Submission
No 452: Chinese Pond Heron Ardeola
bacchus, Christmas Island, Indian Ocean.
17th November 2003 (Unpublished).
This case concerns
observations of a small heron found twice on Christmas Island on 17th
November 2003. On both occasions it was flushed from dry grass near a phosphate
plant stockpile, and observed on the ground or a perch when it relanded. It was
generally wary, not allowing an approach closer than 25-30 metres, but it was
watched for some time through a telescope at c. 80 metres rage; somewhat
blurry photographs of the bird when perched and in flight were also submitted.
The bird was about two-thirds the size of a White-faced Heron (the length of the grass from which the bird was flushed was used as a point of reference). In perched views it had buffy-brown upperparts, dark streaked head, neck and breast; the rest of the underparts were white, as were the areas of upperwing exposed below the scapulars. The upper mandible of the bill was dark slate-grey, the lower mandible yellow at the base, with a reddish flush along the bill and a black tip. The eye was yellow and the orbital skin greenish-yellow. The legs were yellow and the feet were green. In flight, the wings and tail were almost wholly white (contrasting with the dark mantle, back and scapulars). One observer noted pale grey tips (about 1.5 - 2 cm) to the underside of the primaries; at the time the other observers were viewing from a different position above the bird, and none noticed dark primary tips.
The Committee agreed that the description and photographs showed the bird seen to have been a Pond Heron, Ardeola sp., in non-breeding plumage. However, identification of Pond Herons in non-breeding plumage is notoriously difficult; there are several species that are closely similar, and although their identification characters have not yet received intensive study, it is clear that they overlap in most plumage characters. The observers confined their discussion to two of these species, Chinese Pond Heron and the Javan Pond Heron Ardeola speciosa. Characters that they considered indicative of the former were the dark tips to the primaries, absence of a cinnamon tinge in the underparts, and darkness of the upperparts; in addition they considered it unlikely that the supposedly sedentary Javan Pond Heron would occur on Christmas Island. While these arguments are plausible, the committee did not feel that they were conclusive. All of the plumage characters cited are subject to individual variation and the differences between the two candidate species are so subtle that a higher level of detail would be needed in descriptions or photographs for any hope of identification. Furthermore there is no consensus in the literature about the reliability of these features. Finally, several members of the committee did not agree that it is impossible for Javan Pond Herons to reach Christmas Island, with some arguing that a ship-assisted Indian Pond Heron Ardeola grayii might also be a possibility.
We are accordingly unable to accept this record, but we congratulate the observers on finding an Ardeola in Australia, documenting it well considering the viewing circumstances, and calling attention to a challenging and neglected identification problem. We are aware of one previous report of an Ardeola from Australia in December 1995, also a bird in non-breeding plumage on Christmas Island that defied identification to species level (Johnstone & Storr 2005, p460)/
Tony Palliser
Chairman, Birds Australia Rarities Committee
Johnstone, R.E. & Storr, R.M. (2005), Handbook
of Western Australian Birds, Vol. 2, Passerines, WA Museum, Perth.