After eight months without terns I visited Atlit
Saltpans for ringing. The first night of this season (29-30 April) was a complete
success with 64 birds caught; 60 Common Terns including one retrap from last
year, 1 putative White-cheeked Tern, 1 Little Tern, 1 Slender-billed Gull and 1
Little Stint.
The putative White-cheeked Tern is 2nd
calendar bird with typical primary moult and winter / non-breeding-like
plumage. The main features pro White-cheeked Tern are a grey rump, uppertail
and tail rectrices without contrast to mantle colour; in Common Tern there is a
clear contrast between the grey mantle and white rump (see image).
.JPG) |
| Putative White-cheeked Tern |
Second feature
is larger tail / wing ratio (longer tail, shorter wing). Tail length was 169
mm, the longest tail I have measured (n=387). The wing/tail ratio is 1.60 - a
very low figure, lower than the normal range of 1.62-2.85, average 2.00
(n=386); in 2nd calendar birds (n=10) the range is slightly higher,
1.93-2.75, so this bird has a really exceptional wing / tail ratio.
Another aspect of wing/tail ratio is the distance
between wingtip and tail; in this bird the tail is long than the wing. In other
birds caught this night (all adults) the wing was longer.
Other measurements:
Wing=270 mm
Tail=169 mm
Fork=85 mm
Bill(s)=43.5 mm
Bill(f)=35.2 mm
Tarsus=22.5 mm
In the past, this subject was discussed after
I caught terns with rump as White-cheeked and dark underparts. But this possibility
was overruled after images of mating Common Tern X 'like White-cheeked' Tern in
my ringing region were posted. This bird look better for a genuine White-cheeked
Tern, especially the tail pattern. This species breeding in the Red-Sea and
Indian Ocean is little known, especially in respect to the amount of variation
of these features in young birds. I searched for information about
identification and found only a short article from 1982 (Dutch Birding). I will
be glad to receive some information or ideas, especially biometrics.
 |
| Regular Common Tern |
Thanks to Avishai, Rafi and Eilai for their
help this night.