Depending upon how long you hang out watching your birds, you might want to think about having a few small chemical heatpacks in your pannier or underseat bag. Heat Grabber is one of the easiest to find.

If you haven't used them before, keep in mind that it takes the packs about 10 minutes to throw out serious heat once you've torn the seal.If you use a pack for an hour or less, you can usually get a second go-round out of it if you seal it into an airtight container as quickly as you can. (I wrap mine in foil and put them in a small metal can with a tight lid as soon as I finish my ride.)

Contact with air is what causes the pack to heat up, so if you cut off the air supply, it cools off. Even in the container they'll continue to lose a bit of heat, so you'll want to use them a second time within a few days of the first outing. And always take a fresh pair with you anyway, in case the first used pair doesn't heat up enough. Better to be safe than freezing!

I'm sure all of TE's Wool Weenies would also encourage you to wear wool on your outings. Most of my winter wool cycling base layers have zippers so I can adjust the warmth factor as I toodle along. If I stop somewhere for more than a minute or two I can quickly zip up everything to stay as warm as I can, then unzip as needed when I'm pedaling again. Works like a charm, especially when combined with a good-but-light windproof outer layer.

If you're really self-indulgent, snag a pair of Sidi Toasters for your cycling shoes/boots! I nabbed a pair last winter for half price (still $$-y), but they've already paid for themselves in terms of comfort. I tuck them into my hiking boots, too, so get more than just cycling time out of them.

Combining biking with birding sounds GREAT! Have fun!