Friday, 23 June 2017

Greater Crested Terns

I really do get stuck for places to go once the migration is over and (ironically, as it affords me a greater amount of free time) work begins tailing off for the summer. The Qi Gu sandbar holds a few terns, but all now are Greater Crested Thalasseus bergii. Most are first-summers, so non-breeders, though a few adult or 'adult-like' individuals continue to linger. Despite a few of the locals telling me that the youngest-looking of these are 'this year's' on account of high-pitched calls and occasional begging, one look at the long-replaced (yet very worn) mantle and scapulars shows this not to be the case. Moreover (and more interestingly), there are staringly obvious moult limits in their primaries, and a closer look at these feathers reveals something really quite interesting: Some of these birds have up to three generations of feathers present in their primaries, indicating that they are at least one complete moult cycle past juvenile! The first individual below is an excellent example of this, as it has new P1-2 (and covert) and dropped P3, with P4-8 'old' (from a complete post-juvenile moult, which I did not realise Greater Crested Tern had), P9 dropped and in the process of being replaced (as a continuation of the post-juvenile moult), and P10 still juvenile.

Despite some retained orange in the legs (a feature of juveniles), this second bird (below) is more advanced than the first, with all juvenile primaries having been replaced (though P10 is still growing, as part of the post-juvenile moult) and P1-3 new and contrasting with the rest of the wing. So again (and surprisingly for me at least), two moult cycles evident in the one wing!

I had expected that, like the big gulls that winter here, adult Greater Crested Terns would complete their primary moults in their winter quarters and, once done, would then (and only then) head north to breed. However, there has been a surprise (for me at least) here too, as it turns out that only a handful of inner primaries are actually replaced over the winter months and moult is then suspended for the entire period of courtship and breeding. I only snapped two adults this week, and both show the same differences between silvery inner primaries and darker-tipped outers as do the first-summers above. The first individual (below) shows silvery feathers P1-5 and the second P1-6. The individual with the fish is from an earlier post, and shows silvery P1-4. As the number of pale inner primaries varies from individual to individual, the contrast between silvery inners and dark-tipped outers is one created by moult rather than by plumage (it is that of old feather versus new). As no adults in active primary moult occur at this time, it looks as though primary moult in Greater Crested Tern is suspended for the breeding period rather than completed before it. 

The first-summers (with their newly-replaced inner primaries) are arguably at the same stage of feather replacement in respect of these feathers as are the adults (or, alternatively, in the same 'pattern' as the adults), though all retain traces of juvenile plumage. I hadn't realised that feather replacement in Greater Crested Tern proceeded in this fashion (i.e. moult gets suspended for breeding), an embarrassing revelation after having looked at so many! It does at least go to show that there's always plenty to be learnt, regardless of how uninteresting and tedious a day might otherwise appear to be! Above photos taken in Qi Gu, Tainan County 5-6/2017.

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