Leucistic House Finch

House Finches are quite variable in the amount of streaking, the extent of red areas and in the shades of red.  I currently have several bright red males, at least one orange and a couple weeks ago...a partially white bird.

Abnormal pigmentation is usually means there's been a glitch in the bird's genetic processes involved with deposition of pigments when the feathers are being formed but sometimes disease and diet can be factors that interrupt the pigment placement.

According to Cornell Lab of Ornithology's Handbook of Bird Biology, pages 3-50 to 3-52, there are three main types of pigments found in birds: melanins, carotenoids and prophyrins.   Since porphyrins probably aren't involved, I'm going to leave them out of this little discussion. 

Melanins are deposited on skin and feathers as tiny granules.  They are the most common pigment, producing shades from blacks to browns, red-browns, yellows and pale yellows (eg. the yellowish down of young chickens).  Birds synthesize melanins from amino acids acquired through their diet.

Carotenoids produce reds, oranges and many yellows.  They are synthesized only by plants, therefore, birds must acquire them through their diets - either by 'eating the plant substances or eating something that has eaten the plants'

In an article titled Bird Leucism at About.com, Melissa Mayntz says:

The degree of leucism, including the brightness of the white and the extent of pigment loss, will vary depending on the bird’s genetic makeup. Birds that show only white patches or sections of leucistic feathers – often in symmetrical patterns – are often called pied or piebald birds, while birds with fully white plumage are referred to as leucistic bird.

 And

Leucism, or leukism, is an abnormal plumage condition caused by a genetic mutation that prevents pigment, particularly melanin, from being properly deposited on a bird’s feathers. 

Shortly after these photos were taken, my area was hit with a sudden and extreme drop in temperatures.  It was -30 pretty well steady for 3-4 days, with a brisk wind creating a windchill factor of -41C. 

Mayntz again:

While leucism can be unusual and exciting for a birder to see, birds with the condition face special challenges in the wild. Lighter plumage may rob the birds of protective camouflage and make them more vulnerable to predators such as hawks and feral cats. Because plumage colors play an important role in courtship rituals, birds with leucism may be unable to find strong, healthy mates. Melanin is also an important structural component of feathers, and birds with extensive leucism have weaker feathers that will wear out more swiftly, making flight more difficult and eliminating some of the bird’s insulation against harsh weather. White feathers also reflect heat more efficiently, which can be fatal for birds that rely on sunbathing and solar radiation for heat in northern climates.

The warmer weather has now returned, but I haven't seen the white finch at the feeders.  

Reference articles and general reading on Leucism/leucistic birds:

Bird Leucismhttp://birding.about.com/od/identifyingbirds/a/leucism.htm

Sibley's   Abnormal coloration in birds: Melanin reduction
http://www.sibleyguides.com/2011/08/abnormal-coloration-in-birds-melanin-reduction/

Carotenoid Pigments in male house finch plumage in relation to age, subspecies and ornatmental colorations

Caron Y. Inoye, Geoffrey E. Hill, et al
http://www.auburn.edu/cosam/faculty/biology/hill/lab/documents/69.pdf

Plumage variations: Albinism or Leucism?
http://www.birds.cornell.edu/pfw/AboutBirdsandFeeding/Albinism_Leucism.htm

Color Aberration in Birds
http://www.massaudubon.org/printwildlife.php?id=34