Laura Erickson's For the Birds

Thursday, April 26, 2012

Twins and Double Yolks in Bird Eggs

Every year during nesting season, people ask if it’s possible for wild birds to produce twins--two chicks from a single egg. Chicken eggs from the grocery store occasionally include some with double yolks. One researcher in 1849 claimed to have a chicken that laid 10 double-yolked eggs, which he allowed her to brood--9 hatched, producing 9 pairs of twins. However, this has never been replicated. In one study of more than 1100 chicken eggs, double yolks were found only 3 times--that is, in less than 1/3 of one percent. In another, larger study, 2.8 percent of chicken eggs were double yolked. Very few double-yolked eggs hatch. In that large study, 208 double-yolked eggs were incubated and candled periodically to determine if they contained viable embryos, and eggs were examined after the embryos had died. Over 64 percent of the embryos died within the first 7 days. Two of the eggs--less than one percent, pipped, and only one hatched into twin chicks. There are apparently two peaks of mortality, one about a third of the way through development, and another near hatching--apparently the two chicks against each other have a very difficult time bracing themselves properly to pip the egg.

The yolk contains all the nutrition a chick embryo needs to develop to hatching, so that’s not an issue for birds inside a double-yolked egg. The problem is that two embryos need twice as much oxygen as one, and their growing bodies cause the egg to lose water faster. The pores of an eggshell are designed to provide oxygen intake for one developing chick. And as they grow, each bird blocks the other from properly filling the available space.

Peregrine Falcon

There are two ways a double-yolked egg can form. The hen may ovulate two eggs at once, or she may ovulate properly but one yolk gets stuck in the oviduct longer than usual. When she ovulates next, the second yolk moves down with the first yolk, and both end up together where the shell is formed. A case where this seems to have happened in the wild was in 1981, when an unhatched Peregrine Falcon egg from Greenland was examined as part of a study on pesticides. The egg was both longer and wider than a normal Peregrine egg; its volume 35 percent larger. Inside this double-yolked egg were two dead embryos, one that had died at about 24 days of development, the other at 22 days, so both had reached the fourth quarter of incubation. The younger embryo was more deteriorated and decomposed. The researchers believed that after this embryo’s death, decomposition polluted the egg enough to kill the other embryo.

American Goldfinch

Besides that poor doomed Peregrine egg, there are a handful of known cases of twin embryos in the wild. American Goldfinches, Song Sparrows, and Brown Thrashers have produced twin embryos. There are also several known cases of embryo twins in waterfowl, but only because such a large number of duck and goose eggs have been studied for various reasons--fewer than a half of a percent of examined wild waterfowl eggs have had twin embryos. Interestingly, in several cases the twins weren’t the fraternal twins associated with double yolks, but identical twins, caused when a single fertilized ovum, or yolk, divides into two separate individuals. One 1974 study which included both wild waterfowl eggs and duck eggs inside temperature-controlled chambers found that cold temperatures before incubation could lead to this kind of twinning. None of the embryos in this study survived to hatching.

So twins are possible but unlikely in the world of wild birds. Those grocery store eggs with two yolks are genuine rarities.

Three papers about bird twinning:

Thursday, April 19, 2012

Two Chickadee Studies

Transcript of today's For the Birds

Black-capped Chickadee

Every now and then I read a news story that happened quite a while before but I missed it first time around. It turns out that way back in 2007, scientists made a cool discovery about chickadees that I don’t know how I missed. A study from the University of Colorado at Boulder published in Ecology found that chickadees, nuthatches, and warblers foraging in forests can spur the growth of pine trees by as much as one third. These little birds remove so many beetles, caterpillars, ants, and aphids from tree branches that the vigor of the trees increases measurable. Kailen Mooney said that “From the standpoint of the trees, it appears that the old adage, ‘the enemy of my enemy is my friend’ holds true.”

The study was conducted by using mesh to exclude birds from some ponderosa pine limbs for 3 years. The results showed that branches on 42 trees excluding birds had 18 percent less foliage and 34 percent less wood growth by the end of the study.

Anyone who has even a rudimentary understanding of the interconnectedness of life, one of the basic underpinnings of ecology, probably could have figured this out, but it’s good to have actual figures from a peer-reviewed study to back it up.

Another entry in the category of news that was obvious to anyone who pays attention to chickadees and nuthatches but is still really cool to have verified by a scientific study is a doctoral project at the University of Washington. Christopher Templeton is the scientist who showed in 2005 that chickadees have two different alarm calls. The first is a very high-pitched
seee note to indicate a flying bird of prey. This is quite similar to the sound robins also make for the same reason, and whenever I’ve ever heard either species make it, all the other birds have seemed to disappear or freeze, so I’ve always thought other species were reacting to the call.

For lower-level and perched predators, chickadees use the number and intensity of dees in their chickadee dee dee call to indicate the level of threat the bird perceives. In the West, Northern Pygmy-Owls represent a huge threat to small birds, and when chickadees mob one, they use a lot of extra dee notes. A Great Horned Owl represents a threat, too, but not nearly as much to little birds, who can easily out-maneuver one of these huge predators, so they use fewer dee notes.

This winter I myself watched a flock of chickadees mobbing a tethered Great Horned Owl at Houston County’s annual Owl Festival, and they were clearly not all that alarmed, but in past years when I’ve seen them mobbing Saw-whet or Boreal Owls, they go bonkers. It’s always struck me that if I can understand so clearly what a chickadee is saying, surely the birds that spend their lives associating with chickadee flocks most certainly must, and sure enough, Christopher Templeton found, using recordings of chickadee alarm calls reproduced on speakers, that Red-breasted Nuthatches responded with mobbing behaviors, growing increasingly agitated by higher-level chickadee alarm calls. He says that this appears to be learned behavior because the nuthatches’ own mobbing calls are quite different from chickadee calls.

So researchers establish beyond a shadow of a doubt that chickadees are important for forests and also help make other birds aware of danger. It’s always cool when our gut feelings about things that seem like no-brainers are borne up with science.

Personalizing Herons

Transcript of today's For the Birds

Great Blue Heron nest cam

One of the fascinating things about the Cornell Lab of Ornithology’s heron nest chat room has been seeing people’s tendency to look at individuals--human and animal--and jump to conclusions about what is going on in their minds and hearts. This is more focused than simple anthropomorphism. Just as we identify with characters in movies and on TV, and with people we encounter in real life, we can’t help but identify with these birds as we watch their every move up close and personal, and we can’t help but assume that we share some of their motivations.

When the male heron takes his turn incubating the eggs, some people comment on what a great father he is while others attribute his dutiful ways to being a henpecked husband. The male brings lots of sticks to the nest, often handing them off to the female but sometimes arranging them himself. Both of the birds seem to stand up two or three times an hour while on incubating duty. While up, they turn the eggs and rearrange sticks, paying particular attention to the tiny twigs arranged on the floor of the nest where the eggs are, apparently ensuring that the twigs lie flat so no sharp ends can puncture the eggs. As far as I can tell, the male and female put in comparable time in nest repair, but some people pay more attention to one or the other. For example, some people make a point about how the female is fixing up the place after dad messed it up, while others are either impressed with how fastidious Dad is, or amused by how fussy he is. During incubation, while the female is replenishing lost nutrients after egg production, the male spends longer periods incubating than she does.

When he’s off the nest for a few hours, no one comments about his absence, but when the female is gone, there are dozens of comments about her shopping, hanging out with the girls, or even eying the male next door. A great many people say what a “great dad” the male is, with few mentioning what a great mom the female is. When one bird returns and they make contact calls, some people comment on the warm and loving greeting while others think the one on the nest is scolding the other for its absence. These comments go beyond anthropomorphism to individually personalizing these birds to fit people’s own personal histories or the human stereotypes they ascribe to.

There have been some sad and scary moments at the nest. A Great Horned Owl on three separate nights attacked the female when she was incubating. There is no way the male could have helped the situation by flying in--Great Blue Herons don’t fly as fast or maneuver in the air as well as owls do, and their spear-like bill is most effective while the bird is firmly braced on the ground, so there wasn’t much he could do from afar. I was surprised by how many people commented about his “cowardice.” I suspect this owl was a nesting adult female--they’re larger and more aggressive than males, and to attack an adult heron would seem to require that she had young to feed. I am extremely fond of the herons and did not want the owl to succeed, but found it sad and off-putting to read comments about the owl being vicious and even evil.

One of the eggs has become dented and slightly cracked, and it’s fascinating to hear people say one of the birds is clearly sad or distressed or even crying.

Some people so desperately avoid being accused of anthropomorphism that they go overboard the other way. It’s unscientific to ascribe human emotions to birds, but it’s equally unscientific to deny that birds feel those emotions--we have no way of measuring emotions in other humans, much less in other species, so without any concrete way of defining and measuring, discussions of bird emotions are philosophical, not scientific.

Anyway, it’s been great fun for me to observe both the herons I’m so attached to and the other people who for various individual reasons are responding to the herons in various, individual, and extremely human ways. Birds may be my first love, but we humans are oddly fascinating in our own right.

Tuesday, April 17, 2012

Midnight madness in a heron nest

Great Blue Heron
Transcript of today's For the Birds
Ever since the Cornell Lab of Ornithology put up a couple of video surveillance cameras in a Great Blue Heron nest, I’ve been spending a lot of time watching the birds and helping to moderate the chat room at . It was great fun watching the birds fixing up and adding sticks to the nest they built in 2009, and courting and mating, especially because all this takes place on the nest itself. I even got to see the female lay all five of her eggs, which was amazing. Every morning I woke up excited to see what would happen next.

Great Blue Heron -- Third egg!

Then last week, when I was sound asleep and several people were in the chat room, something darted at the mother and got her up, squawking in a loud, strident vocalization we’d not heard before. Next time they could clearly see the bird, she was missing the long, flowing nuptial plumes on the back of her head, though no other feathers seemed to be missing and she didn’t appear to have any injuries. One person on the Cornell staff went through the archived recordings and clipped out the video and played it in slow motion, and we could clearly see that the attacker was a Great Horned Owl. The next night the owl was back again. This time while the female was squawking, the male answered her from a distance.

Great Blue Heron after owl attack

The female heron looked exhausted, stressed, and intimidated. Female Great Blue Herons usually have night duty on the nest, but the male started spending more time near the nest with her. And for two nights the owl did not return. Then Friday morning at about 2:20 am, the owl came back. This time both parents fought her off. Friday night, something alarmed them, but we couldn’t tell what. The male has now been spending time at the nest each night, and last night, Monday the 16th, he was on for at least 13 hours in a row.

It’s impossible to know what the Great Horned Owl wanted, but we’re certain she wasn’t after the eggs--owls can’t swallow large eggs and seldom if ever feed on small ones. She wasn’t after the nest itself. Great Horned Owls do take over old Great Blue Heron nests for their own breeding, but it’s too far into the owl’s own nesting for her to need a new nest. We don’t even know for sure that the owl is a female, but that’s my guess because females are more aggressive and attack larger prey than males. And Great Horned Owls are known to kill and feed on Great Blue Herons. The owls weigh 3 or 4 pounds while the herons weigh 5 to 7 pounds, so obviously an owl cannot carry one in flight, but can strip meat from a heron and carry chunks back to its own nest to feed its young.

Then yesterday, the 16th, we saw that one of the eggs is dented. It’s possible that during an owl attack as the mother crouched or jumped up, she pressed one egg against the stick nest too hard, or that one of the parents stepped on it at some point--we just don’t know.


Exciting as watching the cam has been for me, it’s been fraught. This is no population of birds for whom collecting data and observing mortality is a matter of scientific curiosity. People who have been watching these particular birds interacting and painstakingly turning their eggs and incubating for endless hours are invested in these particular birds. I wake up in trepidation, afraid to turn on my computer in case a tragedy took place over night. Birds devote 100 percent of their time and energy and focus on nesting, and we humans watching the cam are suddenly feeling what this investment really means.

Tuesday, April 3, 2012

Great Blue Heron Nest Cam!

Great Blue Heron

(Transcript of today's For the Birds)

In my lifetime, I’ve been able to watch a lot of birds, and do a lot of writing and engaging with people, and have enjoyed almost every moment of it all. But right now I’m having more fun and a richer experience, both with birds and people, than I’ve ever enjoyed before, thanks to a pair of Great Blue Herons and the Cornell Lab of Ornithology.

In 2009, when I was working full time as science editor at the Lab, a pair of Great Blue Herons started building a nest in a big dead white oak in the middle of the pond right outside the Johnson Center for Biodiversity, where I worked, in Sapsucker Woods. I’d never seen that whole process, and was transfixed--I don’t think I’ve ever spent so many nights up late working because I’d spent so many minutes and hours of actual working time over by the window taking photos. I pretty much documented the herons’ activities every day, and it was thrilling watching them successfully rear four chicks.

Great Blue Heron family

That same male has returned to the nest each year after that and the pair has raised four chicks each time, but I’ve not been around to watch. He’s not banded, but we know it’s him because he’s missing his right hallux--that is, the rear-facing toe on his right foot.

Great Blue Heron

We have no way of being certain about his mate, because she has no unique anomalies, but we’re virtually certain she’s the same one as last year because as soon as she returned this spring, the two of them set up housekeeping with very little preliminary courtship--they were apparently already bonded from last year. I’m in Duluth this year, but I feel like I’m right there, because at the beginning of March, the Cornell Lab of Ornithology worked with a local arborist in Ithaca to climb the tree and get two video cameras in place. One of the cameras, with a pretty wide angle, is set on the tree trunk to catch all the close up action right where the eggs are--when an adult is near the eggs, you can see his or her whole body.

Great Blue Heron -- Third egg!

That camera also shows one of the branches where the adults often perch.

Great Blue Heron

The other camera is above, and that one can be controlled from inside the Lab. A couple of staff members turn it this way and that, able to see what’s happening around the pond from inside. They’ve followed the parents as they’ve flown to trees to gather sticks,

Great Blue Heron

zoomed in on other birds in the pond, and given us amazing close-ups of the herons. It was pouring rain on Sunday, and we got close-up views of the raindrops on the incubating parents’ feathers--how they weren’t penetrating, how they rolled off--it was amazing.

Great Blue Heron in rain

Great Blue Heron in rain

Even more amazing has been actually watching the female laying her first three eggs, 48 hours apart. We can’t see the egg come out, because there’s obviously no camera underneath her, but we’ve seen how her body contorts and thrusts,

Great Blue Heron Laying Egg

how she gulps or pants, and then watched her stand up and lo and behold, there’s a new egg!

Great Blue Heron Laying Egg

Staff people have archived videos for each egg, along with a lot of other clips of fascinating behaviors.

The Lab was planning on setting up the cameras to livestream the whole nesting process starting in mid-April, but with the early spring, the herons jumped the gun. So the cam system is live, with a chat running the whole while, with me, some other Cornell staff members, and volunteers moderating and answering questions. It’s really worth seeing. Just point your browser to and join the fun!

Great Blue Heron Second egg!