Tuesday 4 February 2014

Bum deal?

So I realised I haven’t really gone into any detail as to how I’m spending my days and why. Cue a couple of slightly more science-y posts. You’ve been warned. For this project we’re using the full spectrum of data collection techniques; from very simple and easily repeated sampling in the field, to brand new and genuinely cutting-edge tracking devices. The telemetry is pretty cool and sexy, and is already showing us facets of the white-front’s lives that would be simply impossible to see otherwise. But, you’re going to have to wait for next time to hear about that! For now I’m going to explain how and why I’ve spent a fair bit of time ageing the birds here on Islay, and why I spend a couple of days every fortnight looking solely at goose bums. Yes. I do.

Adult Greenland white-fronts enjoying the Islay sun - it's a surprise, but you have to make the most of it!

It’s a pretty legitimate question to ask why! It might be surprising, but a goose’s bum can tell us a great deal about a goose’s recent history, current environment and its future prospects; to the extent that in some species during spring, it is even possible to predict, with reasonable accuracy, how many goslings an individual will return with the following autumn! Fat is the reason it’s possible to infer so much from a bum – like many of us, geese store fat in their bums. For any long-distance migrant, fat is their most valuable commodity; their fuel for migration. The fatter you are before migration, the likelier you are to successfully complete the journey and be in better condition for breeding. For geese, a bum is like a fuel tank. And big really is bootylicious.

We can, therefore, estimate any given individual’s body condition simply by looking at its bum and giving it a rating, or scoring, against some pre-determined scale. And we have one of those – made earlier of course. It’s called the Abdominal Profile Index (API) and is the basis of a lot of what I’m up to. The beauty of the API is that (with an experienced eye for a goose bum!) it is extremely quick and easy to score the body condition of large numbers of individuals – and from a distance.

When your diet is almost entirely vegetarian, it can take a lot of work to put on weight. Eating high quality food, and/or being able to spend long periods of time feeding mean an individual can fatten fairly easily. However, it may be that there is variation in the ability of individuals, flocks or even populations to access sufficient food resources to put on weight – and, if so, we would expect to see variation between their API’s as a result. Have a quick look at white-fronts in the photos; you should be able to see a clear difference in the size and shape of their bums!

This variation is most visible and important during the autumn and late winter/early spring, when the geese are feeding up rapidly just after and just before migration – to recoup the fat lost on the way here and to put on those extra few pounds prior to leaving. At other times of the winter, being very fat isn’t necessarily a good thing – being heavy slows you down and makes you less manoeuvrable, in turn making you more vulnerable to predation. The geese respond by regulating their body mass during the middle of winter – storing fat, but not compromising their survival.

We are looking at API variation at three scales with the Islay white-fronts. The first is between populations. The API’s on Islay as a whole can be compared with the birds that winter at Wexford in Ireland – a population that is doing much better than any other. If Islay API’s are significantly lower than Wexford, especially at migration departure point, then that would suggest the Islay birds are being restricted by their wintering site. The second scale is between the different flocks (or sub-populations) here on Islay. This will help determine if some flocks are unable to access resources as well as others; which will tie in neatly with work we’re doing characterising the habitats the white-fronts prefer (and where it’s available) and recording disturbance levels through the winter – both  potential causes of body condition variation. The third scale is at an individual level. Having a number of neck-collared birds means we can follow and individual’s body condition through the winter and relate this precisely to the areas and habitats they’ve used through the winter.

a family group in Iceland. Spot the juveniles!

Who knew a goose bum could tell you so much?! Ageing, on the other hand, does pretty much exactly what it says on the tin. It is equally easy, simply counting the number of juveniles and adults in each flock and converting to a proportion or percentage. Luckily, it is quite easy to separate an adult white-front from a juvenile, with a few characteristic differences that can be easily seen in the field. The most striking is the lack of the black belly-bars on the juveniles. Equally, the juveniles (to start with at least) are missing the white “frons”, or forehead, seen on the adults. Finally, the juveniles have a dark nail, right on the tip of their beak. This starts fading to the pale orange-white of the adult fairly quickly though..

2 juveniles with a parent. One in the background too!

Again, proportion of young is informative at different scales. At its simplest, the proportions of juveniles from all the wintering flocks get collated to calculate the productivity of the global population annually. Clearly, this is very useful for monitoring the long-term population trends of the Greenland white-fronts – which make pretty grim reading; a 40% decline in fewer than 15 years. It is also possible to compare between wintering populations. This year (so the 2013 breeding season) the Islay birds seem to have done pretty well, sitting somewhere around 16 – 17% young. I haven’t seen a global total yet, but it seems that other winter resorts have fared pretty poorly again. Last year, for instance, global productivity was about 4%. To put that in perspective, that’s essentially one successful brood per hundred adults. Not particularly clever.  Even productivity of 17% is only five or six pairs returning with goslings. They can get away with that, being a long-lived bird with high annual adult survival rates – they can’t at 4%. And they’ve had poor breeding seasons for a number of years now. We can also look at productivity between flocks on Islay. Given long-term datasets, it may be possible to link some variables – habitat quality, for instance – to variation in productivity within the island.

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