Thursday, 22 September 2016

When He and She Look Different; Birds #1

After quite a hiatus, I am back live in my office, and will be for some time, so my usual weekly postings (not pre-prepared as has been the case recently) recommence now. 

It is an interesting phenomenon in the natural world that in some animal species the sexes are externally identical, in others they differ relatively subtly, though consistently, and in still others they look so different that they could be (and in some cases have been) described as separate species. Our own species of course is a case in point - on average (which of course means there are always exceptions) males are larger than females, and there are obvious physical differences. 

Today though I'm going to limit myself to birds, because this is a very large topic - in fact I'm not going to even attempt to complete it today. In species which do differ physically between sexes - ie are dimorphic - it is usually the males which are larger and more colourful. Not always however.
Australian Painted Snipe Rostratula australia, Jerrabomberra Wetlands, Canberra.
The female, on the right, is larger and more brightly coloured.
(Though I'm not entirely certain that the other bird isn't an immature - it is a feature of such species that the less
colourful sex often has very similar plumage to young birds of both sexes.)
This is a rare and seldom-seen species whose appearance here in 2011 generated considerable interest.
Brown Falcon pair Falco berigora, Sturt NP, New South Wales.
The obviously larger bird on the right is the female; this is consistently true for most diurnal birds
of prey - falcons and hawks/eagles, even though the two groups are not at all closely related,
as well as many owls.
In this case the purpose seems to be to divide up the territory so that the two birds are hunting
different-sized prey from each other, utilising the resource more efficiently.
The colour differences here are simply part of a wide variation in this species, and are not sex-linked.
It is a useful rule of thumb that monogamous species tend to be monomorphic - both are contributing significantly to the ultimate breeding success so neither is more expendable. On the other hand a large proportion of polygamous species tend to be dimorphic, with the dominant sex being larger and more brightly coloured; as noted above this is usually the male. It is glib but nonetheless at least partly true that the more brightly coloured a male is relative to the female, the more socially useless he is likely to be!

Perhaps more helpfully, his conspicuous plumage's role is likely to be primarily for attracting a mate (or several mates) - "I can afford to be so easily seen because I'm strong and smart enough to survive, and isn't that what you want in the father of your chicks?". However, in the broader scheme, the same message helps in intimidating rival males and maintaining the territory. It may even be that by being colourful and loud he is attracting the attention of predators who are thus less likely to notice his more subtly-coloured mate sitting quietly on the nest.

We can say that strong colour dimorphism is commonest among species that nest in the open. As ever in nature, it's all a trade-off - be inconspicuous to predators and you're unlikely to appeal to a desirable female. Be too obvious and you'll end up as lunch before you're a father. An extreme example of this is the peacock's ridiculous tail - the longer and heavier it is, the more females are impressed. Simultaneously the more likely he is to be unable to escape the attentions of tiger, leopard or dhole.

However, I'm going to start by introducing some milder, but nonetheless obvious, examples of dimorphism, where the female is similarly coloured to the male, but generally paler and less intense. Where this becomes a different colour as opposed to a variation on the same shade is of course subjective, and some of these examples could as readily have appeared in next week's offering of more dramatic examples of dimorphism. 
Spotted Pardalote Pardalotus punctatus, female above, male below.
(She belongs to the yellow-rumped mallee race, xanthopygus;
I was a bird bander in a past life.)
She is only subtly more different, with buffy spots rather than his strikingly white ones,
and without his yellow throat.
Tasmanian Scrubwren Sericornis humilis, Freycinet NP, female above, male below.
Like most other scrubwrens, she's a washed-out version of him.

Zebra Finches Taeniopygia guttata, Kata Tjuta NP, Northern Territory.
She, in the centre, lacks his chestnut cheeks and flanks.
Australian Darters Anhinga novaehollandiae, Canberra, female above, male below.
His plumage is richer in colour, especially the chestnut throat and dark breast.

Cockatiels Nymphicus hollandicus, Sturt NP, far north-west New South Wales.
This exquisite arid  land cockatoo is the world's smallest.
Again, males are a more intense version of the grey, white and yellow theme.
Cactus Finches Geospiza scandens, Santa Cruz, Galápagos, female above, male below.
He has much more melanin in his feathers, but it's essentially the same pigment.
White-browed Woodswallows Artamus superciliosus, Canberra, female above, male below.
Again the difference is evident, but is in intensity of shades.

Ducks are particularly notable in dimorphism (though by no means all of them of course), and several will feature in next week's post of extreme dimorphism. Here are a few more subtle ones.
Australian Wood Ducks Chenonetta jubata, Canberra, male right, female left.

Green Pygmy-geese Nettapus pulchellus, Fogg Dam, near Darwin, female left.
These are not really geese at all, but in the mainstream line of duck.

Chiloé Wigeons Anas sibilatrix, Puerto Natales, southern Chile, male right.
Despite the name, this pretty duck is widespread across southern South America.
Some species are subtly dimorphic in a more specific way - they have very similar plumage except for just one feature. This may even be as discreet as eye colour!
Black-necked Storks Ephippiorhynchus asiaticus, Croydon, North Queensland.
He has black eyes, she yellow.
Galahs Eolophus roseicapilla, Nambung NP, Western Australia.
In this case she has red eyes, while his are black.

Magpie-larks Grallina cyanoleuca, Canberra, female above, male below.
In this case the only difference is in the face colour - black for him, white for her.
(Immatures have a black forehead and white throat and sort it out later!)

Magnificent Frigatebirds Fregata magnificens, Galápagos, male left.
She has a white throat, where he of course has the red throat pouch.
(In this case he is flying higher than her, he is not smaller.)
Olive-backed Sunbird pair Cinnyris jugularis, Cairns, Queensland.
It may initially seem that they are very different, but nearly all the difference is in his iridescent throat.
Nankeen Kestrel Falco cenchroides;female, Fraser Island, above, male, near Canberra, below.
Here the distinction is in crown and tail, chestnut for her, grey for him.

And that will do us for today. Next week, as I've flagged, I'll conclude this series (for now at least) with examples of more extreme dimorphism, where the sexes are entirely different. 

(By then I hope to have been able to process my photos from my last two Australian trips, to the western deserts and the tropics, so I can share some of those lands with you.)


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Anonymous said...

As great post as usual Ian but I think you misnamed the Woodswollow.

Ian Fraser said...

OOps, thank you, how embarrassing! I got the scientific name right, but was obviously not concentrating on the English one... Now fixed, sorry I can't thank you by name.