Western Bonelli's Warbler (BBRC)

Western Bonelli’s Warbler Phylloscopus bonelli

Vagrant. Southern Europe.

Western Bonelli's Warbler at Gibraltar Point on May 8th 2016 (Graham Catley).

One record in May 2016 at Gibraltar Point NNR. First heard in song at the north end of the reserve where it favoured varied scrub with Hawthorn, Crataegus monogyna, and the occasional Pine, Pinus sp, not too dissimilar to its habitat in the Mediterranean. The call note (see below) excluded Eastern Bonelli’s Warbler, P. orientalis. The bird showed on and off to about 60 birders before disappearing into the mass of vegetation to the north and adjacent to the golf course at 09.45. It returned early afternoon for about an hour but was last seen at 14.00 that day.

Western (P. bonelli) and Eastern Bonelli’s (P. orientalis) warblers were split in 1997 with DNA analysis and significant vocal differences supporting the split with both species being monotypic (BOU 1997). Following the split BBRC reviewed 121 British records and published 51 accepted as bonelli. The remainder were assigned as ‘indeterminate’ (BBRC 1997). The first accepted British record of orientalis was on the Isles of Scilly September-October 1997 (Rogers et al 1998). To 2019 there have been 150 records of bonelli, averaging 3-4 per year, and just eight of orientalis, the last in May 2016 on the Calf of Man. The world population of orientalis has been estimated at 1% of bonelli so it is likely it will remain much the rarer of the two. Plumage details overlap and even trapped birds may not be assigned; recording the characteristic calls remains crucial in confirming identity – a sparrow-like ‘chup’ in orientalis and a rather Willow Warbler-like ‘hooeet’ in bonelli.

Western Bonelli’s Warbler, Phylloscopus bonelli, at Gibraltar Point NR on May 8th 2016, the first county record.

by K M Wilson.

Note: this account appeared in the Lincolnshire Bird Report for 2016. This record was one of five in 2016, bringing the British total to 139.

On May 8th 2016 I left home at about 0645 with the intention of doing a sea-watch in the hope of a few black terns or arctic terns offshore in the easterly wind. Whilst passing through an area of dune scrub, I heard a strident trill that stopped me in my tracks. It sounded spot on for Wood Warbler, Phylloscopus sibilatrix. I waited for a few minutes and the bird sang again. Sure the trill sounded like Wood Warbler but where was the fore-song?

I briefly mulled over the possibility of these vocals coming from a Great Tit, Parus major. In recent weeks, one Great Tit mimic had been performing Willow Tit, Poecile montanus, song (an extreme rarity now at Gibraltar Point) and another doing a pretty good Sardinian Warbler, Sylvia melanocephala, call close to my current location. After another burst of song, the penny dropped (or perhaps the “spinning coin”)! I was listening to a Bonelli’s Warbler. The bird was singing from the back of three mature hawthorns and I was facing into the strong early morning sun. There was no option of moving my location without the risk of disturbing the bird so I sat tight with scope at the ready. The bird eventually worked its way to the front bush and then luckily appeared near the crown for a few seconds. In the scope I saw the front end well. The characteristic facial expression, with bland features, subdued supercilium and pale surround to the eye, without darker lores and eye-stripe, quite greyish upperparts and clear white underparts – so it was a Bonelli’s Warbler but which one? I knew that pretty much the only diagnostic in field feature for separating Eastern and Western was the call. I had no camera or recording equipment on me but I did have my phone and was able to get hold of James Siddle, who was not far away. He was quickly able to hear the bird, then a handful of other birders arrived to help sort out the identification, including Graham Catley, Dave Hursthouse and Neil Drinkall.

The bird’s chosen area was in varied scrub with occasional pine trees – actually not dissimilar to the habitat where I have watched breeding Western Bonelli’s Warblers, P. bonelli, in Europe. It was quite mobile around about three acres of habitat and narrowly missed going into a mist net on a few occasions. It regularly returned to a mature sycamore where it sang. The upperparts colouration overall seemed very greyish and not olive-y. This raised suspicions that it might be an Eastern Bonelli’s, P. orientalis. On occasions the bird perched side on or back on in full sun and the bright greenish edging to the secondaries formed a bright panel. Additionally the carpal area, tail edgings and rump were bright yellowish green. With patience, some good scope views were possible and the leg colour was determined as being yellowish-pink and the bill looked predominantly dark. The primary length was at least as long as willow warbler.

Mindful that there was a crowd of up to 60 birders on site looking for the previous day’s Alpine Accentor, Prunella collaris, we wanted to establish the bird’s identity as quickly as possible, record it on film and audio and plan access for a twitch in a difficult situation. Graham Catley was able to play the songs of both Eastern and Western Bonelli’s Warbler on his phone app and we agreed that the song pitch was closer to Western Bonelli’s Warbler. Fortunately after about an hour, the Bonelli’s had an interaction with a Willow Warbler, P. trochilus that encouraged it to call. The call “hweet” was a bit like Willow Warbler but slightly finch-like and with a hint of Pallas’s Warbler, P. proregulus. Its bout of calling was not particularly loud but continued intermittently for several minutes. This established the identification as a Western Bonelli’s Warbler.

The bird was in a restricted-access ringing site. Despite the prospect of catching the bird, Dave Vincent kindly agreed to close and furl the nets early so that we could “facilitate” the “twitch” now that the bird had been identified. Before long, many birders were at the site and getting views.

The record was accepted by BBRC (British Birds 110(10): 562-631).

Reference

Wilson, K.M.W.(2018). Western Bonelli's Warbler Phylloscopus bonelli at Gibraltar Point on 8th May 2016 - the first county record. Lincolnshire Bird Report 2016, p 234-5.

(Account as per new Birds of Lincolnshire (2021), included October 2022)