Planet Earth - Prion sea birds

Climate change could be altering the migration patterns of seabirds, and a team, led by the University of Glasgow, is studying one seabird in particular. They’re called prions – a type of petrel, whose beaks filter food from the sea. They’re especially fond of tiny crustaceans known as copepods. Last year over two hundred thousand prions were discovered dead or dying on a New Zealand shore after a major storm. But the species’ decline has also been linked to failing food supplies resulting from changes in the climate.  Planet Earth podcast presenter Sue Nelson went to meet Dr. James Grecian, one of the researchers working on the project, to find out more about these seabirds...

James - Prions are small seabirds, about the same size as a puffin. They’ve got a similar wingspan, but much lighter, they’re only a couple of hundred grams. SO very, very small birds and they breed in large colonies, sometimes up to 2 million pairs around the Southern ocean, in places like New Zealand, the Falkland islands. But, because these birds are so abundant, they’re really major consumers in ecosystems. And so it’s really important that if these are birds that are being impacted by climate change, then there’s a whole range of other species that could be impacted by climate change. I mean, if copepods are moving and birds can’t follow them, maybe fish can’t too and that has implications for global fish stocks.

Sue - How has this sort of, abundance of copepods, affected these particular birds?

James - Plankton species are very, very susceptible to changes in temperature, so as the oceans warm they’ll move towards the Poles. For species that target copepods, this could be a massive change. So, if your main prey species is moving, you’re going to have to adapt and change to try and follow them. And what I’m trying to work out is whether or not they can.

Sue - So how do you do that?

James - Firstly, we can look at stable isotope - non-radioactive isotopes in your body that you absorb as you’re feeding. And they are relatively predictable. And so we can use that as a guide to try and understand where prions are. So if we can sample prion feathers, we can look at the isotopes in those feathers and work out what the prions have been eating. We know it’s copepods, we can try and work out where those copepods were and therefore where the prions were when they were eating them.

Sue -  I notice you’ve got something that looks remarkably like some feathers in your pocket there.

James -  Yeah, so I’ve got a few samples of feathers here from a bird that was sampled in Gough island, in the middle of the Southern Atlantic. And these are feathers sampled from a bird that was killed by skewers and they kill prions and eat prions. And so round colonies in the Southern ocean, you find big piles of dead prions and it’s a really, really good resource for us, we can go along, we can pick up and we can sample lots of birds that we know were killed on that colony, so we know were probably from that colony. And if we then use those feathers and analyse those feathers, we can get an idea of where that bird was before it was killed. This figure I’ve got in my hand here is dark round the outside because it’s one of the outer primary feathers. And so this was probably grown in the centre of the wintering grounds. We believe that primary molt, so the molt of the main flight feathers in a bird starts with the innermost primary and works towards the outermost primary. And that starts as the leave the breeding colony. So if we sample feathers from the outside of the wing, those are the feathers that are form the wintering grounds and those will give us an idea of what the bird was feeding on.

Sue - So what sort of chemicals or what signature in effect, are you trying to get from a feather like this one here?

James -  We put it through a mass spectrometer, working with the Scottish Universities Environmental Research Centre, in East Kilbride. And the two elements that we’re mainly interested in are carbon and nitrogen. We’re interested in different isotopes of carbon and nitrogen, so we’re interested in heavy carbon and heavy nitrogen – Carbon-13 and Nitrogen-15. And they will accumulate in body tissues. So, for example in nitrogen, the heavy nitrogen is much harder to metabolise than light nitrogen and so as you’re eating things, your body will withhold, will store if you like, the heavy nitrogen because it’s much easier to get rid of the lighter ones because it’s easier to metabolise. And so for animals that forage very, very high up the food chain, there will have been this build-up in nitrogen, or heavy nitrogen in their tissues. And so, we can look at the ratio of nitrogen, of heavy nitrogen in their tissues and that will give us some idea of where the bird’s been foraging.

Sue - And then you can tell how far they’re moving as a result of climate change moving their source of food.

James - Exactly. The other isotope we’re interested in is carbon and carbon is much better as an idea of spatial differences because it’s due to baseline productivity. So regions of the ocean that are very productive will have different carbon signatures to areas of the ocean that are less productive. And, it’s actually quite nice in that, in the Southern Ocean because of the way the ocean fronts, the sub-Tropical fronts and Polar fronts build up, there’s actually a relatively predictable gradient in carbon. So, if we look at the carbon isotopes, heavy carbon isotopes, in our feathers, we can, to an extent, within various confidences, get an idea of where, along the Southern ocean, that bird was wintering. So what we want to do is we want to try and get a baseline, to try and look at what the isotopes mean in space, and we can do that through tracking data. We can try and correlate the two to see what isotopes are associated with what areas in the wintering grounds. One of the key things with using the isotope data is that we’ve got museum specimens from as early as 1871. So we can compare the isotopes in feathers today, in 2012, with birds that were killed on colonies 140 years ago.

Sue - And you could match that up with climate data.

James - Hopefully that’s exactly what we’re going to try and do. That’ll give us some idea of how the birds may have moved in the wintering areas that they’re using, which could well be due to climatic change.