Birding News & Views

The Eyes Have It

Pecten: A Unique Structure In The Eyes Of Birds

by Howard Topoff

How much energy does it take to see? The visual system is one of the most energetically demanding systems in the brain. Although seeing obviously doesn't require the energy of running a marathon, the millions of rods and cones in the retina use quite a bit of oxygen and nutrients to convert light into nerve impulses and transmit them to the brain. So you will not be surprised that the human retina (and that of most animals) is infiltrated with a large supply of blood vessels. After all, it's the blood that carries oxygen and nutrients to all retinal cells. But all these blood vessels create their own problem: they block some of the light from reaching the retina.

Birds, which are even more visually dominant than humans, have solved this problem in a most unusual way. The retinas in their eyes contain NO blood vessels - they are avascular. So, the $64.00 question is: how do the eyes of birds, which are among the best developed in the animal world, function without these vital molecular supplies? The answer is their retinas ARE supplied with oxygen, amino acids, sugars, and all necessary nutrients, but not from blood vessels in the retina. Instead, birds have evolved a truly unique structure called the PECTEN.

The Pecten (see diagram) consists of folded tissue and is filled with melanin granules ( the same pigment that darkens our skin and hair - well, it used to darken MY hair). It projects from the retina into the vitreous, is well supplied with blood vessels and keeps the retina supplied with oxygen and nutrients.

Scientists still have a lot to learn about this mysterious structure and have hypothesized several additional interesting functions. For example:

1. Slight warming of pecten due to absorption of light by melanin granules enhances the secretion of nutrients into the vitreous, eventually to be absorbed by the bird's avascular retina.

2. The pecten may shade the retina from dazzling light or aid in detecting moving objects. And since it contains melanin, it may lessen stray light entering the bird eye to reduce background glare.

3. You probably heard of the blood-brain barrier. The cells of the capillaries in the brain are so tightly joined that many (especially toxic) substances are prevented from diffusing into brain cells. Turns out the capillaries in the pecten also have these "tight junctions," so the pecten might constitute a blood-retina barrier, keeping harmful substances out of the bird's retina.

When it comes to relative size, visual acuity, and spectral sensitivity, the eyes of birds are truly unique. The evolution of the PECTEN is just one additional adaptation that gives new meaning to the term "Bird Brain.”


The North American Migration Count

(Courtesy Of Mike Williams)

The North American Migration Count (NAMC) was started in 1992 to give bird watchers the opportunity to enjoy a day's birding during spring migration with the knowledge that the results of their findings, together with the birds counted by others, would reveal the status of bird migration on a specified date. The goals of the NAMC are to get a picture of the progress of spring migration and to obtain data on the abundance and distribution of each migratory species.

(Mexican Chickadee by Robert Shantz) (Thick-Billed Kingbird by Maya Decker)

The Migration Count for Cochise County was held on Saturday, May 9th. 211 species were seen, the highest in a decade. In the Portal area, including up to Rustler, 10 people and 1 tour group were checking birds at feeders and in the field. We saw 132 species and 2423 total birds. 10 of the 132 were only seen in our part of the county. These 10 were:

Band-tailed Pigeon Blue-throated Hummingbird

Cassin's finch Mexican Chickadee

Peregrine Falcon Pygmy Nuthatch

Red-Naped Sapsucker Rufous-backed Robin

Tropical Kingbird White-throated Sparrow

There were 20 species that the majority of the species were seen in our count area of the county. These included Brown-crested Flycatcher, Elf Owl, Gambel's Quail (below) Magnificent Hummingbird and Northern Cardina

(Cecil Williams)


Willow Tank - Helping A Critical Wildlife Sanctuary

by Alan Craig

It is critical tokeep water in Willow Tank to provide wildlife habitat. One wouldhave to camp outat the tank to fully appreciate how manybirds, bats and critters use the water at WT, let alone the foodand cover the vegetation provides. During the 2+ months ofSpring migration and a much longer period during Fallmigration, those birds that migrate by day may stop onlybriefly for a drink or to grab a few insects before continuingout of sight en route to their nesting or wintering grounds.

It is owned by the Larry River's family and we owe a big thankyou to them for their extensive financial and physical help inkeeping this vital water source open.

Cecil Wiiliams

Willow Tank is open to bird watchers. There is water onlyin the deep end now as the banks and reinforcing is completed. There will be multiple viewing stations and stairs, which will be completed in the next month or so, followed by the refilling.

A few days ago there were 4White-faced Ibis feeding on the pond along with a pair ofMexican Ducks.

Check it out!

WT is on Sulphur Canyon Rd. west off of Stateline Road just asStateline intersects U.S. 80.

Friends of Cave Creek Canyon strongly believes in preserving and improving this valuable resource. We have made a sizeable donation and encourage you to also help. For more information, contact Alan Craig at 520-558-2220

Howard Topoff 2011