[texbirds-freelists] [texbirds] Origins of Tropical Mockingbird
- From: Bob Doe <doefamily@xxxxxxx>
- To: texbirds@xxxxxxxxxxxxxxxxxx
- Date: Tue, 24 Apr 2012 16:10:15 -0400
There seems to be little doubt that the unusual Mockingbird being seen at
Sabine Woods is a Tropical Mockingbird (Mimus gilvus). However, the origins,
and thus the "countability" of the bird remain problematic. I suspect that the
Texas Bird Records Committee will have some problems with this one,
particularly since it would be a first U.S. (and ABA) record if accepted.
As I see it, there are 3 mechanisms which would account for the bird being
where it is: natural vagrancy (bird flew there by itself). intentional
assistance (bird was transported as a captive, and escaped or was released),
and un-intentional assistance (bird "hitched a ride" on a ship). There are
probabilities and factors which argue for and against each mechanism.
Tropical Mockingbird is generally considered to be a sedentary species and the
bird at Sabine Woods is about 800 miles from the nearest population. I am not
aware of other significant vagrancy in the species. But even within a
"sedentary" population, individuals may wander or disperse widely. Mockingbirds
(genus Mimus) are known to be capable of prolonged over water flights. Northern
Mockingbirds have occurred on Guadalupe Island, 150 miles off the coast of Baja
California. Socorro Island, 250 miles south of the tip of Baja California has
(had?) an endemic mockingbird species presumably derived from mainland birds,
and the island has been colonized by mainland birds. Northern Mockingbirds have
reached Britain on at least 2 occasions. The Galapagos Islands, 1000 miles from
the mainland, have several species of endemic mockingbirds, presumably derived
from mainland stock(s). Tropical Mockingbirds occur on several Caribbean
islands, confirming the possibility of over water dispersal, over at least
limited distances. And, as Brush has pointed out, there are numerous offshore
oil and gas rigs off the northern coast of the Yucatan peninsula and the
southern coast of the US which could provide a point of refuge for a vagrant
over the Gulf of Mexico. Other "sedentary" tropical species have reached Texas
and have been accepted, Yucatan Vireo being probably the most directly
comparable. And Sabine Woods seems a "likely" location for a naturally
occurring vagrant from the Yucatan to occur.
Whenever a vagrant individual occurs, there is always a question of whether it
is a "real" vagrant, or a bird which has escaped or been released from
captivity. Tropical Mockingbird is known to be kept as a cage bird and it is
known that some caged birds can be transported long distances, either to be
sold, or as personal "property". "Caged" birds often show considerable feather
wear, and such wear can be used to support the contention that a bird has been
recently caged. The Sabine Pass bird does show considerable feather wear,
particularly on the tail and wing coverts, indicating that it MAY have been
caged. But not all feather wear is the result of captivity.
Martin Reid has provided links to a series of photos of presumably wild
Tropical Mockingbirds that show extensive feather wear analogous to the wear on
the Sabine Pass bird.
There seems to be an underlying assumption that caged birds are transported by
immigrants as they enter the country, and thus a popular cage species is more
likely to be an "escapee" rather than a true vagrant. I can understand the
rational and the concern, but I don't think the facts support the assumption.
There doesn't seem to be a "trail of exotics" along heavily traveled
immigration routes and the Sabine Woods bird would seem to be pretty far
removed from major immigration routes (about 70 miles from Houston area and 25
to the closest approach of I-10). I just don't think it is very likely that a
bird transported overland would end up at Sabine Woods. It would be interesting
to find out from Border Patrol/USDA how many and what species are seized from
immigrants. That would help establish probabilities of captive transport.
Transport by sea is another matter. Mariners have been known to capture or care
for pelagic birds and vagrants which may land aboard. There is the possibility
that crew members may buy a caged bird in a foreign port, care for it during
passage and release it prior to making landfall (knowing it is illegal to land
with wildlife aboard). The upper coast (from Houston/Galveston to Lake Charles,
LA) is heavily traveled by ships. But again, I think that the general dearth of
exotics argues that this doesn't happen very often. With the number of ships
visiting the area and their diverse ports of call, if crewmen were releasing
birds, I would expect an almost continuous stream of exotics. Most of the
exotics we do get seem to be escapees of the domestic cage bird industry, with
species origins in Asia or Africa, and not the species which occur in Mexican
markets, for example.
Lastly we have to consider the un-intentional transport of vagrants. Mexico is
the second leading exporter of oil to the US (Canada is the largest, then
Mexico, followed closely by Saudi Arabia). Virtually all of the exported
Mexican oil comes from the northern Yucatan and Bay of Campeche area, and the
vast majority of this oil goes to Gulf Coast refineries. It is about a day and
a half sail from northern Yucatan to Port Arthur. I can see 2 plausible
scenarios for a bird utilizing un-intentional transport to get from the Yucatan
Peninsula to Sabine Woods. In the first, a natural vagrant over the gulf gets
tired and lands on a "floating island". As the ship approaches the Sabine
River, the bird sees land and flies off. In the second scenario, the bird is
aboard (maybe asleep) when the ship leaves port, and awakes to find itself
surrounded by water. It stays aboard for a day and, approaching land, flies off.
We will never know the true origins of the Sabine Woods bird, unless someone
steps up and says "Oh yeah, I released it". The possibility of purely natural
occurrence seems quite high (meaning the bird is capable of reaching Sabine
Woods unaided), but the probability seems low (I can't even find a Tropical
Mockingbird record for Cuba, which is a lot closer to the Yucatan than Texas
is). Depending on how you interpret the feather wear, natural occurrence may be
even more unlikely. The possibility of the bird being a "captive" is high (they
are known to be kept captive), but to me, the probability that this bird is an
"escapee" is very low. As much as I would like this bird to be a natural
vagrant, I think the un-intentional transport scenarios are the most likely, as
both the means and opportunity are present. However, if this bird is determined
to be ship assisted and the record not accepted, then I think the TBRC needs to
re-examine some other records (specifically the Yucatan Vireo record, which
seems to be very analogous).
I'd love to hear other comments, especially from anyone that might have numbers
for species/numbers of birds seized by Border Patrol/USDA, or some insight into
number of birds transported by crew aboard ships;
Bob Doe
Spring Branch
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