Isotopes, and other news

Posted on Nov 7, 2013

CAnMove Course.

I’m now back in the office after an excellent 2 week course at Lund University (Sweden) on the Ecology of Animal Migration. I met lots of nice, interesting, clever people, and learnt a lot about the causes and consequences of animal migration, as well as some of the techniques used to track animal movement. We even squeezed in a trip to Falsterbo Bird Observatory, where there was an awesome push of finches, tits, woodpigeons and sparrowhawks. The course is well summed up here by the course organiser, Tom Evans (who worked on Skomer Island a few years before me).

Falsterbo fyr

Falsterbo fyr

Geolocator news.

Since I last posted here, I’ve been working away with the solar geolocator data, attempting to produce maps which honestly represent our best interpretation of the light data. I don’t know what people generally expect geolocators to provide when you plug them into a computer, but it’s certainly not a map! Great care is required when analysing the data and producing maps, and it’s really important to be explicit about the limitations and inaccuracies inherent to this technique. It’s a process which could go on forever, so for now I’ve put this project to bed until we recover the second round of geolocators next season. But I can reveal that our 4 French birds wintered in southwest Africa (Botswana / Namibia / Angola) whereas the Latvian birds were further east in Zambia and Zimbabwe. I look forward to being able to publish these routes in more detail, in combination with data from additional populations, multiple years, and hopefully the same individuals in different years. In the meantime, maybe the Spanish PTT tagged birds will keep you entertained.

Stable Isotopes – the story so far.

I’ve also been preparing feather and nail samples for isotope analysis, and most of the results are now coming in. So, the rest of this post will provide a bit of background to the isotope project.

What are isotopes?

The concept behind stable isotopes is as follows: atoms of the same element naturally vary in the number of neutrons, leading to different forms of an element having different nuclear masses. Carbon, for example, commonly has 12 neutrons, and more rarely has 13 or 14. These different forms are known as isotopes. [Some isotopes are unstable – or radioactive – but here we’ll deal with the stable isotopes only]. Some isotopes are much, much more abundant than others, but the exact ratio of one isotope to another varies (often predictably) in different environments. This is because atoms of different nuclear masses behave differently under certain biogeochemical processes; some forms are favoured over others, so that an environment becomes either ‘enriched’ or ‘depleted’ in a particular isotope. 

Why are they useful? 

The 2 elements I’m currently interested in are Carbon and Nitrogen. Specifically, I’m measuring the ratio of 12C:13C and 14N:15N. The heavier forms are rarer, and higher ratios (δ or ‘delta’ values) indicate enrichment in the heavier (rarer) form. δ13C and δ15N values tend to be higher in arid environments, and marine foodwebs. δ13C is also higher in foodwebs based on C4 photosynthesisers (grasses for example), and δ15N values increase up the food chain. The idea is, then, that by measuring the isotope values of animal tissue, we can learn something about the environment in which the tissue was formed. Metabolically active tissues like blood carry the signature of very recently encountered environments, whereas bird feathers reflect the environment of the moulting area. The map below shows the broad patterns in plant δ13C values across Africa (taken directly from Bowen, G. J. (2010) Annu. Rev. Earth Planet. Sci., 38, 161-87)

Patterns in plant δ13C values across Africa (taken directly from Bowen, G. J. (2010) Annu. Rev. Earth Planet. Sci., 38, 161-87)

Patterns in plant δ13C values across Africa (taken directly from Bowen, G. J. (2010) Annu. Rev. Earth Planet. Sci., 38, 161-87)

What about Rollers?

Isotopes are particularly useful in studies of animal migration. In our case we don’t know very much about what Rollers do in Africa. So by sampling winter-grown feathers (the outermost primary feathers, in theory) we might be able to cheaply reconstruct the sort of environment an individual was using. We can then compare where birds from different populations fall in ‘isotopic space’, as well as test whether the isotopic environment encountered overwinter influences breeding (i.e. are carryover effects operating?). As with the geolocators, it’s not an exact science – we’re not sure exactly when or where each individual moulted its feather; we’re not sure if all individuals metabolise isotopes in the same way; and it’s difficult to disentangle whether variation in isotope values arises from variation in diet, habitat, climate, or something else altogether. Nevertheless, it’s a useful exercise, as long as we are honest about our assumptions. 

So, with these aims in mind, we collected feathers from 41 adults last season, as well as from ~60 chicks. I spent several painstaking weeks cleaning, weighing and preparing the samples, but most of them have now been run through the mass spectrometer.

Weighing 0.5mg samples into tin cups

Weighing 0.5mg samples into tin cups

Feathers in pots

Feathers in pots

Statistical analyses are still ongoing, and at present the sample size from Latvia is too low to confidently compare the two populations (we’ll get more samples next year). Additionally, before we can reliably interpret the isotope results, we really need to ‘ground truth’  the data by collecting tissue samples from Rollers in known African environments. So if anyone can put me in touch with some ornithologists working in Africa I’d be very grateful! The carry-over effects question remains to be tested (first I want to build an Access database, which is an undertaking I may come to regret), and I will also explore whether chick isotopes can be predicted by local habitat and diet.

So watch this space, I guess!

Tom