After passing Hog Island, I heard a series of high-pitched, descending whistles. I recognized the call as a species I don't hear very often, so we started scanning to see if we could find the bird responsible for the call. Eventually we located two birds perched up high in a tree on the north end of Hog Island. Can you spot them?
We decided to sail past the north end of the island for a closer view. This image, taken as we approached the island, should help you with the identification:
And although we were still pretty far away and the birds were perched at the top of the tree, luckily I had a zoom lens with me which resulted in a couple of decent close-ups:
Bald Eagles (Haliaeetus leucocephalus) are rare migrants, winter residents, and recently re-established breeders in Sonoma County. (Hog Island is in Marin County, but this blog is mostly about Sonoma County, so I'm describing their status in the latter.)
Bald Eagles were more common in northern California in the early 1900s. Their populations suffered steep declines in the mid-late 1900s (e.g., from persecution and pesticide impacts), but with protection they have recovered. They are still rare in this area, but sightings are now regular and there are two breeding sites in Sonoma County — Lake Sonoma (first recorded in 2001) and Laguna de Santa Rosa (first recorded in 2007). [If you're interested in looking for Bald Eagles locally, two have been observed recently on Penny Island near the mouth of the Russian River.]
The call of the Bald Eagle is somewhat unusual. You may see it described as "weak," "trivial," or "insignificant." It's known as a chatter call and consists of several short descending notes sometimes written as kee kee kee kee ker.
Interestingly, if you've ever seen a Bald Eagle in a movie, you may have heard a dramatic high-pitched scream. This is not the true call of a Bald Eagle, but instead a Red-tailed Hawk. I've never known if the movie-makers used the wrong call by mistake (lack of attention to detail) or on purpose — e.g., if the true call of the eagle was too "wimpy" for the powerful image of this majestic bird of prey. But it always distracts me to hear a Red-tailed Hawk call when watching a Bald Eagle on-screen. Movies are hardly ever accurate when it comes to bird sounds, but this is one of the most bothersome examples.
If you'd like to compare the two, listen to the Bald Eagle call and the Red-tailed Hawk call at the Cornell Lab of Ornithology's web pages.
P.S. Now this is a little weird, but it may help with imagining what a Bald Eagle call sounds like. As I was writing this, I was puzzled in that I thought I heard eagle call notes, but I wasn't playing a recording. It turns out that there was a basketball game in the background (Celtics vs. Thunder), and the sounds of sneakers squeaking on the court are somewhat similar to eagle call notes!