Tree Swallows on the Tuolumne River: You See What You Know...

In my usual life as a geologist and professor, I often describe to my students the secret of mineral or fossil hunting: once you've seen something, however difficult to see, you'll start seeing it everywhere. I've spent my entire first year and a half as an amateur birder, and in that time I never saw or noticed the Tree Swallows that live in our area. Barn Swallows, yes. Violet-green Swallows, sure, but never the Tree Swallow (Tachycineta bicolor).

But then we took a walk on Friday evening through the Joe Domecq Wilderness Park on the Tuolumne River, and I saw a pair of unfamiliar swallows in an old snag and got a few pictures. A short while later I discovered it was a Tree Swallow. 

So here it is on Monday, and I take a walk down to the Tuolumne River bluffs at the future park in Waterford, and there is another Tree Swallow in an old snag, hunkered down in the heavy winds. It was kind enough to let me get a couple of close-ups before wandering off. 

Tree Swallows range all the way from Central America to northern Alaska, and are more tolerant of cold temperatures than other swallow species (when their preferred food of bugs aren't available, they'll switch to plants). They arrive from their migrations sooner than the others, and with global warming, they have been nesting nine days earlier than they used to. Sibley's guide suggests that they are year-round residents in the Great Valley. So why haven't I seen them until now?