Song of the Osprey

This year there's been a lot of interest in the various sounds and calls (“vocalizations”) that ospreys make, and in particular their responses to intruders. Take a look at this video with the sound turned well up...

  

 … it's from a camera at an osprey nest in Latvia. The audio quality isn't great but we can forgive that. (The ESTLAT Conservancy do a tremendous job with their monitoring, considering that the nests are among forests in the middle of nowhere.) Listening carefully, we can hear that there are two markedly different calls being given by the adult. Monty and Glesni use these same sounds and it might be interesting to figure out what they are “saying”...

The first type of call is a short, melodious chirp on a rising tone. This seems to be a territorial call and it appears to be specific to ospreys, directed at the incoming intruder as a kind of identifier. Although ospreys don't defend a feeding territory as such, many other bird species do, and they sing to define the range of the territory. A bird's territorial song – which we merely listen to and enjoy – is a very serious matter for it and its neighbours. Millions of years ago some ancient common ancestor of Monty and Glesni might have had similar behaviour, and it could well be that this little chirp is all that evolution has left to us of the osprey's song. It says: “I can see you. We are here, and this nest is occupied.”

The second call is very different: it is the true “alarm call”: a strident challenge and a warning. It says: “Go away! You are too close to my nest and we will chase you if you approach!”

Unlike the “song”, this call is not specific to other ospreys, but is employed generally. The alarm call does something else, too... Watch the behaviour of the nearest chick as the adult's “chirp” song changes to alarm-calling. The chick immediately lies down in the nest and keeps very still. This is an instinctive response to the alarm, technically known as “thanatosis” or playing dead. The young will remain in this posture until the alarm-calling stops – although it can be seen that they are not very good at pretending to be dead, as they keep raising their heads to see what is going on! Osprey chicks tend to do this, even though it rather spoils the effect...

 Chick "doing a Ceulan" and looking around during the alarm call phase.
(Click for larger)  Image: ESTLAT Conservancy

Nest intrusions are a part of life for paired ospreys during the breeding season – and few things are more often misinterpreted by casual observers. On blogs and social networking pages, we regularly see words like “attack” and “fighting off” being used, the users of them assuming that the intrusions are always being perpetrated with hostile intent.

It's much more complicated than that...

“A good place for ospreys is one that already has some ospreys in it.”

With the small (but gradually rising) population density here in the British Isles, it's natural to view the osprey as a solitary nester. But in other parts of the world, nest sites can be separated by only a few hundred of metres, with as many of them occupied as are available and/or can be supported by the local food supply. In this context it would not be wholly wrong to describe ospreys as being communal (or at least, semi-communal) nesters. And in birds, communal living inevitably means the evolution of a social hierarchy and the behaviours to moderate it.

There is plenty of evidence that this is happening among European ospreys, too. Young unpaired birds, both male and female, are fascinated by occupied nests of their own species. They feel compelled to investigate them and it's not just a matter of prospecting for sites: the very presence of a successful nest gives information about the availability of fish, construction material, local prevailing weather and much else besides.

Visiting other nests also enables a roving individual to gauge its own social status as it gets older and more experienced. (You know you're at the bottom of the heap when the local dominant breeding pair can't even be bothered to chase you away!) Although it's true that some “homeless” bonded pairs might attempt to oust an incumbent male from a nest he has just finished building, this doesn't often happen unless the overall pressure for nest sites is very great.

Blue 24(10) inspecting the nest at Cors Dyfi 26/6/13
Click for larger (c) DOP 2013

None of the foregoing means that intruders are WELCOME at an active nest – no matter how peacably-intentioned the visitor might be. The dramatic response of the incumbents demonstrates that - especially if they have eggs or chicks in the nest.

Increasing intruders

Recently, Dyfi Osprey Project revealed that they have seen more than 30 different individuals interacting at the Cors Dyfi nest site (to 2/6/13), compared with 25 by the same period in 2012. Does this mean that the UK population of ospreys has suddenly increased by 17%? Perhaps not...

A significant observation is how few of these visiting birds were ringed. Given the overall UK ringing rate, we might reasonably expect that 30-40% of them would have visible rings but in fact it was only a handful. It's possible that the appearance of these “extra” ospreys is a consequence of the unusual migration weather and the late spring, and that a proportion of them were in transit to other countries – most likely, Scandinavia and the Baltic.