Iris coloration in birds (particularly passerines) is a topic that has intrigued me ever since I started observing the behaviour of different starling species. Some years ago in this publication, Peter Ryan provided an excellent review of the structure and function of birds’ eyes (African Birdlife September/October 2013: 28‒34). My focus here is much narrower, but let’s start with some basic anatomy.
As in humans, the pigmented area of a bird’s eye is the iris, which surrounds the pupil, but the sclera (the white of our eyes) is not visible when we look at the eye in a living bird. Pupil size in animals varies primarily with light conditions and is influenced by internal physiology. However, whereas in our eyes variation in pupil diameter is an automatic reaction controlled by the sympathetic nervous system, birds also have voluntary control over muscles in the iris. So deliberate changes in pupil size can display either a very large area of pigmented iris or a very small area, under exactly the same light conditions. Clearly this can serve as a close-range signal during interactions between individual birds.
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Esta historia es de la edición March/April 2022 de African Birdlife.
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EXPLORING NEW HORIZONS
Keith Barnes, co-author of the new Field Guide to Birds of Greater Southern Africa, chats about the long-neglected birding regions just north of the Kunene and Zambezi, getting back to watching birds and the vulture that changed his life.
footloose IN FYNBOS
The Walker Bay Diversity Trail is a leisurely hike with a multitude of flowers, feathers and flavours along the way.
Living forwards
How photographing birds helps me face adversity
CAPE crusade
The Cape Bird Club/City of Cape Town Birding Big Year Challenge
water & WINGS
WATER IS LIFE. As wildlife photographer Greg du Toit knows better than most.
winter wanderer
as summer becomes a memory in the south, the skies are a little quieter as the migrants have returned to the warming north. But one bird endemic to the southern African region takes its own little winter journey.
when perfect isn't enough
Egg signatures and forgeries in the cuckoo-drongo arms race
Southern SIGHTINGS
The late summer period naturally started quietening down after the midsummer excitement, but there were still some classy rarities on offer for birders all over the subregion. As always, none of the records included here have been adjudicated by any of the subregion's Rarities Committees.
flood impact on wetland birds
One of the features of a warming planet is increasingly erratic rainfall; years of drought followed by devastating floods. Fortunately, many waterbirds are pre-adapted to cope with such extremes, especially in southern Africa where they have evolved to exploit episodic rainfall events in semi-arid and arid regions. But how do waterbirds respond to floods in areas where rainfall - and access to water - is more predictable? Peter Ryan explores the consequences of recent floods on the birds of the Western Cape's Olifants River valley.
a star is born
It’s every producer’s dream to plan a wildlife television series and pick the right characters before filming.
