Saturday, March 21, 2009

on the recolonization of bald eagles to the Bay Area

Birders and other members of the public are regularly seeing bald eagles in the Bay Area and especially the North Bay nowadays. This includes pairs, nests, and lone adults as well as juveniles and subadults. Clearly the recolonization of the North Bay and the Bay Area by Bald Eagles is underway now and we can expect to see them with increasingly regularity and in increasing numbers in the near term and long term.

When I think of this recolonization, I am reminded of a statement regarding peregrine falcons that Greg Septon of the Wisconsin Peregrine Society credited to raptor biologist Fran Hamerstrom. Greg quoted Fran as saying that "peregrines like to be around other peregrines". This simply but astute fact characterized the recolonization of the North American continent by peregrine falcons in the aftermath of DDT and a similar pattern is followed by bald eagles, I am sure. Peregrines clustered in favorable areas and built local populations that then spread further over time from loci determined by concentrations of serviceable breeding locations, food concentration, and other factors that species and populations adapt to in their quest for survival.

Bald Eagles showed up at the Del Valle Reservoir in the East Bay over ten years ago. One of the birds was an Alaska native that had been hacked along the California coast, if I remember correctly. Later, a pair was formed at Lake Sonoma and later yet another at the Laguna de Santa Rosa. Another was formed at Kent Lake in the Marin Water District lands. As these birds succeed or fail, their presence on the land adjusts tropic threshholds. Ospreys are affected and their concentrations no doubt change in relation to the competition and threat from the fish eagles. Production of young eagles by nesting pairs tends to produce wanderers that ultimately will return somewhere near their point of origin, but in their wanderings are no doubt attracted by the presence of other conspecifics. Bald eagles want to be around other bald eagles, just like peregrine falcons want to be around other peregrines and humans want to be around other humans. A few humans were pioneers and spread out to unexplored territories, but most congregate in proximity to their kith and kin. This is natural for various taxa.

It is unlikely that we will ever seen massive concentrations of bald eagles as they do in Alaska or even in the Tule Lake region of far northern California. Bald eagles can congregate by the scores or even hundreds when massive concentrations of salmon or waterfowl provide means of support for many predators in a short distance. This sort of concentration is not likely in the North Bay or the Bay Area. But there is room for several nesting pairs per county in all likelihood, and year round breeders will attract other floaters and younger birds.

In short, be prepared to continue seeing bald eagles in the North Bay in greater frequency as the next few years pass. I could picture perhaps three pairs in Marin County and three to five in Sonoma County and a few in adjacent counties. The East Bay will see a few more pairs and maybe one day we will get reports down on the peninsula. We know that bald eagles need not be wilderness birds and they can become relatively comfortably acclimated to human presence as long as certain threshholds of disturbance are not crossed.

I can speak from recent personal experience that it is exciting to be driving along and to see either a flying or a perched large bird with a pure white head and tail. I could get used to it and I am sure I will. The recolonization of bald eagles to our area is in process and for some it will be almost a daily occurance to see majestic bald eagles in the northern portion of the San Francisco Bay Area.

Stan Moore
Fairfax Raptor Research
P.O. Box 341
San Geronimo, CA 94963
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  1. Stan,

    This morning (3/21/09) Kimya and I saw an adult Bald Eagle over Lake Nicasio in Marin County. This is the first I had seen there, though there have been recent reports. While we were watching, an Osprey began diving on it.

    All the best,

    Chris Conard

  2. Cool Chris --

    I have driven past Nicasio Reservoir three or four times recently hoping to see the eagles and keep missing them. I have seen golden eagles in the area and it would be way cool to see both eagles in the same area at the same time.

    Stan Moore

  3. i saw a golden bald eagle near the sonoma county airport two days ago
    it was really neat lots of marble in the feathers and a white to mable colored head not as big as a turkey buzzard but it was twice as large as our red tailed hawks

  4. Good Stuff. the Eagles are fun. Glad they are coming back... Thanks for posting this.