Petrochelidon condundrum

After cleaning up on many of the continuing vagrants in the El Paso area (Broad-billed and Anna's Hummingbird, Pine Warbler, and Red-throated Loon) along with some more regular species that I still hadn't seen in Texas, I spent today (Feb. 19) in the Las Cruces area birding along the Rio Grande near Mesilla Valley Bosque SP. Though I didn't find anything quite as exciting as the birds seen yesterday in Texas, I did manage to find a sizable (11 individuals) flock of Cave Swallows (along with a handful of Northern Rough-winged Swallows and a Tree Swallow).

Cave Swallow (Petrochelidon fulva) Mesilla Valley Bosque SP, Dona Ana County, NM

Feb. 19, 2017

Cave Swallow (Petrochelidon fulva) Mesilla Valley Bosque SP, Dona Ana County, NM

Feb. 19, 2017
Perhaps a few days early for the Rio Grande Valley in NM, but on par with the arrival of P. fulva elsewhere in the US at equivalent latitudes this year

The distribution of Cave Swallow in NM is rapidly evolving with both the well documented expansion of the species in TX, as well as increased observer competency likely playing roles. Differentiating between Cave and the *other* regular (and more widely distributed) U.S.  Petrochelidon swallow, the Cliff Swallow (Petrochelidon pyrrhonota) can be pretty tricky, especially in mid to late summer when HY Cliff Swallows have fledged and can give a flight call quite similar to that of Cave.
Differentiating between Cave and Cliff Swallows can be hard, which may be why there aren't more Cave Swallow records from further north. It can also be pretty tedious, sifting through often times hundreds of Cliff Swallows in the hopes of finding a Cave. The acrobatic flight of swallows in general can make all but impossible to nail down the field marks separating the two species. This time of year, along with late fall, tend to be the times when most extralimital Cave Swallows are reported. Cave Swallows move north much earlier than Cliff Swallows, as evidenced by the eBird reports of Cave Swallow for 2017:

P. fulva reports in 2017 as of Feb. 19 (eBird)

Notice the Virginia report. Cave Swallows have a propensity to wander, but compared to Cliff Swallows, which spend the winter much further south, they are relatively short distance migrants. This may explain the earlier arrival dates (being closer means Cave Swallows can afford to bump northward as conditions warm up as opposed to waiting for a particular day length as Cliff may do to time their migration). Compare the Cave Swallow map with the Cliff Swallow map:

P. pyrrhonota reports in 2017 as of Feb. 19 (eBird)

 Aside from those Cliff Swallows on the California coast and a few in central AZ (which seem to have an earlier arrival date than the rest of the continental US), Cave Swallows are much more dispersed this time of year across Texas, southern Florida, and southern New Mexico this time of year.
This is an ideal time of year for Cave Swallows to show up because the aforementioned absence of Cliff Swallows (in most of the US) means that locating and studying a single Petrochelidon becomes relatively straight-forward. Its no longer a matter of sifting through a massive flock of swallows, but rather watching a single or relatively low number of birds. This also becomes the case in late fall/early winter:

P. fulva reports Nov. to Dec. all years (eBird)

P. pyrrhonota reports Nov. to Dec. all years (eBird)

 Though Cliff Swallows also do wander some in late fall, there is a much greater number of wayward Cave Swallow records. Furthermore, it is likely that early and late Petrochelidon swallows will receive more attention (as would probably any unseasonal swallow) and this increased scrutiny may also aid in the detection of more Cave Swallows at the "wrong" times of year. So, it seems that if a Cave Swallow were to show up outside of its normal range within New Mexico, the chances of detecting it are better now than later in the season when it becomes a matter of sifting through hundreds of Cliff Swallows.
But what *is* the "normal" range for Cave Swallow in New Mexico? The species has recently undergone a massive range expansion in west Texas, which seems to have also had an impact on the New Mexico population. Hope you're ready for more eBird range maps...

P. fulva reports in NM 1900-2005 (eBird)

P. fulva reports in NM 2005-2010

P. fulva reports in NM 2010-2017

The trend in these maps is quite evident. More than even within New Mexico, it is striking how much more prevalent Cave Swallows have become in west Texas as well. It would appear Cave Swallows have relatively recently widely adopted the tendency to nest under culverts and bridges. Though Cave Swallows nested on man-made structures in the Las Cruces area for some time, one has to wonder whether or not it is a learned tendency, or if Cave Swallows are inherently apt to nest on non-natural structures. I might argue it is a learned trait, as it would make sense that as more "pioneer" colonies were established, and more young produced in nests on man-made structures, they too may return to use a culvert rather than trying to find a natural cave somewhere. As this population increases, more and more Cave Swallow colonies are established in similar situations- culverts and bridges in west Texas and southeastern NM. While a culvert isn't much different structurally from a cave wall, being able to identify a cement surface as a suitable site for nesting does not seem like it would be inherent
This trend within New Mexico is also likely due in part to an increased number of observers in New Mexico going out and checking "suitable" nesting structures along the eastern tier counties in the summer and sifting through Cliff Swallows in the hopes of discovering a Cave Swallow.
From the east side of the state there are summer reports of Cave Swallow (nesting on man-made structures and/or just free-flying) from Lea, Eddy, Chaves, Roosevelt, Curry, and DeBaca counties plus a tantalizing summer report from the Vaughn Sewage ponds (July, perhaps post-breeding dispersal...?) Dona Ana remains the only county in the Rio Grande Valley with breeding Cave Swallows reported to eBird, but perhaps a well timed visit to the Amendaris Caves (no public access) in Sierra County could turn up some breeding Cave Swallows... With an established population at Leasburg Dam SP in the more northerly portion of Dona Ana County, it seems before long Cave Swallows may bump northward to Hatch and ultimately southern Sierra County (where there are a few reports of wandering Caves). A recent report of 3 Cave Swallows from Holloman Lakes near Alamagordo in Otero County officially bridges the reporting gap between those eastern Cave Swallows and those in Dona Ana County. Perhaps that population too will increase.
It seems unclear whether or not competition with Cliff Swallows is a limiting factor. Some places I have checked have had both Cliff and Cave Swallows, whereas others have been monospecific. Ultimately the two species are quite similar, and until recently the Cliff Swallow got away with being more generalist in its nest selection preferences, whereas the Cave Swallow mostly stuck to nesting within its namesake chasms. It will be interesting to see how the relationship between these two species develops in the future. Whether hybrids become regular. Whether Cliff Swallow populations decrease in response to competition with Cave Swallows, or if Cave Swallows will continue expanding northward.
Ultimately, this has just been a long-winded way of saying this- my challenge to New Mexico birders who may not have been thinking about seeing Cave Swallows away from Leasburg Dam SP or the spectacular Carlsbad Caverns should consider checking those culverts and bridges on the eastern side of the sate. Especially when you're making your trips to Melrose Woods and assorted migrant traps, its pretty easy to pull off the road and sift through some swallows. Though there are many who have been doing this for the past several years (to whom I am very appreciative!), more people looking means a greater chance of discovering more colonies and better documenting this range expansion on the northwestern fringe of this species range. It seems like only a matter of time before the first colonies in Quay, Harding, and San Miguel are discovered. Moreover, the areas in between the current colonies will also likely begin to fill in over the next few years, as was the pattern in Texas. So New Mexico birders, when you're brushing up on your identification of female eastern wood-warblers, consider flipping to the Petrochelidon plate in your field guide and reminding yourself of the field marks for Cave vs. Cliff Swallows. Remember also though it can be very aggravating trying to photograph the birds themselves, if it is possible to see the nests, Cave and Cliff Swallows build different nests, so knowing the difference can be a short-cut to documenting a colony, especially if the birds are present but proving camera-shy. Who knows, maybe you'll be the first one to discover that new colony.
So lets all get out there and find some Cave Swallows this year!

Cave Swallow (Petrochelidon fulva) Mesilla Valley Bosque SP, Dona Ana County, NM

Feb. 19, 2017