Olivaceous Warbler - Hippolais elaeica

Eastern Olivaceous Warbler has occurred once at Portland, on July 4th 1999. The record was fully documented in Birding World (12: 284-285), as follows:

Conventional wisdom would place the odds of finding a vagrant warbler at Portland on a hot, sunny afternoon in early July as being similar to those of winning the jackpot in the National Lottery - fortunately, birding is full of surprises....

July 4th had seen Peter Morgan, the most stalwart of Portland's visiting ringers, exhibit typical dogged persistence by keeping the Observatory garden mist-nets open all day for the questionable pleasure of catching and ringing just three local juvenile birds. At 3.00pm he came across a grey and white warbler trapped in one of the nets, which, on extraction and cursory examination, he was able to provisionally identify as an Olivaceous Warbler. Realising the significance of his find he hurried the bird to me: it required only the briefest of scrutiny to arrive at the same conclusion, which was then confirmed by detailed examination and measurements.

The bird was released into the nearby Obs. Quarry, where it remained until the middle of the following day. At times it was remarkably elusive, remaining hidden for long periods in low bramble bushes, but was eventually seen by more than 150 birders.

In the hand, identification was straightforward. Among the Hippolais warblers, Upcher's Warbler, and both caligata and rama Booted Warbler, could be eliminated by wing length and differences in wing-formula. A grey-and-white' Melodious Warbler - if such birds truly occur - could, due to the close structural similarity between the species, present a serious risk of confusion, but that species lacks any conspicuous white on the outer tail feathers. Blyth's Reed Warbler is also superficially similar, but it too lacks conspicuous white on the outer tail feathers, as well as having a shorter first primary.

In the field, important back-up features were the birds tail-dipping and calling: it frequently pumped its tail down from the horizontal, and often gave a Lesser Whitethroat-like tongue-clicking teck' call.

Most authorities (eg BWP 1992, Svensson 1992) recognise five races, of which three occur only in Africa and are, in any case, smaller than the Portland bird. Svensson (1992) gives a number of measurement and wing-formula differences, set out in the table below, which place the Portland bird as belonging to the eastern form elaeica, as opposed to the western form opaca.

	Portland bird	elaeica	opaca
Bill length (to feathering)	13.7	12.8-14.9	14.0-15.8
Bill width	3.7	3.1-3.9	3.9-5.0
1st primary > primary coverts	3.5	2.5-7.0	4.0-9.0
2nd primary < tip of wing	5.0	2.5-6.5	5.0-8.5
Tail length	48	48-59	50-61
Wing length (maximum chord)	69	62-71	67-74
Tarsus length	20	19.9-22.2	22.7-24

Intriguingly, BWP adds that the five forms are "...Perhaps better considered to comprise 3 incipient species...". Since field identification of the forms is speculative at best, it may pay British birders to ensure they see any future trapped Olivaceous Warblers!

With an apparently out-of-season occurrence such as this, it is interesting to consider the mechanism that may have led to the birds arrival at Portland. Its plumage was in a rather poor state: an area of feathers on the crown were in pin', most of the greater coverts on the right upper wing were missing, the under-tail coverts were entirely missing, and the left half of the tail consisted of new, still not fully-grown, feathers. This pattern of feather loss is clearly not the result of normal moult. A possible explanation is that the bird suffered some sort of trauma, perhaps predator attack, on the breeding grounds several weeks previously; this could, in turn, account for its subsequent random vagrancy. As an aside, it is hard to imagine a records committee being very charitably disposed toward such an occurrence, had it related to, for example, a finch or bunting, rather than a warbler.

Finally, is is ironic indeed that this occurrence came just five weeks after the Observatory had been informed that, after 43 years on the Portland List, the species would have to be deleted, following the rejection by the BOURC of the record of a bird trapped on 16th August 1956, which was, at the time, the second British record of the species.

Svensson, L. 1992. Identification guide to European passerines. 4th Edn. Stockholm.

The Portland Eastern Olivaceous Warbler features on 'The Natural History of The Isle of Portland' video which is available from the Observatory.