Sumer is icumen in,As Stephen Jay Gould owlishly observes 1, one of the most famous telegrams in biological history came from the young British biologist W. H. Caldwell who in 1884 wired the Annual Meeting of the British Association in Montreal saying: "Monotremes oviparous, ovum meroblastic." The crowd went wild.
Lhude sing cuccu!
Groweth sed, and bloweth med,
And springth the wude nu.
-- Sung annually at Reading Abbey gateway since 1250 A.D.
Confused? Not standing on your seat cheering? Well this telegram solved an enigma which biologists all over the world had been puzzling over for almost a century since the discovery of the platypus. Translation: Platypuses lay eggs (the meroblastic part needn't concern us here).
There are many interesting trails that lead to and from this discovery but the one I wish to follow here is that many, wonderful and varied are the ways in which creatures reproduce. Making copies of yourself is a complicated affair fraught with many prob lems -- and a certain number of rewards as well! There are several possible approaches each with its own set of advantages and disadvantages. To lay eggs or to give birth; to produce many offspring or few; to take care of them or not? All these are possible strategies and have important ecological and evolutionary consequences.
At various times I have explored some of these issues on the pages of the New Brunswick Naturalist and today I would like to expand on that discussion by considering, no not platypuses, but birds.
As most of us amateur ornithologists know, birds build nests, lay eggs, rear their young and then, once fledged, send them off into the world to fend for themselves. What could be simpler. However this conventional scheme of things (and after all do we not teach our children about the birds and the bees?) has some utterly unconventional variants.
One sexual deviant we are all probably familiar with (no need to blanche here -- I promise this article will not become X-rated!) is Molothrus ater -- the Brown-headed Cowbird. The Cowbird is one of 87 species of birds in the world that are so-ca lled obligate brood parasites. Translation: they lay their eggs in the nests of other birds. Not only do they, they must. These birds do not build nests of their own and must find a host to rear their young. Such birds are from four different families: the Cuckoos (Cuculidae), the African Honey Guides (Indicatoridae), the Blackbirds (Icteridae) and the Parasitic Weaver Finches (Ploceidae) of Africa. The European Cuckoo (Cuculus canorus) is, perhaps, the best known of them all and the inspiration behind our word: cuckold (the husband of an adulteress; one whose wife is unfaithful). Of these obligate parasites only the Brown-headed, Bronzed (M. aeneus) and recently the Shiny (M. bonariensis) Cowbirds 2 are found in North America.
Less well known than these obligate parasites are so-called facultative brood parasites. These are birds that, although building nests of their own and raising young also seize opportunities to deposit eggs in the nests of others. You might be surprised to learn that such familiar members of our avifauna as the Canada Goose, Snow Goose, Wood Duck, Mallard, Gadwall, Pintail, Blue-winged Teal, Shoveler, Redhead, Lesser & Greater Scaup, Canvasback, Common & Barrow's Goldeneye, Bufflehead, Hooded, Common & Red-breasted Merganser, White-winged Scoter & Common Eider all engage in such antics. Most such facultative parasites are ducks but Black & Yellow-billed Cuckoos, Ring-necked Pheasants and Bobwhite also imbibe in such behaviour. All of this brings me to the subject of this month's column, a recent theoretical paper by Bruce Lyon & John Eadie called: Mode of development and interspecific avian brood parasitism 3. Say what?
To understand these author's findings we must first explain the distinction between altricial and precocial birds. Chicks of altricial birds hatch from their eggs blind, are for the most part naked, unable to stand and quite defenseless. The young of prec ocial birds, on the other hand, have open eyes, are covered in down, are alert and able to walk and can soon start foraging for themselves. Ecologically the choice facing birds is between laying eggs with small amounts of yolk that then hatch into little- developed chicks that have to be cared for carefully for some time; or laying eggs with sufficient nutrients to let the chicks develop much more fully before hatching. Each strategy buttresses the species against certain dangers but leaves it open to other problems. Precocial birds tie up more resources in each egg and have to incubate for a longer time but when the young hatch they can soon move and are not, literally, sitting ducks. Altricial birds put fewer resources into each egg (making it easier to recover and re-nest if disaster in the name of some predator strikes) but must care for vulnerable chicks for a longer time.
Lyon & Eadie's observation (backed by a theoretical discussion and an analysis of ecological energetics) is that obligate parasitism is found almost exclusively in altricial birds whereas facultative parasitism is predominant in precocial birds. Why? Well , simply put, for altricial birds, who invest less energy and nutrients per egg and can produce a larger number of eggs, it's to their advantageous to drop as many eggs as they can in as many nests as they can and then let the foster parents shoulder the work and risk of rearing the kids. Precocial birds, on the other hand, generally invest too much energy and nutrients in their eggs (and the kids are less of a bother!) to be indiscriminate about where they leave them. In fact, of these facultative brood parasites almost all (29 of 33) are known to parasitize not only nests of other species but also other nests of their own species. This suggests to the authors that the parents are, literally, loathe to put all of their eggs into one basket. Should disast er strike your brood you can rest easy knowing that a few extra progeny are out there with the neighbours.
Now such reproductive shenanigans may seem peculiar enough to you but ecological and evolutionary challenges have taught birds that there is more than one way to skin a catbird (excuse me for this yolk). The 'sitting duck' principle alluded to earlier is a very serious concern of birds. One of the advantages of being a bird is you can fly away, up into the air, where all manner of snakes, cats, mongooses and the like can't follow. Sitting on a nest makes you and your young very vulnerable. This problem ha s been solved in ways other than parasitism. Take the Brush Turkey for instance. Round about van Demon's Land (Australia, for all of you non-convicts) live twelve species of a group called the Megapodes. These birds actually go through the elaborate procedure of building an enormous compost pile in the centre of which they lay their eggs. They then allow the rotting vegetation to provide the heat that will incubate their eggs! The details of this are endlessly fascinating and utterly beyond the scope of this article but those interested should consult Frith (1959) 4 or Seymour (1991) 5 for some extraordinary accounts.
Equally extraordinary, and less known, are the details of the world's only known marsupial bird. "What," you cry in alarm! "This really is straining credulity!" Ahh, but truth is, indeed, stranger than fiction. Deep in the jungles of Central and South Am erica dwells a curious bird known as the Sungrebe (Heliornis fulica) 6. In 1833 the German ornithologist M. A. Wied 7 reported that this bird carried its young about in pouches under its wings. Subsequent generations of ornithologists viewed this report with, to say the least, 'skepticism.' However in 1969 Mexican ornithologist Miguel Alvarez del Toro 8 spent a summer in Chiapas observing a nesting pair of Sungrebes and stunned the ornithological world when he discovered that almost immediately after hatching the male places each of the two chicks in pouches under his wings and forthwith departs.
What happens subsequently is unknown but presumably the male cares for and feeds these naked, altricial chicks until they are ready to fend for themselves, all the while transporting them safely beneath his wings. Voila! Sitting duck problem at least fifty percent solved! By the way, there are two other species of Sungrebes, the Southeast-Asian, Masked Finfoot (H. personata) and the African Finfoot (Podica senagalensis). To the best of my knowledge no one has ever observed the breeding behaviour of these birds. Any takers? Could be some interesting discoveries waiting to be made!
So next time you have to tell someone about the birds and bees remember that at least the birds have more than a couple of tricks up their feathered sleeves.
The Attic warbler pours her throat,
Responsive to the cuckoo's note.
-- Thomas Gray, Ode on the Spring, l.5
The Shiny Cowbird, formerly a Central and South American species, has been expanding its range to the north. This past summer one was found in Maine and before long we can expect to see them appear in New Brunswick.
Beavioural Ecology, Vol. 2(4): 309-318.
Frith, H. J. (1959) Incubator Birds. Scientific American. August, 1959:
Seymour, R. S. (1991) The Brush Turkey. Scientific American. December, 1991: 108-114.
I cannot refrain from mentioning that one of the ornithological high points of my last year was sighting a Sungrebe in the lagoons of Tortugero, on Costa Rica's Caribbean coast.
Wied, M. A. (1833) Beitrage zur Naturgeschichte von Brasilien 2(4): 827-828
Alvarez del Toro, M. (1971) On the Biology of the American Finfoot in Southern Mexico. The Living Bitd 10: 79-88.