin construction? Has the mud been frozen?
Watch the robins in your neighborhood to see what stage they’re in. Join Cornell Lab’s Nest Watch program and your data can become the basis for studies like James’ and Shugart’s that broaden our knowledge of birds.
(Credits: photo by William Majoros on Wikimedia Commons. Click on the image to see the original.
Today’s Tenth Page is inspired by page 260 of Ornithology by Frank B. Gill, portions of which are quoted(*) in this article.)
After a week near western gulls in San Diego I got pretty used to seeing individual gulls perched high, watching the others fly by. Inevitably, the lone gull would throw his head back and give the long call when other gulls flew over. What did he mean?
The “long call” is used in many contexts, as a greeting between mates or a statement about territory. In this video two great black-backed gulls give the long call when they fight over a fish. Watch the video and I’ll tell you what I think about their interactions.
Their gestures tell the tale.
The hungry gull (HG) approaches, bowed low in a threatening gesture.
The eating gull (EG) sees the threat and opens his wings, “Back off!”
HG turns away and gives the Long Call: He hunches over, bows his head, then lifts it high leaning his body at an oblique angle and calling loudly. You might think he’s not talking to EG because he’s not looking at him. Far from it! By turning away he’s avoiding direct confrontation. Perhaps he’s trying appeasement.
That didn’t work. HG walks past EG without looking at him directly. As he approaches EG’s tail he gets an idea.
Tail pulling didn’t work at all, so the hungry gull bows low (a threat) and walks to the front of EG. Facing him and opening his wings (again, a threat), he tries to steal the fish.
Finally the eating gull has had enough. The two fight. EG quickly wins. Hungry Gull retreats while EG gives the long call in triumph, and then resumes his meal.
What’s the relative stature of these gulls? My guess is that EG (the eating gull) outranks HG (hungry gull), but HG is willing to test the limits.
In Africa sunbirds fill the ecological niche that hummingbirds fill here. Like hummingbirds, they feed on nectar, have long down-curved bills and come in beautiful iridescent colors. The main difference is that sunbirds perch instead of hover.
Like hummingbirds, sunbirds also pugnaciously defend their nectar sources and spend a lot of time chasing and fighting. What is the advantage in doing this? Doesn’t it cost more energy than peaceable feeding?
In 1975 Gill and Wolf studied the energy expended by territorial golden-winged sunbirds in Kenya. Their results were a bit surprising. It costs less energy per day to defend really good nectar sources than it does to feed at undefended low-nectar flowers.
Here’s the math:
Undefended flowers have less nectar because so many birds are feeding at them. Foraging burns 4 kilojoules of energy per hour but it takes 8 hours to get enough food. 8 hours * 4 kilojoules/hour = 32 kilojoules burned.
Defended flowers have twice as much nectar so it takes only 4 hours to get the same energy. 4 hours * 4 kilojoules/hour = 16 kilojoules burned while foraging.
Defending these flowers is energy intensive (12.5 kilojoules/hour) but if it doesn’t take much time it’s worth it. If it only takes 20 minutes to defend those flowers in that same 8 hour period the results are: 0.33 hour * 12.5 kilojoules/hour = 3.7 kilojoules burned in defense.
What does a territorial sunbird do with all that extra time? He sits around and watches his flowers. 3.7 hours * 1.7 kilojoules/hour = 6.3 kilojoules spent sitting.
Therefore his total energy expenditure is 26 kilojoules, a savings of 6 kilojoules in 8 hours.
That’s why hummingbirds are so belligerent at our feeders. They’re making the calculation that defending a great food source is the cheapest way to go.
Photo of a golden-winged sunbird from Wikimedia Commons. Click on the image to see the original.
Today’s Tenth Page is inspired by page 310 of Ornithology by Frank B. Gill.)
The scarlet ibis looks bright orange-red to us but that’s not what the ibis sees.
Unlike humans, birds can see ultraviolet light. This trait was discovered by accident and largely ignored until we figured out that most birds have ultraviolet-reflectant feathers. This opened up a whole new view of plumage.
Above is my poor attempt at showing what this looks like. Instead of orange-red the ibis appears purplish to himself and other birds — more purple than I can show. For an awesome photo of what birds actually look like click here and scroll down to see three views of a cockatiel.
When the ultraviolet colors came to light we uncovered many surprises. The axillary feathers of northern saw-whet owls are UV-reflectant. Who knew their armpits were so flashy! The brightness fades in older feathers so bird banders use UV light to age the owls. Here’s a saw-whet UV photo linked from Washington College’s Chester River Field Research Center where they band the owls. (Click on the photo to read more.)
The world of birds is far more colorful than we imagine.
A scarlet ibis does not look scarlet to an ibis. Really.
Scarlet ibis photo by tj on Wikimedia Commons, retouched by Kate St. John to attempt ultraviolet shades. Click on the image to see the original.
Northern saw-whet UV axillary feathering by Washington College, Chesterfield River Research Center, Northern saw-whet monitoring.
Today’s Tenth Page is inspired by page 100 of Ornithology by Frank B. Gill.)
Much as we’re unhappy with the results, the introduction of house sparrows from Europe began a grand experiment in avian adaptation.
House sparrows were introduced to both the U.S. and New Zealand in the 1850s where they immediately became isolated from their native populations. More than 150 years later they differ based on where they live.
In addition to changes in plumage the birds are different sizes. In locations where winters are harsh, the birds are large. Where the climate is moderate, they are smaller. This effect is called Bergmann’s rule and is true of birds around the world.
Laysan is famous for its bird life, a nesting island for many Pacific seabirds and home of the rare Laysan albatross and even rarer Laysan duck. It was also the home of the Laysan rail, a fearless, flightless bird less than 6 inches long.
Unfortunately, in 1903Max Schlemmer released rabbits on the island as a money-making venture. Instead of making money it was the beginning of the end. The rabbits on Laysan had no predators and in short order they overran the island. (Keep in mind that a rabbit can bear 35 young per year.) The rabbits ate everything. Everything.
By 1918 Laysan was a barren dustbowl on which only 100 rabbits survived. With little to eat and no cover the Laysan rail population was hanging on by a thread. Meanwhile a few rails had been introduced to other islands in the northwestern Hawaiian chain in hopes they could survive elsewhere.
In 1923 the Tanager Expedition eradicated Laysan’s rabbits but it was too late for the rail. The last two on the island died that year. A few hung on at other islands in the chain but the final blow fell in 1944 when a World War II ship drifted to shore on Eastern Island, Midway and the ship’s rats swam ashore. The rats ate the last Laysan rails on earth. That was that. Extinction.
In the broadest sense, loss of habitat killed the Laysan rail. In a narrow sense it was a case of extinction by rabbit.
(drawing by Walter Rothschild from Wikimedia Commons. Click on the image to see the original. Today’s Tenth Page is inspired by page 640 of Ornithology by Frank B. Gill.)
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!