Table of contents

August 2000 - Volume 84 - 8

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Cover illustration: In the pink The greater flamingo (Phoenicopterus ruber) is a large bird and a member of the Phoenicopteridae family. This family is widespread with five separate species, including both old and new world flamingos. This species may stand as tall as six feet, and all have the long legs and flexible necks typical of wading birds. Probably related to herons, spoonbills, and ibises, they have unusual hooked-shaped bills allowing them to feed by dipping their mandibles under water and scooping backwards with their heads upside down. Commonly seen on the salt pans and shallow soda lakes of Africa, these birds have enthralled people from emperors to peasants for thousands of years. The birds live in large colonies and may live up to 50 years. Like many birds, flamingos are monogamous throughout their life. Flamingos will breed on small hillocks situated in shallow standing water as a protection against predators. Some of these species are endangered because of the reduction of wetlands and habitat destruction.   All species are varying and beautiful shades of pink, but it is not an intrinsic coloration. Flamingos feed on small amphipods, polychaetes, small crustaceans, and related species of shrimp. The lovely pale pink of the body, tinged with vermilion along the wing edges, as seen in this photograph, results from the shrimps and other invertebrates that are consumed. If the shrimps are bred to be white without coloration, so the flamingo will become white too.   A flamingo typically feeds with its head upside down immersing the mandible (and sometimes the whole head) under water. The upper mandible is narrow and fits into the lower mandible much like the lid of a box. The edges of the mandible have transverse plates called lamelli, which are used to strain invertebrates from the water. As the flamingo walks forward with its head upside down, it filter feeds and sifts through stirred up sediment, using its thick, spine-covered tongue to sieve liquid through the mandible, leaving only the crustaceans.   Flamingos share with gulls, albatrosses, and shearwaters an interesting adaptation for vision. These birds all have an intraocular structure known as an infula. This is a single, linear, trough-like fovea which continues in a horizontal meridian nearly throughout the entire retina. Although humans have a concentration of photoreceptors in the horizontal meridian, the vague increase in photoreceptors in this area offers us little known advantage and may be evolutionary detritus. Although the purpose of this concentration of photoreceptors in this horizontal meridian is not well understood, it appears that the visual acuity of flamingos is in the range of our own and this horizontal band may presumably be used for protection against predation. This linear strip of sharply focused visual acuity would allow a flamingo to see an approaching bird from a considerable distance. Even though these birds do congregate in flocks, they are preyed upon by eagles and large hawks that hunt by isolating injured or diseased birds. When a flamingo feeds, its head is in such a position that the infula would be just above and parallel to the flat horizon of a shallow lake allowing for a panoramic view of the horizon for much of the 360° surround. How these foveae are neurologically integrated is not known. Such an evolutionary variation in foveal construction must be a clever approach for protection of this species.DR IVAN SCHWAB, Sacramento, California, USA