From my electronic files, I reproduce (untouched except a few words -- shown in parentheses -- to make the meaning clear) my second and fourth round CBRC votes in support of accepting the record of Swallow-tailed Gull from Pacific Grove in June 1985. The CBRC ended up with a split decision at that time (as did the ABA Checklist Committee later), and rejected the record on "natural occurrence questionable." Subsequently the record was rereviewed after an additional sight record accumulated that was accepted. I reproduce my original votes here so that readers may evaluate the reasoning prior to the additional occurrence.  -- D. Roberson

Validation Form

Record number: 79-1985   Species:  Swallow-tailed Gull
Member's name:   D. Roberson   Date of review: 27 Apr 1990
Circulation:  Second
DECISION: Accept:  X
   identification questionable:
   natural occurrence questionable:
   establishment of introduced population questionable:
I rather like Bevier's discussion in his draft of the 11th [CBRC] report: "In the end, those decisions (where natural occurrence was a question) are a collective opinion based on the information available and are certainly not established fact (see Anhinga, Ruddy Ground-Dove, and Barnacle Goose in this report)." Whatever our decision, it should not be confused with "established fact". However, we would do well do make our "collective decision based on the information available."

Our Committee is structured conservatively. It takes 8 of 10 votes to get a record of this nature accepted. Given this construct, it seems inappropriate to add yet another layer of extreme conservatism to the review (the >95% confidence proposed by some, such as Marantz, or the preference for a "pattern of records" before acceptance cited by others for their rejection vote, such as by Jon Dunn). It should be noted that the AOU has accepted1 Florida records of the Band-tailed Gull as occurring naturally (although there is plenty of room for uncertainty). In their comments they say that the "possibility of the Florida birds being escaped captives or man-assisted vagrants remains". This is appropriate, and is a comment that we could use for this species, or others , even if we accept the record. As I have pointed out before, the standard used by all the important committees in the world (e.g., AOU, BOU, Dutch & Finnish committees to whom I have talked) is whether the bird in the disputed record was "probably" of wild origin, even though the "possibility" of captive or man-assisted origin cannot be ruled out.  We can never know for certain  whether this bird, or many other birds, occurred naturally.  It is certainly reasonable to raise doubt about numerous records already accepted:  White-tailed Tropicbird (bizarre onshore behavior; tropicbirds are commonly caught on ships offshore when they run into the lights at night - I have photos and videos of several such occurrences for tropicbirds, as well as storm-petrels and Pterodromas; indeed, hardly any seabird vagrant is without some element of doubt about this type of ship assistance), Wandering Albatross (albatross are known to be commonly captured aboard ships), Whooper Swan, Smew, Garganey, Baikal Teal, any Tufted Duck, Zone-tailed and C. Black-Hawk (numerous captives in zoos; some held by falconers), any plover or shorebird (considering the occurrence of Kittlitz's Sandplover in San Diego, from a source that was never traced, almost any shorebird could be a captive), southern hummingbirds, Painted Buntings, Brown Shrike (the Farallones are next to major shipping lanes and there is much documentation for ship-assistance of landbirds to England across the Atlantic; it most also occur in the Pacific), and others. There is some doubt, exceeding 5%, for any of the above accepted records or for any extraordinary vagrant. But we accepted all the above, choosing the most "probable" explanation for their individual occurrences each time. In many cases we did not await a pattern of intermediate records, nor should we.  We can only weigh and balance the evidence at hand and choose the most probable scenario. This is the proper standard for this and all other similar records in California. I would hope that this Committee would focus on that question. Indeed, Marantz' argument is for accepting the bird, as he finds only 20% chance of escape from captivity (from where? there are no known captives) and 20% chance of man-assistance on a ship. But the probable scenario, even under his arguments, is wild occurrence. It seems a pity that an accept vote does not follow from such a careful analysis.

Paul Lehman wrote this regarding Ruddy Ground-Dove (1/31/89):

"I also think Don is stretching reality to propose that vagrant species can go only as far in the wrong direction as they do normally in the correct direction -- seemingly to the mile, in fact!!! There are countless examples of short-distance migrants (or even (largely) quasi-residents) turning up well away from their normal range. If all of these were called   escapes, Don and the rest of us would have much shorter lists...." [emphasis in original].
But now Paul says that the Swallow-tailed Gull is an exception, that it is totally confined to the Peru Current and cannot wander, and someone suggested (I cannot refind the statement now) that adults accompany young, precluding their vagrancy.2 This attitude seems contrary to the analysis used to eventually accept the Ruddy Ground-Dove and other species; indeed, all the arguments pro and con on the ground-dove discussed probabilities. I heard no one argue that there was a 95%+ confidence level in the records reviewed. We should be consistent in our approach. Approaching the record to determine whether it is more probable that it occurred as a wild bird than by some type of unnatural assistance, I turn to the possibilities:

Captive status: Through the birding grapevine, we heard rumors of captive birds at the Bronx Zoo, New York or the National Zoo, Washington, D.C., and of a presumed escape in Massachusetts. [Upon checking] the Bronx and National zoos did not have the species and we were unable to locate any captives in North America.  I have written Alan Barron three times asking about his rumored bird in Massachusetts. He has not responded; the Mass. rumor stands as totally undocumented and must be ignored. While it is possible (though there is still no evidence) that Swallow-tailed Gulls were once maintained in American zoos, it appears there were none in captivity during the summer of 1985.

Ship assistance: Several commentators have speculated that a Swallow-tailed Gull might have been captured either around the Galapagos or off Peru and thence brought to California (S. Cardiff, D. Dittmann, R. Stallcup).  Those favoring this theory cite the substantial shipping traffic between western South America and California, and note that the bird might have escaped or been released at other West Coast ports and simply wandered to Monterey Bay.  Doubts about assisted passage were cited by the A.O.U. (1983) and the CBRC (Roberson 1986) in relegating a Black-tailed Gull Larus crassirostris taken at San Diego, California (Monroe 1955) just after the Korean War to less than full status on their checklists.3 A similar debate surrounds the recent appearance of a Gray Gull L. modestus in Louisiana, and the several Florida records and a California report of the Band-tailed Gull L. belcheri, a race of which is a Peru Current specialist. The Band-tailed Gull is an easily approachable Larus  (pers. obs.) and has been known in captivity (e.g., see Murphy 1936). However the Gray Gull may well be a good record, as it has been collected on Cocos Is., well north and in the tropical zone (Slud 1967).

Swallow-tailed Gulls are attracted to lights at night. On several days in the Peru Current off Ecuador, when our ship [I spent four months aboard a NOAA ship doing bird surveys in the eastern tropical Pacific in 1989] would stop for oceanographic readings for an hour each night, this species was attracted to the lights and would try to pick squid and Myctophids (lantern-fish, its other favorite food) from the surface. I have a copy of a video in which Bob Pitman dip-netted one during attracted to lights at night, held it in-hand for the camera, and released it unharmed. Certainly it is possible that one could have been captured at night on a ship and brought to California, but those possibilities appear rather slim. Except at night, it does not habitually follow ships off Peru (T. Schulenberg & pers. obs.), though occasionally may  approach ships and fly by (A. Small, K. Garrett, pers. obs.). It is completely protected on the Galapagos and officials at the Charles Darwin Research Center knew of no illegal exportation of the species [in response to a letter asking this question]. Certainly the opportunity to capture a healthy bird is minimal, given the small world population size and its disinclination to be attracted to man's activities except at night.

If the Pacific Grove gull was ship-assisted, it showed no signs of recent captivity. Its plumage was rather immaculate and it must have undergone a molt since captivity. No unusual wear or soft part damage was visible and it was unbanded. The bird flew well and left the roosting rocks at night, apparently to feed on the abundant squid in the area.  Unlike the Larus gulls cited above, the Swallow-tailed is a specialized feeder (Belopolsky & Tsigankova 1985).  Further, it seems quite unusual that an escaped captive would come ashore and engage in prolonged bouts of breeding-associated behavior, such as "foot-staring" (similar to noddy terns; Cullen & Ashmole 1963). I think it of some interest that other coastal records of vagrant seabirds were of adults (Wandering Albatross, Brown Boobies, White-tailed Tropicbird) and most were either attracted to seabird nesting colonies or engaging in courtship behavior (e.g., tropicbird).

Natural distribution: The species regularly migrates 3000 miles to waters off Peru. It is regular south to at least off Mollendo, southern Peru (about 17°S) and quite probably to northern Chile (Johnson 1972); Jehl (1973) collected a bird in June 1970 off Concepcion, central Chile (36°12'S, 70°10'W). Species with long migration routes are prime candidates for vagrancy (Sharrock & Sharrock 1976). The lack of intermediate records is cited as a reason to be suspicious of this record, although it did not present an obstacle to the acceptance of a Wandering Albatross Diomedea exulans  in California (Paxton 1968) by the A.O.U. or by the CBRC. Strangely enough, the only North Pacific precedent was from the Bay of Panama (Murphy 1938), which is also the site of the only northern occurrences of the Swallow-tailed Gull (see below).  Of the Wandering Albatross, the A.O.U. (1983) said "[w]hile there will always be uncertainty with respect to possible transport by man, vagrancies in our area by other southern albatrosses (e.g., D. cauta  and D. chlororhychos) lend support that the foregoing reports are based on natural wanderings."   The referenced record of the Shy Albatross D. cauta  was of an adult collected some 39 miles offshore of the Quillayute Rover mouth, Washington (Slipp 1952).  Slipp's discussion of its occurrence is instructive:

"Efforts to account for erratic occurrences of sea birds far from their normal range are   usually futile.  However, we can say positively that in the present case there is no doubt that the specimen was a wild bird which had arrived on the scene by means of its own powers of flight, for its size and nature render it a most unlikely captive, and its immaculate plumage testify to freedom and normally good health. It seems inconceivable, likewise, that it might have been tolled so far from its native waters by any one ship, although it showed no very great reluctance to joining the flock of "goonies" around the JOHN N. COBB and may well have traveled long distances in the company of ships encountered in its wanderings.  Cyclonic wind storms are known to be a frequent cause of long distance transfers of sea birds from the tropics to higher lattitudes (Murphy, 1936: 50-59), possibly accounting for a part of the extreme dislocation in the present case. The unprecedented nature of this record suggests that  whatever combination of the above agencies may have been operative, a large residual allowance must be made for the individual caprice on the part of the bird itself. We can hazard little more to account for the appearance of a relatively localized subspecies (cf. Murphy 1936) some 8,000 miles away, across the tropic barrier, from its native islands."
The distribution of pelagic species is influenced by water temperature, salinity, and the availability of food (Ainley 1976). To date, all extralimital records of the Swallow-tailed Gull are directly attributable to El Niños.4 The 1982-83 El Niño was the strongest climatic anomaly in the southeastern Pacific during this century (Arntz 1986), surpassing even the recorded El Niños of 1891 and 1925 (Rasmusson 1985). Species associated with the Peru Current were widely dispersed away from areas of normal occurrence. In summer 1983, numerous Inca Terns Larosterna inca were recorded in the Bay of Panama, the first confirmed records north of Ecuador, along with an assemblage of normally Peruvian species (Reed 1988). Among these was an immature Swallow-tailed Gull photographed on 14 June 1985 near the Perlas Islands, Bay of Panama (Reed 1988). This confirmed record supports a July 1957 report by Robins (1958), who saw the species off Panama but did not publish a description, which prompted the A.O.U. (1987) to relegate the record to their Appendix B (probably erroneous or unverifiable records). However, this species is easily identified and it was only the expansion of the AOU area to include Panama that caused any concern over documentation. We use numerous sources that include undocumented records where there is no reason to doubt the i.d. Back in 1957 there was no reason to document the gull because there was no review committee involved, the paper listed pelagic records of several species, and peer review by the ornithological literature was considered quite adequate. In fact, 1957 was the previous strong El Niño (see papers in Wooster & Fluharty 1985, a 300 page book on the El Niño of '82-83). The 1957 El Niño was less disruptive than 1982-83, but likely accounted for the Panama record. Reed (1988) theorized that seabirds respond to a collapse in their food base by long-distance dispersal, perhaps following prey-density gradients, to the sites of upwelling, such as in the Bay of Panama.

Attached [to my vote but not reproduced here] is a map from the National Geographic showing the geographic scope of the relevant El Niño. You will note that it was strongest over the entire pelagic range of the Swallow-tailed Gull. All guano birds were dispersed away from the Peru Current (Arntz 1986) except in local upwelling near Chimbote. Surveys that regularly found all the Peru Current specialists in normal years found only storm-petrels and the occasional Waved Albatross (no Swallow-tailed Gulls; Arntz 1986). Many of these birds, including specifically the Swallow-tailed Gull, immigrated into Chilean waters (Alival et al. 1983). Peter Harrison has provided us a letter of his observations of the species south to about 42°S! He also found Inca Terns well south; it appears that both the tern and the gull widely dispersed away from their normal ranges during the El Niño. The evidence we have suggests that virtually the entire population dispersed away from the Peru Current, both north and south.

Jon [Dunn] takes the El Niño event much too lightly and would do well to read something of the unprecedented phenomena. Its affect on seabirds was substantial and ongoing for years after the peak. Ainley et al. (1986) summarized as follows (at p. 11):

"The post-El Niño behavior of seabirds was complex, depending on species and locality. Reproductive success in all species on Christmas Island, except Sooty Tern and Black Noddy, returned to 'normal' in 1984 -- even if breeding population size did not. By 1985, however, reproduction in the latter two tern species was still depressed and still only small numbers of the other species were present. At the Farallon Islands, reproductive success continued lower than expected for three species in 1984, and there were some indications that Tufted Puffins may not have bred that year. In Alaska, the reduced success of Black-legged Kittiwakes continued from 1983 through 1985, indicating a one year delay from patterns of affect elsewhere."
When one considers that Swallow-tailed Gulls do not return to their natal islands for up to five years, one might suggest that a El Niño-dispersed young bird could become lost and wander pelagically, until making landfall at Monterey Bay.  The bird behavior suggests it was responding as if on the breeding grounds. Monterey Bay is unique among coastal locales in that the 6000 foot deep Monterey submarine canyon closely approaches shore at that point, making it justly famous as the best land-based site for viewing pelagic species in western North America. Numerous rare and vagrant pelagic species with both warm  and cool water affinities have been recorded there. There are extensive breeding colonies of seabirds along the Monterey coast. Indeed, if one were to pick a hypothetical point for a land-based occurrence of Swallow-tailed Gull, one can hardly choose a more likely locale. Some effects of the El Niño in Monterey Bay lingered through summer 1985, including an exceptional abundance of pelagic red crabs Pleuroncodes plannipes, which were being washed up in droves on Pacific Grove beaches during the bird's occurrence. An El Niño-influenced dispersal is certainly a reasonable scenario.

CONCLUSION:  I am most impressed with the specialized nature of this species (Hailman 164a,b, 1965; Snow & Snow 1967, 1968; Harris 1970, 1979). In virtually every respect it is much more like a migratory tern or a strong-flying procellarid than it is to the coastal Larus  gulls.  Indeed, I don't understand why all members are not making comparisons to kittiwakes, Sabine's Gull, or terns, all of which have been recorded widely in their pelagic range. The bird is in its own genus - honestly, do you guys really think that it is best compared to the coastal Larus ? If we talk about terns, there are plenty of accepted records which were unsupported by intermediate records, including Aleutian Tern and Elegant Tern in Britain, and a Pacific race of White Tern off Bermuda. If we talk about pelagic birds in general, perhaps the most similar record is of Greater Shearwater (also in Monterey Bay), which was unprecedented and was (and still is) unsupported by intermediate records supporting a pattern of occurrence.

Given the undeniable possibility that the Swallow-tailed Gull could occur in California, we balance that possibility against other alternatives. It is unknown in captivity. No evidence supports captive assistance aboard ships (and, of course, "passive" assistance is no obstacle to acceptance, as virtually everyone commented when voting on the Red-footed Booby invasion). Certainly it could  have been transported illegally, been released, and eventually made it to Monterey after a molt. The same can be said for Wandering Albatross, Greater Shearwater, and White-tailed Tropicbird, to name just a few. At any time a "large residual allowance must be made for the individual caprice on the part of the bird itself" (Slipp, op.cit), but when the impacts of the "most prolonged and catastrophic El Niño visitation of recent times" (Rasmusson 1985) are considered, I feel the scales are clearly tipped toward acceptance. The desire to await additional records is laudable, but since it appears dispersal is attributable to major El Niños, which occur only a couple times each century (Rasmusson 1985), we will likely have to wait many years, perhaps beyond our lifetimes.  I suggest we serve science best by making a "collective opinion based on the information available" (Bevier), which is the best we can do.

1The AOU has given the species an AOU number (54.2), which they do only when it has properly occurred in the old AOU area (i.e., birds from Panama which have not occurred farther north do not receive an AOU number; see AOU Checklist, 6th ed., p. 216).

2This is not always true, as I photographed a subadult alone a hundred miles off Ecuador, and I don't believe there is any evidence supporting the supposition that adults accompany young on migration.

3Jon mistakes the vote on the Black-tailed Gull. It was not 2-8 on natural occurrence questionable. I was one reject vote on i.d. questionable, since we had no documentation on i.d. before us (i.e., no  photo of the specimen). If I had reached the origin question, I would have found the issues far from clear, though probably I would have felt the odds of a wild bird being a bit less than 50%, given the locale and the lack of theory supporting vagrancy.

4Intriguing, and nothing more than that, are the records off San Diego in Apr 1895 (Anthony 1895). While unacceptable, if they were good it might not be coincidental that they were reported during the post-El Niño period following "the great El Niño of 1891" (Rasmusson 1985).


Ainley, D. G. 1976. The occurrence of seabirds in the coastal region of California. W. Birds 7:33-68.
Ainley, D. G., Carter, H. R., Anderson, D. W., Briggs, K. T., Coulter, M. C., Cruz, F., Cruz, J. B., Vallé,  C. A., Fefer, S. I., Hatch, S. A., Schreiber, E. A., Schreiber, R. W., and Smith, N. G. 1986. El  Niño - southern oscillation 1982-83 and effects on Pacific Ocean marine bird populations. Proc. I.O.C. (abstract presented at I.O.C., Toronto, June 1986).
Alival, A., Fuenzalida, R., Herrera, G., Prado, L., Soto, R., and Zapata, B. 1983. Presencia del   fenómeno "El Niño" en la zona costera de Iquique, con especial referncia al period 1982-1983. Amb. y Des. 1:133-136 [in Spanish, with English summary].
Anthony, A. W.  1895.  Probable occurrence of Cregrus furcata  off San Diego, California. Auk 12:291.
Arntz, W. E. 1986. The two faces of El Niño 1982-82. Meeresforsch. 31:1-46.
Belopolsky, L. O., and Tsigankova, Z. K. 1985. New data on the ecology of the Galapagos Swallow-tailed Gull.  Vestnik Zool. 1985:76-77 [In Russian, translation by V. Volmensky].
Cullen, J. M., and Ashmole, N. P. 1963. Black Noddys on Ascension Island. Ibis 103b:423-446.
Hailman, J. P. 1964a. The Galapagos Swallow-tailed Gull is nocturnal. Wilson Bull. 76:347-354.
Hailman, J. P. 1964b. Breeding synchrony in the equatorial Swallow-tailed Gull. Amer. Naturalist  98:79-83.
Hailman, J. P. 1965. Cliff-nesting adaptations of the Galapagos Swallow-tailed Gull. Wilson Bull. 77:   346-362.
Harris, M. P. 1970. Breeding ecology of the Swallow-tailed Gull, Creagrus furcatus. Auk 87: 215-243.
Harris, M. P. 1979. Survival and ages of first breeding of Galapagos seabirds. Bird-Banding 50: 56-61.
Jehl, J. R., Jr. 1973. The distribution of marine birds in Chilean waters in winter. Auk 90:114-135.
Johnson, A. W. 1972. Supplement to the Birds of Chile and adjacent regions of Argentina, Bolivia, and Peru. Platt Estab. Graficos, Buenos Aires.
Monroe, B. L., Jr.  1955. A gull new to North America. Auk 72:208.
Murphy, R. C. 1936. Oceanic Birds of South America. Vols. 1, 2. Amer. Mus. Nat. Hist., New  York.
Murphy, R. C. 1938. The Wandering Albatross in the Bay of Panama. Condor 40:126.
Paxton, R. O. 1968. Wandering Albatross in California. Auk 85:502-504.
Rasmusson, E. M. 1985.  l Niño and variations in climate. Am. Scientist 73:168-177.
Reed, J. R. 1988. Inca Terns in the Bay of Panama during the 1982-1983 El Niño event. Am. Birds 42:172-173.
Roberson, D. 1986. Ninth report of the California Bird Records Committee. W. Birds 17:49-77.
Robbins,  C. R. 1958. Observations of oceanic birds in the Gulf of Panama. Condor 60: 300-302.
Sharrock, J. T. R., and Sharrock, E. M. 1976. Rare Birds in Britain and Ireland. Poyser, Berkhamsted, England.
Slipp,  J. W. 1952. A record of the Tasmanian White-capped Albatross, Diomedea cauta, in American North Pacific waters. Auk 69:458-459.
Slud, P. 1967. The birds of Cocos Island. Bull. Am. Mus. Nat. Hist. 134.
Snow, B. K., and Snow, D. W. 1968. Breeding behavior of the Swallow-tailed Gull of the Galapagos. Condor 70:252-264.
Snow, D. W., and Snow, B. K. 1967. The breeding cycle of the Swallow-tailed Gull Creagrus furcatus. Ibis 109:14-24.
Wooster, W. S., and Fluharty, D. L., eds. 1985. El Niño north: Niño effects in the eastern subarctic Pacific Ocean.  Washington Sea Grant Prog., Univ. of Washington, Seattle.

Validation Form

Record number: 79-1985   Species:  Swallow-tailed Gull
Member's name:   D. Roberson   Date of review:   13 August 1993
Circulation:  Fourth [and final]
   identification questionable:
   natural occurrence questionable:
   establishment of introduced population questionable:
At the annual meeting back in January, I presented a short slide and video-tape show illustrating why I feel this bird is clearly within the category of records that should be accepted. As in any rare species where issues of origin arise, one cannot "prove" conclusively whether or not the individual arrived on its own volition, responding to forces of nature, or was carried here "against its will" by human agency. One can, at best, simply evaluate the facts and make the most reasonable judgment possible. The rule of parsimony (i.e., accept the explanation that requires the fewest assumptions) is very helpful.

Having again considered all the evidence in this record, the balance clearly favors the hypotheses that was a wild bird, likely displaced here by the most powerful El Niño conditions in this century. The 1982-83 El Niño conditions in the Peru Current dispersed, as far as we can tell, the entire world population of this species away from its regular habitat.  Some birds are proven to have gone well south and others well to the northeast (Panama). It seems extremely probable that others were dispersed to the northwest, into the waters of the Northeastern Pacific Ocean. Young birds were dispersed as well as adults (e.g., Panama record and the dearth of any birds in the Peru Current during the peak of the event); young birds dispersed to the northwest would not seek a landfall until they reached breeding condition.  This might well occur in 1985. When an adult bird is in breeding condition, it is likely to come ashore at a seabird breeding colony and engage in breeding behaviors. This is exactly what occurred in Monterey Bay in June 1985 when other effects of El Niño were still occurring (e.g., pelagic red crabs). The behaviors observed in the Swallow-tailed Gull in 1985 were standard and well-known breeding behaviors; they best correspond to an adult female.

 During my slide presentation, I showed some previously unreported correlations to El Niño events over the past century, including the dispersal of Black-vented Shearwaters to British Columbia during previous El Niños. There is a good correlation with extraordinary bird records and oceanic events. To answer Mr. Dunn's third round question: the stronger the El Niño the longer the effects will linger. El Niño is not a brief affair. It takes 18 months to work out the basic elements such as the Kelvin wave (see previously-attached material). The lingering effects of the 1982-1983 event continued for over 3 years.  Most El Niños are much less impressive and have much less impact. Only "once-in-a-lifetime" El Niños have these sorts of effects. The last truly "great" El Niño was in 1891. Its impacts might continue to be felt for half-a-decade, and might account for the 1895 San Diego reports of Swallow-tailed Gull. Perhaps someone in 2080 or so, when the next "great" event occurs, might correlate the next Swallow-tailed Gull with the sequence described herein.

El Niño aside, acceptance of this record is supported by similar treatment to all other records of pelagic species. The CBRC has accepted records of adult pelagic birds in breeding condition at seabird colonies, or engaging in breeding behavior, for numerous previous records, including Wandering Albatross (adult female on flat headland similar to nesting habitat), White-tailed Tropicbird (male on beach attempting to copulate with a model glider), Red-footed Booby (adult on Farallones in seabird colony), and Sandwich Tern (adult engaged in courtship in tern colony). These were exceptional records. The Swallow-tailed Gull is within this same category; it is more exceptional mostly due to its smaller world population size.

At the meeting I showed a video of the capture of one adult Swallow-tailed Gull by active dip-netting at night under lights. It is "possible" to capture one, but it takes enormous effort. Nighttime shrimp or squid fisherman "could" do so, but no boats catching these commute from California. They are short-distance fisherman from the mainland of South America. I cannot image a fisherman intentionally capturing this gull (which competes for the same squid) and then transporting it to California by means which have never been located. There is zero evidence to support day-time capture by a casual tourist (the Stallcup theory). In other words, all alternative theories of capture and transport to California (a) lack evidence and (b) fly in the face of known reality. Thus the rule of parsimony strongly supports the acceptance of this record.