Integrating the Impossible

The complexities of resource exploitation in a littoral ecosystem

Estuary of Afon Dyfi looking south-west.

(Picture E. Evans, used by permission)

It's not the Amazon rainforest, despite all the rain. It's certainly not the high Serengeti under Ol Doinyo Lengai.  And yet these estuaries around the British coast are some of the most biologically productive ecosystems anywhere on the planet.  Each cubic metre of glorious glutinous silt down there contains over 25000 kJ of calorific energy – about the same as a dozen (large) bars of chocolate.  Fish base their entire life cycles around these places.   Wading birds flock to them in winter because there's no food resource that compares.

The Dyfi estuary supports not just one food chain but hundreds.  At the bottom of each are microscopic organisms living in the mud and, near the top, one osprey nest.   It seems like a perfect location for ospreys to live and breed, so why only one nest so far?  That's an interesting question – but first, a shaggy dog story...

 This is Polky, a good-natured little pooch who lives with one of my neighbours across the road. Polky and his almost-identical brother Cobblers (don't ask) go for walkies twice a day, and their favourite game is catching a thrown tennis ball on the grassy area between our respective apartments.  To be honest, this is the only time I can tell the two dogs apart because Cobblers is brilliant at catch ball. He makes it look easy and he never misses.  Polky, on the other hand, cannot catch ball for toffee. His eyesight is fine, his technique is sound and his timing appears to be good. He really tries but he just sucks at it.  If the ball hits him on the nose and rolls away, that's a good effort by Polky's standards. Both dogs have been playing this game since they were pups.  It's a game of skill and co-ordination; one that involves instinctive AND acquired abilities.

Some animals are better at learning how to do stuff than others.

In Britain, many osprey nests are close to reservoirs or freshwater lakes. Catching fish in such places is by no means easy, but at least the birds know where the fish are.  Given the innate hunting skills they were born with, any reasonably competent male osprey ought to be able to produce fish for his nest.   However, rivers and their estuaries are different.  The overall food resource there is greater and more varied, but accessing it can be much more complicated.   In a tidal estuary and river, the behaviour and distribution of fish changes on an hour-by-hour basis, AND varies over seasonal time scales as well.   Weather patterns far upstream can change the salinity of the seawater offshore, or affect its temperature and transparency.  At any given spot, there may be dozens of sizeable fish present on one day, and not a single one at the same time on the next.

In such a complex environment, instinctive hunting skills are not enough – not by a long chalk. There is no inherited information that says..

“Flounders can be caught along the sandy areas south of the estuary, for an hour either side of low tide.”

“Hunting migratory sea bass in mid-May is a waste of time because there aren't any.”

“If visibility is poor today, there's a well-stocked trout lake about 12 kilometers north-east from here."

“If it's been sunny for three days, shoaling mullet will be taking plankton near the surface in the mornings.”

All this, and much else, has to be learned.  The reason that Monty - and his neighbour at the Glaslyn nest to the north - are so successful is that they have achieved a complete mastery of all these factors.  They know and understand the dynamics of foraging on the Dyfi, or Tremadog Bay, and it has taken them years to amass all the detailed information needed to do this.

Other male ospreys may not be so proficient, because it's one thing to catch sufficient food to feed yourself – quite another to collect enough for a female and a brood of ravenous youngsters as well.

The nest-provisioning stats collected in Wales illustrate this problem.  A solo osprey only needs about 400g of fish per day to maintain condition.  That's two small trout or one very moderate mullet.  This graph shows the weekly take required when three chicks are being supported.   And it's not just about quantity: if conditions are unfavourable, the solo osprey might go hungry for a couple of days until fishing becomes easier.   This state of affairs won't do when there are young to be fed – the incumbent male has to be able to produce something every day, and only a bird that knows his home range like the back of his own talon can do this.

Unringed osprey "Dai Two-Dots" 26/8/12  ( (c) Dyfi Osprey Project)

Perhaps this is the real reason that “Dai Dot” and other satellite males have not attracted roving females and set up nests of their own in west Wales.   They can live along the estuaries in the summer quite comfortably, but they have never acquired the EXTRA skills and knowledge to be successful providers for a family.

Like Polky, they just don't have what it takes to catch the ball.

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.