Air Itam Dalam has had an exceptional run of hawk-cuckoo records this spring, and Mr and Mrs Hum have been doing a sterling job of documenting them photographically. One of the birds they photographed on 19 March showed several features associated with Northern Hawk-cuckoo, and it was with the aim of studying this bird that I headed to the site early in the morning.
There was also a ‘Pale-legged type’ Leaf Warbler, which showed well at times. Recent records in the Gulf of Thailand and Singapore suggest that Sakhalin Leaf Warbler may be the commoner of this difficult species pair in the Peninsula, so I was eager to see how it would respond to playback of the songs of both species. The result? It showed no visible response to either! Big disappointment!
Tim and I were hosting a visitor from England who was keen to see some shorebirds, so we headed for the coast at Bagan Belat, where the tide was low and the shorebirds were predictably distant.
However, I was surprised to hear the familiar tink call of another ‘Pale-legged’ Leaf Warbler coming from the coastal scrub, so in I went again with my playback of the songs. First I played the song of Pale-legged for about 3 minutes. No sign of the bird. Then I played the song of Sakhalin Leaf Warbler. The response was almost immediate!
There’s no certain way to differentiate these in the field other than by song. Even in the hand, there is apparently quite an overlap in wing length, with only birds at either end of the spectrum (Sakhalin has longer wings, Pale-legged shorter) being identifiable with certainty.
Several observers have noted that Sakhalin’s tink call is lower-pitched than Pale-legged, and if you compare the calls of birds identified by song, such as this Pale-legged and this Sakhalin (in Singapore!), and their accompanying sonograms, the difference in pitch is quite evident, with Sakhalin being around 5kHz and Pale-legged around 6kHz. In addition, to my ears, Pale-legged sounds thin and weak, whereas Sakhalin’s call is appreciably fuller. I was not able to record the call of the Bagan Belat bird, but all my recordings of birds in Perlis are lower-pitched, some even lower than the Singapore bird. The only fly in the ointment of this potential differentiating character are two recordings of apparently the same bird (though it was not actually seen) giving both call types, made in Hangzhou Bay, China by Frank Lambert. This recording is of a thin high-pitched call (around 6kHz), while this one sounds identical to that of the Singapore bird to my ears (and is around 5kHz). This anomalous example apart, a survey of calls of Pale-legged Leaf Warbler on xenocanto in Vietnam and Cambodia reveals that all of the recordings are of the thin, higher-pitched call associated with Pale-legged. Another bird recorded in Hangzhou Bay sounds much more like Sakhalin, a possibility noted by Frank in the notes accompanying the recording.
So, are there any certain records of Pale-legged Leaf Warbler in Malaysia? Wells (2007) gives the range of wing lengths of 7 skins from the Thai-Malay Peninsula as 59-68mm. Birds at the lower end of this range should be Pale-legged, but, for the time being at least, Pale-legged should probably be square-bracketed as a Malaysian bird.
In the meantime, anyone heading to Perlis in the next few weeks would be well-advised to carry a recording of the song of both species (downloadable from xeno-canto), and a recording device to document any response.