Egg Shape

Why do birds’ eggs (or any eggs, for that matter) vary in shape?

At least one very intuitive shape exists. Murres, an alcid (the same family as puffins), lay very pointed eggs that roll in circles, which is important as to avoid the eggs rolling off the small ledges on cliffs that they lay the eggs on, without the protection of a nest.  But most birds build nests and avoid that problem.

Rock cormorants, not murres, but the same nesting situation

Important to all eggs is the amount of surface area/volume – a ratio that you may remember from cells, but is important at all levels of biology. Insects are limited in size by this ratio – that is why a walking stick might be 6” long, but no insect is 6” wide. Bird eggs, like everything living, respire and the eggshell, though it might seem solid, is actually somewhat permeable and while an egg needs to exchange gasses, it does not want to lose too much water – as it is laid with all the water it needs, but very little extra. Scientists have predicted that, if this is the only important factor, all eggs should be round as this is the lowest SA:V. Add to this that round eggs are the strongest and you might think all eggs would be round. But they aren't.

Eared Dove nest, Chascomus, Argentina

The shape seems influenced by the female reproductive tract of the particular bird, and this probably accounts for egg shape variation between species. But bear in mind this is a proximate cause – i.e. the egg shape is determined by this, but ultimately, the shape given is the result of evolution of the reproductive tract. But then why do eggs differ perceptively in shape between members of the same species, or even more puzzlingly, between the same female?

 

These three eggs are all of Tachycineta stolzmanni, the Tumbes Swallow, lit to illuminate yolk area. 

An interesting idea has been floating around since the 1940’s. Shape might have to do with how effectively eggs can fit under the incubating bird. Most female birds (and some males) possess a “brood patch” (or several), an area of skin with very good blood supply which, during the breeding season, is featherless and used to heat the eggs. Perhaps the best shape of eggs is one that the most eggs can fit under the brood patch. This model, most recently simulated by Barta and Szekely in 1997, predicts differently shaped eggs for different numbers of eggs and shapes of brood patches. Their predictions are somewhat borne out by observational data – birds that lay one, or many, eggs generally have rounder eggs and ones that have four eggs generally fit together very tightly with their tips pointed in.

Penguins are one of the only groups of birds without a brood patch,
they have a pouch formed by the skin.

But look at any dozen eggs you get from a grocery store and you can see an exception to this. These are all different shapes and chickens are indeterminate layers, meaning that they can lay many eggs (and the number is not known to the chicken when she begins, thus how does she compensate for shape?), and if you keep taking eggs from a chicken, the chicken keeps laying them. And yet shape differences persist in the species. A study of Blackbirds and Great Tits in Spain showed that shape and clutch size were independent, another thorn in the side of this prediction.

Rufous-collared Sparrow and nest

Perhaps small differences in shape have no functional difference and thus selection wouldn’t operate on them. An entirely plausible, and possible, solution and perhaps a really pointed or perfectly round egg might be disadvantageous, but all the variation between two points is similar. But before deciding this, bear in mind the Grant’s finches (dealt with wonderfully in Jonathan Weiner’s book The Beak of the Finch), where a millimeter of beak difference, barely perceptible to an observer, meant life or death for the finches. It seems likely that some other force, or some combination of the discussed forces, determines the shape of a bird’s egg. 

Mystery skull

Ok, not really a mystery, I know what it is, but this is a challenge - and entirely possible with the information contained in the picture (click on it to make it bigger). The only information you get is that it is from Chiloe Island.

I'll post an explanation later. But I expect a couple of you to figure it out first. 

Las mariposas de Chiloe (butterflies of chiloe)

During my stranding in Santiago, I went to a few bookstores and was surprised to discover a guide to Chilean butterflies in one – clearly I needed to buy it. Published in 1996 by two Chilean naturalists, it is a well-illustrated and informative book and all text is in both English and Spanish besides the two forewords one of which is in English and the other in Spanish – and they are entirely different.

Vanessa carye, Chiloe Island

In Santiago, I caught up to Vanessa carye and Phoebis sennae (cloudless sulphur), and on to Chiloe I went.

The overnight bus ride to Chiloe gave me time to scope out the possible species on the island According to the guide, there are 10 species on Chiloe Island – and a detailed survey of the center I am at during 2002 and 2003 turned up 9 of them (Concha-Bloomfield & Parra 2006).

Butleria elwesi, Chiloe Island

The first day here just walking around the center, I found 4 species – including one not found during 2002-2003, again, Vanessa carye, which may be fairly irruptive, like its northern relatives the painted and American ladies - and this year it is common here and in Santiago.  Since then I have found another two species and the other four should be around later in the summer. My goal now is to photograph them all.

Neomaenas poliosoma, Chiloe Island

The radiation of Chilean Satyrnidae, to which this guy belongs, is a group that very little is known of. For what it is worth, I find this species commonly in little forest breaks where fern and bamboo grow. 

Colias vautherii, female

Colias vautherii, male

The above species belongs to the same family as the cabbage white and sulphurs at home. The male is bright orange on the upperwings and the female white/black. 

Eroessa chilensis

This guy is by far my favorite butterfly here. A large, high-flying and gaudy species, this was the only picture I could get of it - resting and nectaring on "michay", Berberis darwini, a holly-like barberry common here and one of many plants that Darwin first collected for western science. 

Sexual selection: Darwin musings

Being at a center named “Senda Darwin”, having just reread the Chiloe section of Voyage of the Beagle, and having tackled his biography – a 700 page tome – this summer, I have been thinking of the great man a bit. What most impressed me of his biography was not the theorizing he did; surely great, but also the part we learn so much about. What impressed me most was his experimentation. When he wanted to learn how plants settled distant islands, instead of immediately embracing completely ridiculous notions of land bridges which many contemporaries and later scientists did (remember that plate tectonics is very young, as theories go), he went about soaking seeds in saltwater for varying lengths of time to see if they would survive, which most did. Of course, as experimentation begets more questions, he found that even while perfectly viable, most of the seeds became waterlogged and sank – which makes drifting to islands pretty difficult.

But what he will be most remembered for, of course, is selection: both “natural” and “sexual”. After saying something along the lines of “the sight of a peacock’s tail makes me sick”, Darwin figured the puzzle out, which he put down as sexual selection in his book The Descent of Man, which I will not profess to have read the entirety of (unlike the Origin of Species and Voyage, Descent is not a short, easy read). Simply, he posited that the choosy sex (usually the female) chooses the opposite sex based on some character and thus that character can become quite ornate. Various hypotheses have been put forth to exactly why, one says that an ornate male – for instance, a peacock male that can survive with such a long tail – is a signal to the female that the male must have good genes.

Peacock, Santiago zoo

And when I was going back through some pictures, I realized I have quite a few pictures of sexual selection – or at least the products of it. 

Fork-tailed Flycatcher, Chascomus, Argentina

Male Fork-tailed Flycatchers display their absurdly long tails in this sort of acrobatic display (accompanied by noise - I'll not call it song) pretty much all day, or at least when any female comes by. 

Saffron Finch pair, Chascomus, Argentina

The Saffron Finches above show a fairly normal sexual dimorphism probably related to sexual selection that basically everyone is aware of: the male is the pretty yellow one on the right and the female is the (also pretty) brown one. Interestingly, in Peru, the same species is also common and both males and female show the bright yellow coloration. Explain that...

But since this is not just about birds, onto other taxa!

Cicada sp., Nantucket

Sexual selection acts not only on coloration and ornamentation, it also acts on behavior. Male cicadas, grasshoppers and katydids (among many, many other animals of many, many taxa) sing for mates, at an increased risk of predation because of this behavior. 

But onto my favorite example of sexual selection, bar none!

Strap-toothed beaked whale, male left, female right

This beast, Mesoplodon layardii, the strap-toothed beaked whale possesses what I believe to be the most ridiculous sexual ornamentation of any animal. Two teeth grow upwards from the lower jaw in males, eventually crossing the top and allowing almost no opening of the mouth, thus adult males limit the size of prey they can eat because of this ornamentation - which is thought to be important in fighting for mates. I talked to a grad student who had dissected at least one of these and she thought the musculature in the throat/lower jaw might be increased and thus allow it to suck food in with negative pressure. 

Let's not forget that we, too, are animals and are thus always trying to impress mates and will dress in ridiculous outfits and do ridiculous things to that end. But a tasteful portrait of costume and ritual:

Gaucho parade, Argentina, Nov. 2010

Cool beetles

As I mentioned earlier, many insects here are bigger and more colorful than those at home. I was walking on a trail near the edge of a forest, when I spied what looked like a big wasp flying by me. Naturally, I netted it and was incredibly surprised to find this in my net.

Long-horned beetle, Nov 14, 2011

Interestingly, it stridulated the whole time it was in the net as well as when I grabbed it. Stridulation is the production of sound by rubbing parts of their body together though I don't exactly know which part it was on this guy (on grasshoppers/crickets it is the wings). The stridulations were high pitched and almost bee-like, which I think may be a form of mimicry, I certainly thought that with the orange body and long legs, it was a wasp and though I did not hear it until later, that may only add to its display. (see here for an older post that discussed bumblebee mimicry in another beetle).

The last picture I took before it zipped away. 

To hear stridulating, the easiest species to find is the red milkweed beetle which occurs on common milkweed basically wherever it occurs. Pick it up and lightly squeeze it and it will cry out! I tell children that it is telling me to let go, and then when they all get a chance to hear it, I say "ok" and let it fly away.

Ceroglossus sp. (magellanicus?)

That picture shows a big, beautiful ground-beetle (Carabidae) that I have found a couple of. I'll detail these guys more, as they are a well-studied genus and quite awesome, but I was excited enough that I figured I'd share it now.

Crazy Harvestmen

Among the many things that are bigger, better and simply crazy here than at home are the harvestman. What on earth is a harvestman? Harvestmen are Opiliones, arachnids but not spiders. Usually referred to as “daddy long-legs” they are common everywhere as scavengers and omnivores. Doug Morse related a story to me of watching one trying to steal the prey from a crab spider! They appear to only have one body segment, unlike spiders which obviously have two, but they actually also have two – they are simply joined together across the width of the body.

Harvestman, 13 Nov 2011

I have found a number of this spectacular species around the center, mostly around rotting logs, though also in our bird boxes. I assume that like the species in New England, these are primarily nocturnal. One of the largest misconceptions – well, let’s say the largest – is the pervasive rumor that harvestmen are incredibly venomous, but cannot pierce skin. This is entirely, completely, and totally false. They do not possess venom glands and having picked up hundreds, I have never been “bitten” (I would guess it is more like stabbing, actually, if it ever happens), so perhaps the second part of that is true. 

This guy makes our species look like wimps!

“Spider bites” in general are grossly over reported and one study I read found that everything from small skin infections to flea bites among others were diagnosed – by real doctors – as spider bites. So enjoy your spiders and harvestmen, don’t be afraid (don't pick up black widows, either...). Scorpions, still another arachnid, stay far away from!

These live in our swallow boxes in Peru. I dropped one on my lap once -
I may never move that rapidly again in my life. 

A Mystery Decapod

The Senda Darwin (Darwin’s trail) center lies up a hill and separated from the road by a river, over which there is a quaint little bridge that often has Ringed Kingfishers on it and Cincloides picking at the banks. On hot days, the water is tolerable so I walk down it sometimes and look for dragonflies, birds and whatever else is around (I found a dead lamprey today!).

The first time I did this, I noticed what I thought were little crabs. So I picked one up and lo and behold – the mutant offspring of a crab and a crayfish. I tried googling “Patagonian flat crayfish” which was incredibly uninformative, but eventually I figured it out. It is a member of the genus Aegla, which are related to the hermit crabs, and are their own family. Apparently, they are pretty much restricted to southern South America.  

They seem to be scavengers (one was eating the dead lamprey) and mostly just sit motionless on the bottom of the stream. Once you get the knack of what they look like, they are seemingly everywhere – a few per square meter. The lineage is ancient (70+ million years old) and there are fossils from marine environments, suggesting they might have evolved there, then moved into freshwater after which the marine forms died out. 

A fascinating little creature that only a week ago, I did not know existed!

Petaluridae

In any new country, I get to see plenty of new birds, insects, plants, etc… Very rarely do they excite me as much as this dragonfly did.

Phenes raptor, male, Nov 14, 2011

I was out walking along one of the trails in the wet scrub habitat here when over my head came this GIGANTIC dragonfly. It landed not 15 feet from me and when I got it in my binoculars, I was able to see the widely separated eyes – the mark of only one dragonfly family: the petaltails (Petaluridae).  I rushed over and quickly caught it and once in my hand, I realized that this guy made Anax junius – the big species back home – look tiny. It was too big to hold out and get the entire dragonfly in the frame with the 100mm lens…

the widely spaced eyes are unique in dragonflies

But it wasn’t just the size that was amazing about this dragonfly, the petalurids are the most ancient of the dragonfly families, and one of the rarest. Worldwide, there are only 11 species, most of which are in Australia/New Zealand, though there are two in the US and one in Japan as well as this one in South America. Little is known about the family and their habits, though their larvae live terrestrially (in wet soil) and some burrow, which is a marked departure from the fully aquatic habits of all other odonate larvae.

Look at the appendages on the abdomen for holding the female during mating!

That day, all counting, I saw three of these and the next day I was able to catch a female at a little boggy area in the forest here. Compared to other big dragonflies (big Aeschnids and Gomphids), these are downright lazy and easy to catch. They seem to spend a great deal of time perching and they don’t seem to fly patrol routes like many other big species, but neither did I see them sally out for insects near their perch.

It took them a long time to recoup from being handled. Usually, dragonflies pump up the fluid in their veins and take off within a minute, often in under 10 seconds. The male, whom I did hold for longer than normal, took about 20 minutes before he flew off weakly to a sheltered perch. The female, who I only held for 30 or so seconds, took a little bit of time, too.

I have caught almost 10 odonate species here so far, but I have to put in the time and key them out (I know a few of the genera by sight) which will take quite a while. More on that later. 

A handsome emerald (Cordulidae) from the river here

Bird Specimens

Studying birds does not solely consist of looking at them in the field or in cages in a lab, much science occurs deep in the bowels of museums in cabinets containing thousands of dead birds. The logical next question is: how does one go about storing a bird for posterity?

Grassland Yellow-finch (male), Nov 9, 2011

Carefully (the obvious answer, right?). You make a study skin, pictured above. Now, of course, there is an art to this process that I have not mastered - mine look decent, not great, and I take forever and a day to create one - I have heard that good ornithologists view the seven-minute-skin as the mark to aspire to, whereas an hour is what I generally shoot for. Basically, to make a skin, you take out all the guts, fill the bird with cotton and sew it back up. Simple as that.

Dorsal view, same specimen, thread temporarily on to hold wings in place while it dries

I was shown the ropes this past summer by a very experienced (and most importantly, very patient) coworker at the Maria Mitchell Association, Julia, who has prepared hundreds of these and who makes them all look perfect without seeming to try and generally doing two while I bumble through one mediocre attempt. So without further ado: beginning your bird. The first thing you must do, while the bird is still whole, is take measurements. Wing chord, total length, tail length and bill length are fairly normal ones, though a whole host of others could be taken, too. Take the mass and assess the molt on flight feathers, tail and body feathers.

A Chucao, thawed, ready for stuffing!

The pictures I will include from here on are of a Chucao Tapaculo, a really cool antbird. At Senda Darwin, they want to begin a collection, as they have a freezerful of birds and mammals, but no one here knows how to do it. So despite my own inexperience, I am the teacher here (and I don't speak the language well!). I prepared the finch pictured previously to show how to do it and Gabby, a veterinarian here, prepared the Chucao. This particular bird was really ratty, it is in severe molt (and molting feathers fall out really easily) and had been frozen awhile and may have beat itself up in the trap, but despite this, Gabby did a good job with it.

Molt. Notice the sheathing on the outer pirmaries (and the secondaries). Also
notice the tiny wings that this bird has for its size. 

After measurements, you must take out all the insides. This involves cutting down the sternum and peeling back the skin from the muscle. Then you must cut bones at the hips and the shoulders in order to pop the body out, which basically inverts the skin at the neck. You can then peel back the neck all the way to the back of the head and pull out the tongue. After which you cut off the back of the skull and remove the brain.

All the insides of the bird from the tail to the base of the skull (neck at right). 

Neck skin inverted, back of skull cut off, ready to take out the brain!

Fill the eyesockets in with cotton and pull the skull back through the skin (it was inside out!). Then you have a skin without any meat on it, which you can stuff with cotton and sew up.

The yellow-finch with cotton eyes. When I showed specimens to kids last summer,
they all wanted to know where the eyeballs went.  

Sounds easy, doesn't it? It isn't, but much can be learned from dead birds, and now, unlike in times past, most specimens are salvaged - from window strikes, car hits or cat kills. The Chucao died in a trap set for monitos (an rare marsupial) and the yellow-finch was found with a broken leg, bleeding, on the ground near one of the swallow nest boxes and died while I was bringing it back to the center.

You have always wondered what bird eyeballs look like, right? Blueberries.

When you are done, a specimen with proper curation should last for hundreds of years (though mine are not of that quality, surely).

Finished, pre-drying, Chauco Tapaculo - the molt made it very ratty  even before
the loss of some feathers during prep - but a damn good first specimen for Gabby.

Peru's arid forests and swallows

Chile is awesome, but more on that later. 

For three months this spring, I was in Pacora, Peru in the northern desert region. Because of the Andes, Peru has a huge rainshadow on the western side. Portions of it are sand as far as the eye can see, while other parts are dry tropical forest. Dry “forests” might be classified in the states as scrubland, as there is a lack of large trees (where you draw the line between a shrub and a tree might be important here), but no matter the name these areas are incredibly interesting and rare areas worldwide.

Bosque de Pomac Sanctuary, Lambayeque, Peru

During my time there, rains fell only twice and each was simply a light drizzle. Rain farther up in the mountains, which this area relies on for agriculture as it flows down the rivers were scarce as well because of the la niña event, therefore many fields went fallow this year. However, the forests teemed with birdlife and lizards. Endemism – the existance of a species in one place and nowhere else – is high in these areas, because of the narrow historic range (the Andes are close to the Pacific) and because of extensive human development in these areas.

White-tailed Jay, a dry-forest endemic

Historically, the Sican and other cultures lived in the area, growing what crops they could on the land and bringing seafood in from the coast. You can still find clamshells scattered around the ruins that were brought and eaten there 1200 years ago. Nowadays, the local economy gets by – certainly does not thrive – on farming rice and other water intensive crops by either damming rivers during the “wet” season and retaining the water or by pumping out the deep water table and irrigating that way. Additionally, they utilize some local resources – Acacia seed pods are harvested to make a sickly sweet syrup much like maple syrup (but as a New Englander, I feel it is inferior).

I was there to study the Tumbes Swallow, an endemic swallow to the area from Chiclayo to about the Ecuador border – basically 6 hours in a car. It exists only in the remaining dry forests and a few towns. Whether it has historically had higher populations, we don't know, but most of the dry forests in unprotected areas (and even in some protected areas) are exploited heavily for firewood – a necessity of life – by the local population. On a more positive note, within the national park of Bosque de Pomac, the Tumbes Swallows are thriving.

Tumbes Swallow fledgling, you would look that upset if you had a parasite on your face, too.

They belong to the genus Tachycineta, which includes species from Alaska to Tierra del Fuego and essentially everywhere in between. You are probably most familiar with the Tree Swallow or the Violet-green Swallow, the two members of the genus in the USA. The Tumbes Swallow is nothing like these, however. It is about half the size of a Tree Swallow and lives in deserts (most Tachycineta live near water).

It seems most likely that their water demands are met solely by the insects they eat, which is a difficult thing in that environment. In a normal water year, there are some mosquitoes around (and dengue), and a few other ariel insects. However, in the la niña year last year, almost no insects were present – the only ones around were these litte gnats that tried desperately to get in your eyes and mouth, probably for the water. How the swallows managed, I don't know, but they did – about 50% of the clutches fledged babies, and more would have if not for predation.

Lesser Nighthawk - a common desert species from the US to Chile

Normally, when a predator hits a nest box, we can usually deduce the culprit. In Chascomus and parts of the US, snakes will eat eggs or chicks. In Tierra del Fuego, southern House Wrens will steal the eggs and eat them, though occasionally just puncturing them and leaving them there with two little holes in them. In Bosque de Pomac, we had no idea what ate the chicks (eggs were never touched). We had two guesses: Peruvian Pygmy-owls, a tiny, day-flying owl, to which the swallows responded violently, or el raton de campo (literally, “the rat of the field”) a rat that would climb trees and may have eaten babies. Upon my return to this cool ecosystem in February I hope to figure out this mystery as well as another interesting phenomena occurring with the swallows – that they breed and molt at the same time, a very rare phenomenon in birds and even rarer in birds that feed on the wing.

Peruvian Pygmy-owl, red morph

Lizards, from tiny house geckos to gigantic tegus, were everywhere in the desert.

Pearl Kite, another raptor around the site that pissed off the swallows



Andre Burnier with the commenest species of bird observed: Black Vultures