I've been birding in CT for over 15 years now, so there isn't much I haven't seen in this small state. The official state list currently stands at 436 species, and I have seen 371 of them. Most of what I haven't seen are legitimate mega rarities. But there are some birds that are "regular" vagrants to the region, and maybe even one or two surely annual migrants that just happen to be super difficult to detect. I thought it might be interesting to pull out the state list and find some regular vagrants that I'm missing, focusing on species that occur often enough regionally to be worth specifically searching for, and discussing possible strategies on finding them.
In taxonomic order:
Swainson's Hawk - I've probably put in as much time looking for this one as any other bird on this list. Not that I sit at Lighthouse Point just hoping for one bird to fly over; autumn hawkwatching at Lighthouse is a ton of fun regardless. But I have logged dozens of hours at LHP in New Haven, one of the prime hawkwatches in New England and the one with the best track record for Swainson's Hawk. So I'm just going to keep doing what I'm doing, and one day I'll be there when a Swainson's flies by. To put things in perspective, this species is not even on the state review list. It is not quite annual, but one is reported more years than not, on average, in recent years.
Yellow Rail - This is a frustrating one because this secretive species undoubtedly moves through the state each and every autumn, but it is just so tough to actually find. Since we know they move through here, it is worth an effort to look for them. The best time is probably late fall or early winter, when a walk through a saltmarsh would give me the best chance to see one...if I'm lucky enough to get it to fly instead of running away. Early spring may provide a chance for migrants or wintering birds to be heard calling at night, though very few people are out listening at this time of year. Think about it...overnight temps in April are still pretty damn cold, so how motivated would you be to drive to a marsh and stand outside to listen for the oh-so-slim chance of hearing a Yellow Rail?
Black Rail - Another secretive bird, and a declining one at that. East coast Black Rail numbers are dwindling overall, and so are the frequency of "overshoots" from their breeding range to the south. However, they still to pop up every now and then, as proven by one bird in nearby Rhode Island this past spring. The way to get this bird in CT is to listen for them in marshes, fresh or salt, from about mid-May to early-June. Then, when you hear one, call me.
Wilson's Plover - A couple similarities with Black Rail here - declining on the east coast (though not drastically, as far as I know), and seen most often as a spring overshoot. Wilson's Plover is a bird of sandy beaches, so should be looked for at places like Milford Point and Sandy Point. Anytime in May is worth looking specifically for this species.
Curlew Sandpiper - With the amount of shorebirding I have done in July and August, plus a moderate amount in May, I'm really surprised I haven't found one of these yet. Those are the prime months for this species, and one can appear in any shorebird flock along the coast during migration. Such a distinctive-looking species in those months, this bird is unlikely to be misidentified unless one turns up late in the autumn when molted adults and juveniles can be confused with Dunlin.
Little Stint - Only one state record, but this species could easily be overlooked in a flock of peep. The best strategy is to work the Semipalmated Sandpiper flocks hard once they begin to appear in early-mid July. An adult in alternate plumage, even a fading or molting bird, could be identified confidently with good views by an experienced observer. Milford Point and Sandy Point would be the two best bets (in fact Julian Hough had the one state record at Sandy Pt in West Haven).
Pomarine Jaeger - I missed my chance during Hurricane Sandy! Most of the jaegers seen during that storm were Poms, though the one we had that day was pretty clearly a juvenile Parasitic. Jaegers of any kind are tough to come by in Connecticut. My best bet would be to spend as much time looking at the water in eastern CT during nor'easters from October through the end of the year.
Dovekie/Thick-billed Murre/Black Guillemot - I'm grouping these three together because the strategy is about the same for each. I haven't had much alcid luck in the state. I did actually, with others, see two distant Dovekie during Hurricane Sandy but have wavered back and forth on whether I should submit them to ARCC or not. Aaaaaanyway, there are three ways to find rare alcids in Connecticut. 1) Seawatch during a nor'easter in late fall through winter, 2) monitor regional listservs for post-storm wrecks, then search coastal nooks and crannies, 3) take the New London-Orient Point (NY) ferry and hope that you cross paths with one en route. I have done all of these and will continue to do so.
Franklin's Gull - Franklin's Gull has always been a really tough bird in Connecticut, with only four accepted records to date. This is a bit strange, since it is regular enough in surrounding states to expect more of a showing in CT. It is most often seen as a late autumn vagrant, so the best strategy would be to scour gull flocks in October-November, particularly along the coast. This species can be prone to "invasions" east of their typical range during migration when strong weather may influence their movements. Each time there appears to be an eastward displacement I try to look for this bird in CT but have come up empty handed thus far.
Arctic Tern - This is another real toughie in CT despite being regular in May-July along coastal Massachusetts and Long Island. The paucity of CT records is undoubtedly due to our lack of an open ocean, which is why Cape Cod and the south shore of Long Island get this species so regularly. So, my best chance would probably come during inclement weather (particularly with easterly winds) from mid-May to early June. One could make the argument, interestingly, that checking INLAND bodies of water may prove more successful than the coast. There is a track record of spring inland sightings in the region during poor weather, perhaps reflecting downed overland migrants versus storm-blown birds. Unfortunately Common Terns do not begin roosting in numbers along the CT coast until mid-late July, so there are no COTE flocks to scour along the coast when Arctic Tern migration is peaking. Thus, inland might be the way to go.
Boreal Owl - Yes, it's a mega but there are a couple recent (i.e. in my lifetime) records. There are winters in which they push further south than other years. I do not have the patience to do much owling, day or night, so I don't see myself finding a Boreal Owl anytime soon...
Gyrfalcon - May have missed my chance this past winter, when several were kicking around the northeast. Late fall through winter is the time to find this arctic raptor, probably feeding on waterfowl and/or gulls, either inland (open country) or along the coast. With solid numbers of waterfowl wintering along our coast, particularly in the Stratford-Milford area, it's probably only a matter of time until one spends a winter in Connecticut again.
Say's Phoebe - I'm really looking forward to seeing this bird in CT. It's going to happen eventually. This western vagrant is annual in states that border CT. There was one recent record, just a couple years ago, of a bird on very sensitive private property which was understandably not made public. This bird tends to show up anytime between September (early for a western passerine vagrant) and December. It can really turn up anywhere, so I don't have much of a strategy for finding this one, other than landbirding in open habitats during the right time of year. Hammonasset has always struck me as the perfect place to find one of these.
Bell's Vireo - There seems to have been a recent uptick in records of this western vagrant in our region, so I'll include it on this list despite there being only one CT record to this point. As is typical for most western passerine vagrants, late autumn is the best time to find this one. I love birding thickets at that time of year (who doesn't?!?), so I'm expecting one of these to pop up right in front of me someday.
Henslow's Sparrow - Weedy fields in November is the ticket for finding this one. This tends to be one of the later autumn sparrow vagrants. I can't think of one spot in particular that looks better for this species than another; it's just a case of hitting the right habitat at the right time of year, like so many western passerines.
Black-headed Grosbeak - We are long overdue for another one of these in Connecticut. It has been many, many years. This is one I'm not expecting to find. Rather, it's probably more likely to turn up at someone's feeder in late fall or winter. Still, while in the field when western vagrants peak in late autumn, I will double-check every Rose-breasted Grosbeak with this species in mind.
Brewer's Blackbird - Something tells me this species slips through New England more often than we realize, probably due to its ability to blend into large blackbird flocks in fall and winter. Brewer's Blackbirds enjoy open country, so I look for this species in autumn blackbird flocks as they feed in plowed farm fields. I expect to find one someday in the Durham Meadows area.
So there you have it. These 19 species I can go out, look for, and feel like I have a legitimate chance of finding in Connecticut. An exercise like this gives me extra motivation to get out there and look, and it adds a bit more strategy to an already strategic hobby. Logistics and strategy are a big part of the draw for me, hence my love of big days.
I could easily have expanded this list if I included species that are not yet on the Connecticut state list. You could certainly argue that a few of those birds, such as Townsend's Warbler or Black-bellied Whistling-Duck, are more likely than several of these 19 already on the state list.