On
the morning of 11th October 2017 I went looking for migrant
birds at Sri Nakorn Kuen Khan Park. Although this park is in
Samut Prakan province the area it is in is almost completely
surrounded by the city of Bangkok and as such it is a refuge
for migratory birds passing through this vast urban area. In
the past this site has hosted a number of regionally scarce
migrants including Ruddy Kingfisher, Malayan Night Heron, Siberian
Thrush, Narcissus Flycatcher and it is an excellent location
to study Phylloscopus warblers as they pass through
Central Thailand in spring and autumn.
I have frequently found that migrant birds are rather scarce
in the early morning, and it was the case on this occasion too
with heavily overcast skies and some heavy rain showers throughout
the morning, at first there were very few migrant species to
be found but around 8.30am I came across a few common migrant
species including 2 leucogenis Ashy Drongos and a single
Asian Brown Flycatcher but as a sunny spell began I located
a small flock of Phylloscopus warblers in a thickly-wooded
part of the park. Two Eastern Crowned Warblers Phylloscopus
coronatus were easily picked out but several other birds
stayed higher in the trees and were much harder to observe properly.
Three or four Phylloscopus warblers that were clearly
not Eastern Crowned, by their lack of bright yellow undertail
coverts or crown stripe, were hard to identify at first but
one, then two birds called, both obviously the calls of Arctic
Warbler Phylloscopus borealis and with poor light preventing
me from getting to grips with the remaining birds a rain shower
began and I moved away a short distance to some shelter.
As the rain began to ease I located a Sakhalin Leaf Warbler
Phylloscopus borealoides on call and as I watched this
bird at close range a second bird flew in and attacked the Sakhalin
Leaf Warbler uttering a harsh "click" call as it came
in. Although this second bird did not stay long I could clearly
see it was an Arctic-type warbler and I also instantly recognized
the call as that of Kamchatka Leaf Warbler Phylloscopus
examinandus having seen and heard this species in May 2017
in Southeast China and having listened to recordings of the
call many times. Although there are an increasing number of
records of Kamchatka Leaf Warbler in Thailand, as birders learn
to identify it on call, almost all of these records are in spring
and I had never previously positively identified this bird in
Thailand so I was keen to get a better view.
I used some call playback but received no response or interest
but after about 5-10 minutes of waiting I once again heard the
call and was able to track the bird down and watch it feed over
a period of about 30 minutes during which time it called again
twice more. You can hear a recording of Kamchatka Leaf Warbler
calling, which matches almost exactly the bird I heard in Sri
Nakorn Kuen Khan Park, here - Kamchatka
Leaf Warbler Call.
The light conditions were very poor so the photographs I obtained
do not shed much light on any possible plumage traits that Kamchatka
Leaf Warbler may have that could separate it from Arctic Warbler.
Although
I did notice, while watching the bird, that the undertail coverts
had a distinctly yellowish hue from some angles. This feature
was not as pronounced as in Eastern Crowned Warbler, but enough
to make me look twice and it is something that Dave Gandy noticed
in his photographs of a Kamchatka Leaf Warbler that he found
in Bangkok on 4th October 2017 and identified by call which
he discusses here - Kamchatka
Leaf Warbler in Autumn. These yellow tones to the undertail
coverts can be seen in the photograph below.
I
only point this feature out to encourage other birders to make
observations regarding plumage details of both Kamchatka Leaf
Warbler and Arctic Leaf Warbler and do not suggest that this
is a diagnostic trait, indeed, Phil Round observes that the
two species are not reliably separated on in the field or in
the hand without hearing the call or song as there are overlaps
in both morphological trends between the two species.
"The
five P. examinandus were scarcely distinguishable
in morphology from P. borealis (Table 1, Appendix
1 ), though tended to be slightly larger and longer-winged.
One of these birds (band no. 1A04402) was markedly yellower
on the supercilium and throat than typical borealis
and brighter green above (Figs. 4a, 4b, and 5 ), apparently
agreeing with descriptions of P. examinandus in Ticehurst
(1938) and Alstrom et al. (2011). However, another
individual, 1A04411 with virtually identical colouration to
1A04402, and caught on the same day (Fig. 6 ) proved to be
a borealis on DNA (Appendix 1 ), illustrating the extreme
difficulty of reliably separating these two species on morphology."
However, although separation of Arctic and
Kamchatka Leaf Warblers on plumage is currently considered
unreliable the yellowish underparts of this bird and Dave
Gandy's other autumn bird does agree with the description
given in Handbook
of the Birds of the World. It is worth noting that most
of the literature currently available concerns birds on spring
migration or on the breeding grounds and perhaps the plumage
in autumn might be more distinctive? Perhaps not but it is
worth keeping an open mind on this and not completely discarding
observations on plumage while vocalizations remain diagnostic.
Indeed, it is currently considered that it is not possible
to positively identify to species level any Arctic-type warbler
that is seen in Thailand without hearing the call or song.
Currently these two autumn observations are less common than
spring records and hopefully others will be encouraged to
look for this species on its southward migration and on its
wintering grounds.
Some more useful information on identifying
these difficult birds can be found in the following:
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