Moonchild, you've gotten great advice! I'll add a couple of things that come quickly to mind because some would consider me a fairly hardcore birder, at least when opportunity arises, although you would never guess it from my puny "binos" (binoculars)! So as evidence I offer my life list, which is
here.
All kinds of people become birders, but I think it's safe to say that most are very attentive to details of their environment, especially if they bird a spot regularly. If something changes, they are likely to notice. A birder will pay attention to not just the birds but to the plants, insects, and other living things, as well as other conditions (weather; bodies of water; human intrusion etc.) that somehow have an impact on the birds.
Your MC is likely to have strong opinions regarding the practices of: pishing (making a particular noise, a bit like the one used to call a cat, which can cause some birds to come out of hiding) and playback (use of a recording of a bird song/call to draw in birds); allowing cats--and dogs, but especially cats--to roam outdoors; and bird banding (among others). How willing is he to trespass to get a bird? Is this set in the present day? If so, he might use
eBird, or else he might think that eBird has made things too easy or he might question the ethics of (some) eBird reports (e.g., do you reveal the nesting location of an endangered bird?). Especially if he keeps a formal life list, which not all birders do, he will also have an opinion about the
American Birding Association rules as to what can and cannot count on one's list.
Some birders are interested only in accumulating species for their life lists: once they've seen a bird well enough for it to count on their life list, they're after the next. Others are more interested in observing (and perhaps photographing) behavior in greater detail. Your MC surely falls into this second group.
Because your MC is a photographer, and because birds are small and fast subjects, he will be fussy about his lenses ("glass"). If he doesn't have the sharpest and longest lens available, he'll likely be wishing he does. How deep are his pockets for buying camera gear? Think about whether he's more often the, say, 600mm-and-tripod kind of guy or if he prefers to stalk handheld with a 400mm (or, if his means are modest, a 300mm; shorter than 300mm and he either needs to resort using a teleconverter on a 200mm lens or resign himself to birds usually being tiny subjects in his photographs). He's going to need some muscles--or a monopod--to get sharp shots with a handheld setup. Does he use flash or does he prefer the look of only natural light despite the challenges it can pose?
Besides what others have already suggested, take a look at some (auto)biographies of/by birders, such as Kenn Kauffman's
Kingbird Highway or Mark Obmascik's
The Big Year, on which the Steve Martin/Jack Black/Owen Wilson movie was based. The book tells the true stories of three birders who find themselves indirectly in competition with each other as they try to see as many different species in the USA as possible in the span of one year. It's quite different from the movie and gives portraits of three very different men. Those might help you flesh out your character and his motivations.
Good luck with your story!
(For an enjoyable movie about a group of teen birders, check out
A Birder's Guide to Everything.)