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Case Summary for:
Submission No 197: Pacific Swallow Hirundo tahitica, Mossman, QLD. 28 March 1995. Verdict: Not Accepted A submission of outstanding quality presented very professionally, dealing with birds thought to be Pacific Swallow that have been seen regularly in the Mossman - Daintree River area of North-East Queensland since 1994. Photographs, measurements and descriptions of two birds captured in June 1995, one considered to be a short-tailed adult male Pacific Swallow and the other an adult male Welcome Swallow, constitute the core of this submission, and are further supplemented by details from birds watched in the field and from three juveniles (also considered by the observers to be Pacific Swallows) mist-netted subsequently. In summary the account carefully and convincingly details the differences between Welcome Swallow Hirundo neoxina and assumed Pacific Swallow, listing differences in tail length, extent of spotting on the tail, undertail covert pattern, underdown colour and bill size. The first round of voting appeared at first glance to be straightforward with six members voting in favour after viewing the excellent array of material provided. Two members however voted against a positive vote on the first round requesting that we seek expert opinion before making a decision, basing their decision on doubts in material contained in present day literature. One of these members suggested that the tail measurements of the Mossman birds were incorrect for Pacific Swallow (noting that typographical errors in Turner & Rose’s "Handbook of the Swallows and Martins of the World’ made comparisons difficult) and went on to stress that sexual dimorphism in Welcome Swallow (e.g. Disney 1991) could complicate identification. In May 1997 after further discussion between members and visits to museums both in Victoria and NSW a decision was made to approach Dr Richard Schodde (Australian National Wildlife Collection, ANWC) for expert opinion. Schodde’s findings were indeed enlightening and he concluded that the birds referred to in this submission were actually female Welcome Swallows. Key elements of his analysis were that: (1) "The differences between the two species are not so much in colour of the bases of dorsal contour plumage or the undertail covert pattern as in the proportional length, tip shape and white-markings in the outer rectrices"; (2) some female Welcome Swallows in glossy-backed plumage (i.e. older than juveniles) but not apparently fully adult (judging by gonad dissection) have much less white in the tail than males; "the bar is reduced to a broad wash along the inner edge of the outermost rectrix" (though the white tail markings "quickly contract to discrete spots on inner rectrices") "and the rectrices themselves are more bluntly attenuate, shorter and slightly bowed; they match the tail pattern in the photograph of the presumed tahitica perfectly". We note that analagous age and sex-related variation has been documented thoroughly in the tail of the Barn Swallow Hirundo rustica, in which the length of t6 increases by about 8mm from 2nd to 3rd calendar year, by 9 mm from 3rd to 4th calendar year, 4.5 mm per year from 4th to 6th calendar years (e.g Cramp 1988, Moller 199__). The next step in the process was to double check these findings and commence a second round of voting. David Eades (BARC secretary) spent a day at the ANWC in Canberra checking and photographing a selection of specimens of both Pacific and Welcome Swallow and preparing notes ready for the second round of voting. David’s findings confirmed those of Dr. Richard Schodde and in particular once again highlighted the importance of the rectrices in this identification. The additional information prompted a change in the committee’s view after completion of the second round to a unanimous one of non acceptance. A brief summary of the results of our investigations of the characters used by the observers in their identification follows: (1) Tail length (measured to the tips of t6) in the 14 New Guinea Pacific Swallow specimens examined ranged from 40 to 52mm, consistent with the few measurements published in the primary literature by Mayr (1955) and Diamond & LeCroy (19__). The average tail length of 59mm given by Turner & Rose (1989) is inconsistent with all other information available and we consider it to be a typographical error. The tail lengths of the swallows captured at Mossman ranged from 58 to 73 mm and were within the range of tail measurements of Welcome Swallow (66–75 mm in breeding females, shorter than this in younger birds and up to 83 mm in males; Brown & Brown 1989); this was also manifest in the shape of the feather-tips, which were more tapered than in any Pacific Swallow skins examined. (2) The white markings on the outermost two rectrices of Pacific Swallows are reduced to discrete white spots on the inner webs, sometimes just meeting the inner edge of the feathers. In Welcome Swallows the white markings on these feathers are centred and most extensive on the inner edge, particularly in younger females which can have a broad white wash restricted to the inner edge (as was the case with the bird photographed in the hand at Mossman). In both Pacific Swallows and younger female Welcome Swallows, the white markings on t2–t4 are reduced to discrete white spots. (3) Undertail-covert pattern varies considerably in both Welcome and Pacific Swallows and is a difficult field character to interpret as its appearance is greatly affected by posture, moult and so on. We cannot rule out the possibility that there is an average difference in appearance of undertail coverts of these two species, but at best this is a subtle distinction requiring subjective assessment. Undertail-covert patterns in the photographs supplied were within the range of variation shown by Welcome Swallows. (4) "Underdown" - a misnomer for this character, which really refers to the bases of the contour feathers of the upperparts. In Pacific Swallows these feathers have a broad white subterminal band merging to pale grey feather-bases; in ‘classic’ Welcome Swallows the white subterminal band is much narrower and rather sharply demarcated from the medium grey bases of the feathers. The character varies with age and sex in Welcome Swallows, with the white subterminal band tending to be broader in females and probably broader in younger birds. Opinions on how seriously this variation affects identification varied within the committee, with some members considering that the character has real potential and others not so confident. Despite this diversity of views it was unanimously [check] agreed that the photographs of the Mossman birds did not show this character clearly enough for identification purposes. (5) The observers were correct in believing Pacific Swallows to have a proportionately larger bill in Welcome Swallows, and were apparently the first to report this. This difference lies more in the width of the bill than in the more readily measured length. It is in any case a rather subtle and overlapping difference; we feel that what can be seen of it in the photographs of the Mossman birds could fit the variation occurring in both Pacific and Welcome Swallows. In short it was determined that the tail lengths of the captured birds, the shapes of the outermost rectrices and the patterning of white subterminal spots were all within the range of variation shown by female Welcome Swallows, and outside the range of variation shown by skins of Pacific Swallow (particularly of the race frontalis) from New Guinea. Under these circumstances we are not able to accept the record, which would be a first for Australia. It should be stressed that not all questions about these Mossman birds have been answered. One could quite rightly ask, why have such short-tailed Welcome Swallows not been reported by birdwatchers elsewhere in North-East Queensland or Australia? Why is it that most people that visit these birds have been impressed by the patterning of the undertail-coverts? Does this vary with age? This case highlights the gaps in our knowledge about age-related variation in tail-pattern and length of Welcome Swallows, and about the migrations of sub-adult birds. There is also room for further study to find out if geographical variation in Welcome Swallow or perhaps hybridisation with Pacific Swallows might explain some of these anomalies. We once again congratulate the originators for providing the committee with such an outstanding presentation and for bringing this problem to the attention of the ornithological community.
References and Bibliography
Tony Palliser |