The rolling hills of California's interior valleys are dotted by many species of oak trees. Oak trees, of course, produce acorns in abundance, a fact not lost on native American people that inhabited these areas before the arrival of Europeans in western North America. Acorns were used as an important part of their diet. Another animal species that inhabited that same range, also evolved to make full use of the abundance provided by oaks. The acorn woodpecker (Melanerpes formicivorus), found from central California down into Central America, not only uses acorns for food, but also has learned to store acorns, and protect those caches from various species that would pilfer them.
Living in large groups, which is unusual for woodpeckers, they breed cooperatively, and work together to gather acorns by the hundreds. Using their powerful neck and back muscles, and stout beaks, they drill holes in dead trees, dead branches, telephone poles, wooden building, and even fence posts, as can be seen in this week's photo, in which to wedge the acorns. These collections of acorns are referred to as granaries, which are typically guarded by at least one member of the communal group. Typical cache robbers include Stellar's jays (Cyanocitta stelleri) and western scrub jays (Aphelocoma californica).
Acorns are such an important part of the diet of acorn woodpecker populations in California may nest in the fall to take advantage of the acorn crop. Nesting at this time of year is unusual for birds, but the acorn woodpecker is unusual in another way as well. As mentioned, these birds tend to live communally, breeding cooperatively, which is thought to occur in less than 10% of bird species. Communal groups can consist of up to seven co-breeding males, and up to three co-nesting females, but most nests are made up of three males and two females, which are often closely related. While the males are typically brothers, and the females typically sister, very little inbreeding occurs since co-breeders of the opposite sex are never closely related.
This photo was taken with a Canon EF17-40 f/4.0 lens zoomed to 40 mm on a Canon EOS 5D Mk III. The exposure was set to 1/350 sec at f/16 and ISO 800.