Last night, again, I heard the loud screaming cry of some nocturnal bird species just after dusk in my garden. I have heard this on many occasions and eventually gave up on its identification. I even thought it might be some other mammal and not a bird. Written descriptions of bird calls are notoriously unreliable but I made the mistake of discounting the Short-tailed nighthawk (Lurocalis semitorquatus). I listened carefully to all recordings of Costa Rican owl species and could not come up with a match, but I had neglected the nighthawks until now. However, a recording made by Peter Boesman sounds awfully like the sound I have heard with limited regularity over the past few years. Since this is supposedly the flight call, it may explain why I have never been able to locate the bird.
The photo shown above does not look much like the illustrations in the field guides, but the comments at barloventomagico confirm the species, explaining that it is a bird in moult.
All I can do now is spend some time at dusk or just before dawn hoping to catch a glimpse. The Common pauraque (Nyctidromus albicollis) is the only other member of the nightjar family that I have seen in the San Antonio area, though several others are possible (see range maps in Garrigues & Dean, The Birds of Costa Rica). I have seen the Lesser nighthawk (Chordeiles acutipennis) on the CATIE campus. One used to roost next to the offices of the Monitoreo de Aves programme led by Alejandra Martinez. Although many Caprimulgidae species have been recorded in our region, no local bird list includes the Short-tailed nighthawk, with the exception of the Rancho Naturalista checklist at Tuis.
Update as of May 2020: I can now confirm that this ‘scream’ is attributable to the Mottled Owl (Strix virgata), a common owl of our area but one whose more typical call is quite different.