Cloacal or blood sampling? What kind of biomarker (testosterone, sperm, other hormones...) ?
One might measure telomere length from somatic cells to estimate age. If you have a variation in ages, you should see shorter telomeres in the older birds. With enough data, you could begin to create a database. If you measure reproductive hormones, measurements need to be made at a fixed time in the annual breeding season -- otherwise seasonal variation will confuse interpretation. Measuring gonadal steroids is probably best because of their longer half-life.
Jack, thank you very much, this is a new theme to me, and trying to improve knowledge about a bird population, I argue this could be the tool to hit the target. May you indicate some recent references of these methods in order to collect details on different opportunities?
Dear Fabrizio,
I know about using cloacal anatomy in the poultry industry, but I could not find any serious reference about it!
Could you be more precise as to which species you are planning to study? I would expect the reproductive hormones to have very different dynamics depending on the seasonality or not of their breeding cycle. I would think comparing the hormones dynamics over a year on a good sample of individuals of known stage might allow you to differentiate differnt types of dynamics or hormones levels between stages. I do not know about any such study on wild long-lived birds, but you might find something on domestic birds to get a first idea of the interest or not of the measure.
Directly using telomere length to estimate chronological age would also require a good calibration and in most species, the variation in initial telomere length and telomere dynamics among individuals prevents using telomere length as a direct marker of chronological age : http://www.ncbi.nlm.nih.gov/pubmed/21429128 Some more sophisticated methods have been developped but they require an intensive lab and data analysis workload per sample: http://www.ncbi.nlm.nih.gov/pubmed/21585768
Whether they relate to sexual maturation, I am not sure. If interested, you should get in touch with Mark Haussman from Bucknell who did a lot of work on telomeres and life-histories in birds : http://www.bucknell.edu/academics/majors-and-minors/biology/faculty-and-staff/mark-haussmann.html
Good luck!
Very interesting reply, Charlotte. As I thought, it is a challenge. I am planning to apply a method to marine birds, in order to assess if individuals are first-year (immature) or adults which are attempting to breed. I suppose some physiological parameter should be very different between the 2 classes, but I am not so experienced to easily determine which parameter is better to use as indicator.
Have you checked to see if the birds you are interested in are covered in the Pyle guide (Identification Guide to North American Birds)? Part II covers marine birds and may contain plumage characteristics that could be useful in aging or classifying birds.
Alternatively, I have heard of people doing a small surgery to assess the development of testes or ovaries, but that may be more invasive than you would like.
Thank you Claire. The species is endemic of Mediterranean, and ageing is not possible by observing plumage. I found interesting telomere method, but it seems less sound when the birds are long-lived. Even, storm-petrels seem to have an opposite pattern than expected! I'm searching for non-invasive methods, but admitting blood sampling. So hormonal assessing could be the way. But what hormones? I have to compare adult birds very close to egg-laying or mating, with immatures about 1 year old.
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