A student in one of my classes asked why, in many species of animals, if one sex is brightly colored and the other is not, or one sex does mating displays and the other does not, that sex is male. I answered, but won’t repeat the answer here; readers should learn the explanation for what is, in effect, one of the “laws” of biology. (Unlike physics, though, this law isn’t unbreakable, as we see from the gaudy female seahorse.) But here’s a nice case that adheres to the “law”:
Reader Bruce Lyon sent a series of photographs (and a 7-second video) of an amazing bird, the Montezuma Oropendola (Psarocolius montezuma), a denizen of the neotropical rainforests of the Caribbean coast, ranging from southern Mexico to Panama. Like other oropendolas, it builds a hanging woven nest. That, and the fact that these birds nest in colonies, presumably helps deter predators, but that’s just a guess on my part.
Wikipedia describes the male call, given during its display, as “unforgettable,” and I’ve managed to find a 1.20-minute recording of it here. As the Sound Recordings Blog notes:
The song of the male Montezuma Oropendola is one of the most unforgettable sounds of the Central American rainforests. Named in honour of the 16th Century Aztec ruler, Montezuma II, this New World bird weaves his song into an elaborate courtship ritual that can be heard throughout the day during the breeding season. With his claws wrapped tightly around a branch, the male will swing forward, displaying his bright yellow tail feathers and performing his distinctive, gurgling song. The aim of this behaviour is to maintain mating privileges with the females in the colony; If the display is not up to scratch however, females will choose to mate with a superior performer.
Bruce’s descriptions are indented below, and the photographs and video are his. He concentrated on the gaudy male and its sexual display.
Montezuma Oropendolas had a nesting colony very close to one of the cabins I stayed in during my recent Costa Rica travels. Oropendolas are the largest blackbirds—they are related to the more familiar red-winged blackbirds and orioles—and they also have the distinction of being among the most size dimorphic birds in the world. Males are more than double the weight of females.
Oropendolas are regular visitors to bird fruit tables at ecolodges throughout Costa Rica so lots of tourists get to enjoy these fabulous birds up close and personal. Below: An oropendola perched near a platter of bananas put out for the birds near our cabin.
The oropendolas colony was in a small gumbo limbo tree (Bursera) growing in a pasture:
The nests are hanging woven bags, similar to those of their cousins the orioles. The nests are huge; these ones were about four feet long. A male hanging on a nest:
The extreme size dimorphism [JAC: I couldn’t find a picture to show the size difference] is likely a consequence of the mating system. Mike Webster (Cornell University) studied this species for his doctoral work and showed that they have a harem defense mating system more like that of mammals than most birds. He found a clear dominance hierarchy in males and the alpha male alone displayed in the nest tree when there were lots of receptive females. This extreme mating skew likely favors the evolution of large males that can rise to alpha status.
Similar to Webster’s observations, at the colony I watched there was only ever one male displaying in the nest tree, but other males displayed from nearby trees. Oropendola displays are wonderful things—both visually and acoustically. The male bows over until completely upside down, while giving an unforgettable bubbling and gurgling call that is a very evocative sound that says ‘tropics’. The male then rights himself from his upside down position with leg power alone.
Here’s Bruce’s very short video of the male display:
Male launching into his display:
Next, the wings flash out:
Almost at the bottom:
Perfectly executed half somersault:
The male typically did his display perched on a branch but a few times he flew to a nest and displayed while clutching the nest:
Gorgeous bird & display! Kind of like a neotropical bird of paradise wannabe! (now go forth & radiate, my little ones!).
Colour scheme and hanging display resemble Paradisea, indeed. Interesting that there’s little or no size dimorphism in BoPs; also no communal nesting, male contribution to nest-building, or harem-defence as far as I recall, and those differences seem to fit together well: group nesting by females constitutes a major resource unit that is therefore worth males defending. (So much behavioural evolution depends on the size of things worth having!)
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Awesome photos! Thanks.
We sure have some excellent photographers hanging out at this site.
Beautiful.
The wildlife photos are some of my favorites. Along with cat pictures, of course.
Of course in sea-horses it is the males that provide parental care whilst the females are free to go off and seek additional mating opportunities.
Great pictures – love the gymnastics the bird performs!
Singing, swinging gymnastics.
Now I want a bird brain!
http://en.wikipedia.org/wiki/Phalarope
Here’s one female bird that does the courting.
There’s an exotic species of bird where the female is larger, fancier and does elaborate courtship preening, but I can’t remember the name (saw it on a recent nature doc). She had big fluffy black feathers on her hind quarters, and shook and preened with her back to the male bird.
It could have been some type of moorhen, maybe gallinula chloropus.
G chloropus is the Common or Eurasian moorhen. It is not significantly dimorphic.
Polyandry occurs primarily in two orders of birds, the Gruiformes, and Charadriiformes. In the second order, the jacanas and some sandpipers are notable here in the New World. Female Spotted Sandpipers, for example, average 25% larger than males. And they compete for any defend territories. The same is true for phalaropes, in which the females are both larger and much more colorful. These species fooled John James Audubon, who mislabeled the sexes in his paintings of these birds. Obviously he was a greater painter than an anatomist.
Yes, in some seahorses the females are much gaudier than the males, and no, many are not monogamous as a rule.
First, well done on the photography, Bruce!
What I’ve wondered about is why humans have the gaudiness thing backwards. Human males are almost always very drab in their dress, and human females generally dress very colorfully, very showy, and use makeup to apply even more color to their faces.
Has anybody explained that discrepancy?
It’s not even a primate thing; look at mandril baboons, for example. Very colorful and showy males, much more drab females. And gorillas, our very close cousins…it’s the silverback males with the more striking coloring. And male orangutans have all that extra “stuff” going on around the face.
b&
Isn’t the usual explanation that all those dress and makeup variables are meant to emphasise females characteristics: breasts, bottoms, eyes, hair etc that males are meant to use to home in on as indicators of youth, fecundity etc that sum up ‘sexiness’? From that point on just about anything can get exapted into social norms of clothing differences between men and women.
Yes, but that’s the fundamental question: why is it that the females are the ones drawing attention to themselves, while the males are drab, but in almost every other species (including some of our closest cousins) it’s the other way around?
b&
Bower-birds would seem to form a good parallel with humans. We males tend to gather shiny objects to supplement our drab ‘plumage’.
I’ve heard prominent displays of the most-desired cellphone described as lekking….
b&
Lovely photographs, Bruce! I’m quite fond of Montezuma Oropendolas.