That is until you see all the little bastards at 100 yards whipping around as swallows do, so I will make a point of the little things that field guides cannot teach succinctly in the quarter page each bird gets. I spent three weeks in the Bahamas, on Abaco Island, erecting nest boxes for Bahama Swallows. I found out how difficult separating these two species in flight can be, when a good birder, a local guide, called me and reported "thousands of Bahama Swallows". Being without a car, I hitchhiked the hour drive and came upon a group of thousands of Tree Swallows, a bit disappointing, to say the least.
Unfortunately, this post comes a bit late for the post-Irene fallout of Caribbean birds all over the east coast. I will muse a bit more on Bahama Swallows in the future, but suffice to say that the sightings of these birds drop way down between August-January and I don't have a clue why. So the chances of one having been whipped up were pretty high, in my opinion.
The easiest ID feature on the Bahama Swallow is that the white underside includes the underwing coverts, whereas there is no white on the underwing of a Tree Swallow. The second good feature is the tail of a Bahama Swallow is highly forked, whereas on Tree Swallows it is only mildly forked. However, keep in mind that alot of the time, Bahama Swallows fly with the tail somewhat “closed” and the forks cannot be seen well. They also have much more white on the “cheeks” than the tree swallows, which makes their heads look smaller.
Tree Swallow: Notice that white stops on body, none on wings, outer tail feathers are not significantly longer than the adjacent ones.
Bahama Swallow: notice the white underwing coverts and the deeply forked tail.
The color of Tree Swallows, in good light, is much more blue and Bahama Swallows more green; however, be careful with this as Tree Swallows can appear green in the right light. A more useful dorsal characteristic is that Tree Swallows appear very shiny while Bahama Swallows have bright colors, but little shine to the feathers on their backs with a little more towards the wings.
Bahama Swallow: notice the flat color of the dorsal side of the body, with iridescence on the coverts and the rump as well as the two outer tail feathers being far longer than the
Tree Swallow: note the iridescence on the entire upper body from any angle.
I have not seen Bahama Swallows feeding from bushes, whereas Tree Swallows often do – Myrtles on the east coast and some similar plant in the Bahamas. I heard a report that Bahamas will walk on the beach picking up prey, but I have not seen this behavior myself.
Now the more subjective tips to pick them up from afar: after watching a mixed group for several hours, I have found Bahama Swallows to appear more tapered and thinner. I think this is a result of holding their longer tail feathers together during most flight. The wings appear longer – whether this is true or simply a result of the body looking thin, I cannot say. They seem to forage higher than Trees, though the Tree Swallows here do fly very high while flocking, but not while foraging. They sound slightly different (Bahamas have less of a grating call). While I have seen good sized flocks of Bahama Swallows (20-30 individuals), they are not always in flocks, often there will be 1-6 in an area, whereas I have not seen any Tree Swallows in flocks smaller than ~50 in the Bahamas.