Wildlife photos: the bizarre Horned Screamer

This tweet isn’t from a reader, but from wildlife illustrator Jessica Roux via Matthew Cobb.  When I first saw this, I thought that some wag had glued horns to a bird:

Friends, we need to talk about the Horned Screamer, the unicorn of the bird world / likes to attack with spurs!!! on it's wings. pic.twitter.com/Sl4bOKmuG4

— Jessica Roux (@jessicasroux) November 8, 2017

But no, the Horned Screamer is a real bird! Wikipedia says so!:

The horned screamer (Anhima cornuta) is a member of a small family of birds, the Anhimidae, which occurs in wetlands of tropical South America. There are three screamer species, the other two being the southern screamer and the northern screamer in the genus Chauna. They are related to the ducks, geese and swans, which are in the family Anatidae, but have bills looking more like those of game birds.

Neither Matthew nor I had any idea that such a bird existed:

Here’s a photo of three of them:

And a photo from Arkive:

And the range from the Cornell site; the purple area is the year round breeding and feeding range, and they don’t migrate:

Now as for that “horn”; it really is a horn (my emphasis)

The horned screamer is a massive 84–95 cm (33–37.5 in) long, 3.5 kg (7.7 lb) bird, with a small chicken-like bill. The upperparts, head, and breast are black, with white speckles on the crown, throat and wing coverts. There is a long spiny structure projecting forward from the crown. This structure is unique among birds and is not derived from a feather but is a cornified structure that is loosely attached to the skull and grows continuously while often breaking at its tip.

From Animal Diversity Web (note that they also have bone spurs on their wings):

Horned screamers are large, heavy bodied, fowl-like birds that are most recognizable by their two bone spurs at the bend of each wing and the 15 cm, yellowish-white horn-like projection at the top of their heads. The 2 to 5 cm long bone spurs are a result of fused carpel bones and are covered with keratin. The horn-like projection, which gives these birds their name, is composed of cartilage. When young are born they lack the horn but it slowly grows as they age. Horns seem to be ornamental as they do not have a defensive purpose. They are not firmly attached to the skull, swing back and forth as the birds’ heads move, and are easily broken off. After breaking off they will grow back over time.

A video (you can hear its vocalizations here):

Now your first question will be: why do they have these unicorn horns?

And the answer is, “I have no bloody idea!” My research this morning (granted, enacted at 5 a.m., and without coffee) gives no good answer. Both sexes have the horns, which suggests that it’s either mutual sexual selection or perhaps a form of species recognition, but the males don’t seem to “joust” with these horns, even though the species is territorial.

It seems that little is known about this species. If the horn really is “ornamental”, why it’s there is still unclear. “Ornaments” can be involved in sexual selection, but it’s unlikely that only females use the horn to choose males, for it’s found in both sexes, and natural selection would seem likely to eliminate the metabolically expensive and cumbersome horn in females if they don’t “need” it themselves. Animals don’t have such “ornaments” just to look pretty, so it almost certainly evolved for a reason we don’t yet understand.

Regardless, the horned screamer is the only unicorn bird I know, and the only bird that has a cartilaginous horn.