The recently accepted record of Blue Mockingbird from Weslaco, Hidalgo County, is probably one of the more controversial decisions made by the TBRC during the time I have been on the committee. The bird was first discovered in early May 1999 and was observed sporadically through June. It was "rediscovered" in October of 1999 and reported irregularly through the winter months to apparently disappear for the summer. It was relocated again in the fall of 2000. All of these observations were from the same location and it is possible, if not probable, that the bird was present either there or nearby during the entire period. There may even be sightings during these intervals when the bird was unreported that the TBRC never received.
There are two accepted records of Blue Mockingbird for Arizona. The species has also been documented near Las Cruces, New Mexico. The New Mexico record is currently under review. There has also been one documented in southern California that is believed to be an escaped cage bird.
Blue Mockingbirds are known to have an altitudinal migration in northeastern Mexico, but do not breed at lower elevations. In southern Tamaulipas, this species breeds from as low as 900 meters and is routinely found as low as 150 meters during the non-breeding season. Blue Mockingbirds occur with some regularity as close as 150 miles south of the Lower Rio Grande Valley (LRGV). Obviously, the origin of any unbanded bird is unknown and origin is always a question for primarily tropical species occurring in the LRGV because of the proximity to bird markets in the towns just across the border in Mexico. Blue Mockingbirds are fairly commonly observed in these markets. Ted Eubanks addresses the question of possible origin from these markets in a post to TexBirds on 12 May 1999. Below is an excerpt from that message.
"If most of the LRGV vagrants originate in the bird trade, then their rate of occurrence should be proportionate to their prevalence in the trade. In other words, birds that are the most commonly sold should be the birds that most commonly appear in Texas as escapees.
However, that has not been the case. For example, I know of few birds that are more commonly sold in Mexico than Orange-breasted Buntings. This seedeater is sold in virtually every bird market and by every bird vendor. I have seen cages filled with these buntings (along with Painted Buntings and Northern Cardinals) for sale. However, to my knowledge there have only been two instances of Orange-breasted Buntings appearing in Texas. Shouldn't they be among the most common "vagrants" if the bird trade is truly the source of these rarities in the Valley?
I also find it interesting that although the Brown-backed
Solitaire is an incredibly common cage bird in Mexico (just roll
down your window and
listen as your drive through any Mexican village), I am not aware of the
bird EVER occurring in Texas. This makes no sense. They are among the most common birds in the trade, yet they never escape?"
When the Weslaco Blue Mockingbird was originally discovered,
the origin was a natural topic of discussion. The bird was examined
by many observers looking particularly for signs of cage wear.
The bird's plumage, including the retricies and flight feathers,
was in excellent condition and showed none of the adverse effects
normally associated with birds held in small cages. No cage wear
was evident from examinations of the bill and feet either.
Drought conditions have persisted throughout much of Texas and northeastern Mexico for the past several years. The effects of this drought on the vegetation and subsequently on bird populations in these areas was also taken into consideration. There was an influx of other "Mexican" species into the LRGV during the winter of 1998-99 that persisted into the spring. Most of these birds were of the more regularly occurring visitors, such as Clay-colored Robin and Blue Bunting. But there were also some unexpected visitors such as Green-breasted Mango, Violet-crowned Hummingbird and Gray-crowned Yellowthroat. The Violet-crowned Hummingbird was discovered by birders searching for the Blue Mockingbird.
The extended stay of the Weslaco Blue Mockingbird has raised additional questions about its origin. The majority of the committee did not find the extended stay a compelling reason for rejecting the record. However, John Arvin does find this to be a troubling factor. Although not a member of the TBRC at the time this record was accepted, John did provide the following reasoning for not agreeing with the acceptance of the record.
"I have always held the opinion that an unlikely species must be considered of natural origin unless there was some evidence that it wasn't. Just being unlikely is not enough, as basically all the birds on the review list are unlikely; that's why they are on the list. In the particular case of the Blue Mockingbird, now present continuously for two years, I think that this very fact is evidence that it did not arrive as a consequence of natural vagrancy. The pattern shown by Mexican passerine (and other) vagrants is that post breeding dispersal, probably mostly by hatching year birds, takes them in random directions due to inexperience and/or genetic misprogramming and especially during seasons with high environmental stress (drought, freeze, etc.) that makes food scarce in their normal range. They remain where they end up for varying lengths of time and disappear toward the next breeding season. The continual presence of the mockingbird throughout two summers is outside this normal pattern of vagrancy and therefore is a bit of negative evidence toward natural origin in my opinion."
There have been other long staying "Mexican" birds
(Rose-throated Becard, Clay-colored Robin, Gray-crowned Yellowthroat)
in the Lower Valley, but all of those have breeding populations
at similar elevations to the LRGV in northeastern Mexico, unlike
the Blue Mockingbird.
Most of the vagrants that have been documented in the LRGV are found as far north as southern Tamaulipas. This includes species such as Crane Hawk, Collared Forest-Falcon, Mottled Owl, Masked Tityra, and Orange-billed Nightingale-Thrush. This is also the case with the Blue Mockingbird. The TBRC has accepted records of other species that are common in the bird markets. These include the various species of robins and Yellow-faced Grassquit in particular.
All of these factors were considered by the committee in reaching its decision. As I mentioned previously, this is a controversial record that will likely not be recognized by everyone. If that is the case, it will not be the first time a record accepted by the TBRC was not endorsed by the ABA and/or the AOU checklist committees (e.g. White-chinned Petrel).
Secretary, Texas Bird Records Committee, February 2001
Please address comments about our web site
to the webmaster.
All original contents copyright, 1997. All rights reserved. .