Identifying Sakhalin Leaf Warbler Phylloscopus borealoides

We just received the latest Forktail and there is an article talking about the separation of Pale-legged Leaf Warbler Phylloscopus tenellipes and Sakhalin Leaf Warbler Phylloscopus borealoides by call.

These two species are extremely difficult to be identified without song as previously thought, so we didn't tick any Sakhalin Leaf Warbler even Wing Tung and I saw plenty good birds in field (like one during butterflying with very long primary projection or some birds in hand). The long primary projection maybe a clue to tell but still some overlapping, especially genetic analysis often prove long PP birds are in fact Pale-legged Leaf.

Now, the call can be a good (formal, scientific) way to separate these two, which Sakhalin calls at a lower pitch (at or below 5k Hz) while Pale-legged much higher. The difference can be told by human ears but still better to record and generate a sonogram to see the exact pitch, like
 the following two sonogram we recorded in Hong Kong (which two birds were seen well but not a chance to ID if no call!).

Sakhalin Leaf Warbler calls at a lower pitch (at or below 5k Hz)

one note of call of Pale-legged Leaf Warbler, higher than 5k Hz (at around 6k Hz)

To tell from other Phylloscopus, that will be easier. This species complex show much paler leg and dark bill with pale tip and pinkish base of lower mandible. And these two species often forage at lower level of trees and bushes.

Sakhalin Leaf Warbler, identified by call in Hong Kong autumn

The two calls can be listened here:
Sakhalin:
https://macaulaylibrary.org/asset/77337291#_ga=2.17219641.128496227.1533826953-143354930.1529308329

Pale-legged:
https://macaulaylibrary.org/asset/75404771#_ga=2.80635799.128496227.1533826953-143354930.1529308329


Reference:
Chananya, H., Jenjit , K., Sontaya, M., Andrew, J. P. & Philip, D. R. (2017) Distinguishing between Sakhalin Leaf Warbler Phylloscopus borealoides and Pale-Legged Leaf Warbler P. Tenellipes on call. Forktail 33: 77-80.