Are they hybrids of Sterna bergii × Sterna bernsteini?

Introduction

Since for the first time the Chinese Crested Tern (Sterna bernsteini) (hereafter CCT) was recorded at the Min River estuary of Fujian coast, SE China on 8 August 2004 (Jiang et al., 2005a, 2005b), field studies have been undertaken there and the bird been seen regularly. Since 2008, several morphologically anomalous CCTs were noted, suggesting hybridization with the Great Crested Tern (Sterna bergii) (hereafter GCT), as detailed below.

Records of photos from the wild

At about 16:00 p.m. on 23 July 2008, a tern bird was seen which had a black-and-white forehead with a narrow white frontal band, dark brown primaries with white shafts, grey of its mantle ("mantle" herein used to indicate the back and entire upper surface of the wings) almost as dark as that of the GCT, dull yellow feet black-tinged, and the black portion on its bill tip smaller and much duller than normal, particularly the maxilla. The bird was considered a second year CCT at the time (Photo 1).

Figure  Explanation of photos.  (1) The tern bird found on 23 July 2008 used to be considered a CCT juvenile of its second year; (2) A tern bird (center) in a GCT bird flock but having more orange-colored and black-tipped bill and with relatively lighter the color of its back and coverts; (3) A strangely-looking tern bird (left) compared with a CCT bird (right) in "normal" plumage; (4) Two tern birds are mating, the upper one with a white band on its front; (5) After mating, the two tern birds standing together, they all have black-tipped bill; (6) A strangely-looking tern bird flying; (7) A "black-tip billed" tern mating a Great Crested Tern; (8) A "black-tip billed" tern (left) with two CCT birds; (9) A male GCT bird standing by and displaying towards a female bird; (10) Eight CCT birds with three GCT birds (the left three). Photos by Lin Chen.

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At about 16:00 p.m. on 31 May 2009, in a GCT flock, a remarkable adult tern bird was noted with a black-tipped bill and somewhat lighter mantle, and yet with the white frontal band of a GCT (Photo 2).

At 17:00 p.m. on 21 April 2010, two tern birds were seen resting on mud together, the right one on the photo is a typical CCT (i.e., solidly black bill tip, black front and very pale mantle), while the second bird had the black-tipped (though duller) bill of CCT, white front of GCT, and grey of mantle perhaps intermediate between CCT and GCT (Photo 3).

At about 14:30 p.m. on 8 May 2011, two tern birds were observed mating and later on standing together, and they were nearly identical and appeared to be typical CCTs with black-tipped bills and very pale grey mantle, except that the presumed male had the white frontal band of a GCT (Photos 4 and 5).

Then, on 21 May 2011 at about 15:00 p.m., two terns were seen mating and later the presumed male joined a GCT bird flock, the presumed female appeared to be a typical GCT, while the presumed male had the white frontal band of a GCT, but the very pale grey mantle of a CCT and some black on the bill tip (though not as much as a typical CCT) (Photos 6–8).

Finally, at 16:18 p.m. on 22 June 2011, a typical GCT was seen to display to a tern that had the black-tipped bill of a CCT but the white frontal band and dark grey mantle of a GCT, and the behavior of the two terns suggests that the anomalously plumaged one was a female (Photo 9).

Discussion

The field differentiation of the similar-sized GCT and CCT is straight forward: GCT has a greenish yellow (tinged orange at base) bill while CCT has a black-tipped orange bill; GCT has a black crown and crest with a white frontal band while the CCT has the black of the crown extending to the base of the maxilla; GCT has a dark grey mantle while the CCT has a very pale grey mantle (often appearing whitish).

The photos herein show several terns with features of both GCT and CCT which we suggest represent hybrids between the two species rather than individual variation.

The putative hybrid birds shown on Photos 1, 2 and 3 taken in 2008, 2009, and 2010 could be possibly the same individual. The putative hybrid shown in Photos 4+5 taken on 8 May 2011 appears to have more black on the bill and a paler mantle than the birds on Photos 1, 2 and 3 and is likely a different one. The putative hybrid on Photos 6+7+8, taken on 21 May 2011, has less black on the bill than the bird in Photos 4 and 5 and thus appears to be a different individual, while it also appears to have a paler mantle than the birds on Photos 1, 2 and 3, suggesting that it is thus a third individual. While the putative hybrid in Photo 9, taken on 22 June 2011, appears to have somewhat more black on the bill than the bird in Photos 1, 2 and 3, but it could be the same bird. Thus, there are at least three, and possibly four, putative hybrids on our photos.

Previous field studies, though limited, have revealed that CCT usually nest in GCT colonies (Chen et al., 2005, 2009; Yan et al., 2006; Jiang et al., 2010). The CCT is considered Critically Endangered (CR) due to its tiny population (BirdLife International, 2001, 2008; Chen CS, 2007; Chen SH, 2007; Zhang, 2007). Hence, if there is hybridization between CCT and GCT, it would be one more threat to the survival of the CCT.

Between 9 May 2007 till 22 June 2011, a total of 120 working days were undertaken at the location (to be exact, 19 days in 2007, 26 in 2008, 32 in 2009, 27 in 2010 and 16 in 2011), and the highest number of the CCT seen at one time were 7 birds on 5 July 2007, 8 on 10 May 2008, 15 on 29 May 2009, 13 on 5 May 2010, and 8 on 24 April 2011 (11 birds recorded by Guanghui Ni from Fujian Bird–watching Society on 23 April 2011). Whereas, the putative hybrids photographed were the only ones observed during the entire field study. Those three (or more) putative hybrids represent a significant proportion of the existing CCT population.

In 2010, it was suggested, per current knowledge, that the CCT birds may consist of three flocks, they are, the (Taiwan) Straits flock, the (Greater) Zhoushan (Archipelago) flock, and the northern (Chinese coast) flock, according to the current status of occurrence of the bird far as known (Jiang et al., 2010). Whilst, the Min River estuary wetlands are considered functioning the main resting place for those birds in the (Taiwan) Straits flock, as they had been observed regularly and constantly visiting the locality during the entire breeding season (Jiang, 2007) (Photo 10). Nevertheless, so low the ratio of appearance of those putative hybrids suggests that those CCT birds (of the Taiwan Straits flock) might have other places for resting.

This phenomenon, fundamentally, might be of incredible potential value and significance for further supposing, understanding, and outlining the distribution, range, and possible population of flocks of those CCT birds, particularly the (Taiwan) Straits flock, far above and beyond our current poor knowledge to expect.

Acknowledgment

We sincerely thank Mr. Ben King from the American Museum of Natural History (AMNH) in New York, for his kind proofreading and comments to our draft.