Cactus Wren Campylorhynchus brunneicapillus
USA and Mexico. Various dates 2004/05

The Cactus Wren is the largest North American wren, and is 18-23 cm (7-9 inches) long. Native to the south-western United States southwards (southern California, southern Nevada, Utah, Arizona - it’s Arizona’s State bird, and western Texas) to central Mexico it is as might be expected a bird of arid regions, and is often found around yucca, mesquite or saguaro. It nests in cactus plants, sometimes in a hole in a saguaro, or where its nest will be protected by the prickly leaves of a cholla or yucca. It rarely drinks water, getting its moisture from its food (mainly insects).

Cactus Wrens form permanent pair bonds, and the pairs defend a territory where they live all through the year. There is little sexual dimorphism.

Two main groups occur: the nominate brunneicapillus is found in NW Mexico and includes couesi which ranges from SW USA, N Baja California, and northern parts of NW Mexico; and affinis which is found through the Baja peninsula (”Birds of Mexico and northern-central Mexico”, Howell and Webb).

Near Mexico City, Mexico, June 2004:

 

 

 

 

 

 

 

 

 

 

Joshua Tree National Park, California. June 2004:

 

 

 

 

 

 

 

Leucistic individual, Papago Park (near Phoenix), Arizona. November 2005:

 

 

 

 

So what’s the difference between ‘leucism’ and ‘albinism’?

Many birders, myself included, would happily describe the bird above as a ‘partial albino’ - but, speaking in a strictly scientific sense, we’d be wrong. Simply put, albinism is a genetic aberration where there is a total absence of pigment in the plumage and/or bare-parts. An “albino” bird therefore is one with no pigment whatsoever: it will have totally white feathers, and it will have a pink bill and legs, and pink or red eyes, caused by the blood supply under the pigmentless skin being visible.

So what is ‘leucism’ therefore? Scientists and aviculturalists use the term slightly differently, but in essence ‘leucism’ is a partial loss of pigment which can affects all of the colours present and ‘dilute’ their intensity, or cause asymmetric and randomly-arranged patches of white feathers. In a leucistic bird some/most of the ‘normal’ colours and associated plumage marks are typically still present, therefore.

A diluted or washed-out appearance can be caused by another aberration called ’schizochroism’. Here, though, one or more specific pigments (perhaps red or green) is missing: every other pigment is present at normal levels, but where a dominant colouration is lost this can result in the bird looking abnormally “pale” (if the dominant colour was black, and the genes for creating black pigment is faulty, then the bird will have white feathers instead of black ones, but will still more than likely have normally-coloured bare parts).

The Cactus Wren in the photos has white patches scattered throughout its plumage, but does have normal - not ‘diluted’ or washed-out - colour in eg the folded primaries and in the retrices (tail feathers). It has no pigment in the bill or legs, but does appear to have a normally coloured eye - though because of the rich blood supply to the eye “full albinos” often appear to have dark eyes. Clearly, though, this is not an ‘albino’ and is therefore ‘leucistic’: but is it a ‘partial albino’ - after all many birds have the odd white feather or feather tracts? Well, technically speaking there’s no such thing as a ‘partial albino’ because an albino is the result of two recessive genes - you’ve either got two recessive genes or you haven’t. I suspect the next time I see a bird with white splodges all over it I’ll be thinking ‘partial albino’, but maybe - like using meter instead of foot - I’ll get my head around ‘leucistic’ eventually…

All photographs © Charlie Moores