The call differences between Olive-backed Pipit and Tree Pipit are of intense interest to many birders, with some saying that call identification is possible and others doubting this to varying degrees. Magnus Robb has studied the call differences, in particular trying to work out just how much overlap there is in pitch and other characters of the main, buzzing flight calls of both species. It is important to point out that there is a second, much less conspicuous call type in the repertoire of each species. It is easily missed, but if heard it can be a reliable means of identification.
The sip call, as we refer to it, is often heard from migrants of both species. It is very short, and in its purest form it lacks any modulations or buzzy quality. Typically, a migrant will give one every now and then after a couple of the better known buzzing calls. A very similar if not identical signal is used differently in the breeding season, and is often called an alarm call (eg, Alström & Mild 2003). In a breeding context, a pure-sounding version is repeated monotonously, usually from a high perch in the vicinity of the nest.
In Olive-backed Pipit, the sip call is much higher-pitched than in Tree Pipit. Alström & Mild 2003 transcribed the OBP version as sit, and wrote that it is readily distinguished from the süt of Tree Pipit. The pitch of Olive-backed’s sip (or sit) call is similar to that of a Goldcrest Regulus regulus. It does not carry very far and sounds weak and difficult to hear. Tree Pipit’s sip (or süt) call is much lower, similar in pitch to the short trilling call of a Dunnock Prunella modularis, and therefore easier to hear. Despite this, it is such a short, unremarkable call that it is easily missed.
When listening to wintering flocks of Olive-backed Pipit in India, the sip call is a very common sound, and there are also many calls intermediate between it and the longer, buzzing flight call. This flock of around ten was moving along a forest edge in the foothills of the Himalayas. The second call in the recording is a very pure sip, and towards the end there are some similar but slightly longer calls.
Knowledge of Olive-backed Pipit calls led to Magnus discovering Portugal’s second on the island of Berlenga in 2011. This individual was only seen well for a couple of minutes during a stay of at least two days, during which Magnus had to digiscope record shots and make sound recordings simultaneously! The recording is a brief medley of three short cuts, leaving out some clumsy footwork and camera clicks in between. At the very start of the final part you can just make out the important sip call.
Finally, a recording of two Olive-backed Pipits that spent several weeks only a few hundred metres from Magnus’s house in Portugal during the winter of 2012-13. He heard the birds while collecting pinecones for the fire at dusk, but only saw them well after many hours of searching the next day. In this recording they are taking off from a pine tree. A loudish, buzzing flight call is followed after a gap by three or four calls at the same high pitch, intermediate between sip and short flight calls.
For comparison, here are two recordings of Tree Pipit. In both cases the birds are migrating. The first concerns a single bird, and contains two sip calls, the second of which is very low-pitched.
In this one from Bulgaria, two Tree Pipits are migrating along a lake shore. Although Tree Pipits sometimes give very short flight calls, the distinction between their flight and sit calls is usually clearer than in Olive-backed Pipit.
Alström, P & Mild, K 2003. Pipits & wagtails of Europe, Asia and North America. Christopher Helm, London.