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Wednesday, March 21, 2007

"Rassing"

Since 1st March my time has been almost exclusively occupied by “rassing”. This means participating in a programme of ornithological research with the acronym “R.A.S.”, namely Retrapping Adults for Survival. The scheme is directed by the British Trust for Ornithology (BTO) and a full description of the scheme is found here.

It is to do with measuring survival of a nominated species, of which mine is the Marsh Tit Parus palustris. In this context, survival means the proportion of a population which survives from one year to the next. In a stable population, the number of new births in a year is balanced by the number of deaths. If the survival rate should decline, a population might spiral into extinction; on the other hand, if it increases we could end up with the eponymous proverbial “knee deep in Marsh Tits”. My study area is the Roudsea Woods and Mosses National Reserve which I have previously described here .

The Marsh Tit - recognised by its glossy black head and small neat black bib

The fieldwork is conducted during the “RAS Season”, nominally the breeding season which for Marsh Tits is designated as March to August. However, it is during March that the birds are most readily captured as they still come to feeders, and that explains the intensity of field observation during that month.

Survival rates usually differ at different stages of the life cycle. This is called age-specific survival rate and it is known that young birds have lower survival rates than adults. So “RAS” specifically investigates the survival rates of adult birds. Once we can measure survival, we are in a position to examine factors which affect it, for example habitat management practices.

In order to determine the number of birds which survive from one year to the next, individuals have to be caught and ringed with uniquely numbered metal rings issued under licence by the BTO. However, it may be difficult to catch every bird in the population every year, and so the task is facilitated by fitting each bird with a unique combination of coloured rings (up to three, in addition to the metal ring). The colour combinations can be seen in the field with binoculars, especially as they come to feeding stations placed around the habitat. This enables individual birds to be recognised without the need to retrap them.
A ringed Marsh Tit. Each individual has its own unique colour combination

The research is ongoing, and should persist for at least five years. This enables the accumulation of a sufficient sample size to minimise the margin of error in the survival estimate. The objective is to ring, in any one year, enough birds to generate at least 25 retraps (or re-sightings) the following year. So far this season we have accounted for 60 individuals of which 28 are retraps from previous years, so we are well on target.

This year, 2007, is the fifth year of my study period, during which time I have caught and ringed (to date) 155 adult Marsh Tits. Preliminary analysis of the results from the first four years indicates that adult Marsh Tits have a comparatively high annual survival rate of about 70%, compared to about 60% for other tit species.

These results are fully analysed by advanced statistical computer packages operated by the BTO and the outcome may give an insight into the natural history and conservation of this attractive little bird.

26 Comments:

Blogger lorenzothellama said...

Very interesting. How long is the average life span of a Marsh Tit? What is that little mark on your finger? Is it where a tit pecked you?

9:35 pm  
Blogger Maalie said...

> How long is the average life span of a Marsh Tit?.

That is a good question - just the sort of thing this study can estimate. If the average survival of adults is 70%, a hundred birds in year one would leave 70 in year 2; 49 in year 3; 34 in year 4; 25 in year 5; 18 in year 6, and so on.

So 18% of adult birds might expect to live five years; 8% might expect to live for 7 years. Young birds would have a lower life expectancy.

Dunno about the spot, I never noticed it aforewards. It's not there now.

9:55 pm  
Blogger simon said...

great entry mate and educational too!

thanks!! I learn even more!

11:19 pm  
Blogger Tortoiseshell said...

Good to see you back in the blogosphere! In your absence I've had to make do with looking at Welsh politics blogs!

12:42 am  
Blogger Kiwi Nomad 2006 said...

Well, I think I need to apologise. All the references I read about rassing..... I thought it was some kind of seasonal celebration you were involved in!!! Instead you have been busy freezing in the outdoors!

4:58 am  
Blogger Maalie said...

Kiwi, seasonal celebration? I do that too! We have just had the Vernal Equinox - our days are now longer than our nights, huge celebration!

5:48 am  
Blogger TCA said...

- eponymous
adjective [before noun] LITERARY
An eponymous character in a play, book, etc. has the same name as the title.

- eponym
noun [C] FORMAL
the name of an object or activity which is also the name of the person who first produced the object or did the activity

"What becomes . . . of the Herakleid genealogy of the Spartan kings, when it is admitted that eponymous persons are to be canceled as fictions"?

8:14 am  
Blogger Maalie said...

TCA: Absolutely. "Knee deep" is the title of a book

3:24 pm  
Anonymous ann said...

But then shouldn't you say, we may end up with the eponymous situation of Butler's novel? Rather than say 'eponymous' and the title aswell?

6:08 pm  
Blogger lorenzothellama said...

Afraid all this English Language pedantry is beyond my grasp.
Pedant: insisting on strict adherence to formal rules or literal meaning.
Wot do eponymouse mean anywhay?

6:23 pm  
Blogger May said...

Nice little bird.

8:26 pm  
Blogger Maalie said...

Nice to see you here May. I've been spending the last half an hour trying to attach a standard error to my estimate of 70% survival - based on ^-2 * p(1-p)/(n-1) - but it complicates in successive years. It really need a computer program.

8:51 pm  
Blogger TCA said...

Can you estimate how many retraps (from previous years) there ought to be out there waiting to be caught this year? Based of course on our preliminary estimate of annual survival (i.e. attrition)

W

;-)

11:10 pm  
Blogger TCA said...

This comment has been removed by the author.

11:11 pm  
Blogger Maalie said...

This comment has been removed by the author.

6:54 am  
Blogger Maalie said...

Yes, it's 48. 20 more to catch, it seems.

6:55 am  
Blogger lorenzothellama said...

I do wish you wouldn't erase your comments. They are obviously rude, suggestive or downright beligerent. They could very well brighten up an otherwise dull day for your many fans and admirers.

10:10 am  
Blogger TCA said...

It was me - I pressed 'go' twice, didn't think you'd want to read the same comment again... nothing rude.

3:29 pm  
Blogger lorenzothellama said...

Oh shame. That accounts for the first deleted comment, how about the second one? Was that you too?

I do feel quite concerned about the black spot on Maalie's finger.
He's not been consorting with Blind Pew lately has he?

7:06 pm  
Blogger simon said...

crikey!

12:39 am  
Blogger TCA said...

Are you SURE they are Maalie's fingers?

7:34 am  
Blogger lorenzothellama said...

Not absolutely sure, no, but I can't imagine anyone else holding a tit like he does.

8:23 pm  
Blogger Dr Michelle Tempest said...

RAS, sounds good. I think I have been doing too much RSS on blogs! Loved the Marsh Tit photograph. Michelle

9:27 pm  
Anonymous Ellee said...

I think I would like to do something too, I do not have your expertise and background, but I am a member of the RSPB, and what a beautiful bird - and in very good hands too.

Interesting info too about the average life span of a marsh tit.

7:38 pm  
Anonymous Ellee said...

Thanks for your comments Joe, I heard from Simon that you are busy in Scotland.

6:59 pm  
Blogger Merisi's Vienna For Beginners said...

Thank you for visiting my blog! :-)
I am so glad you did, that way I could beam myself over to yours and bookmark it.
I spent my childhood at the edge of the marshlands and alluvial woods of the river Inn ("Innauen") in Austria, about 30 miles before it disembogues into the Danube at Passau. Since that was during "the good old times" (*smile*) we kids had a lot of time for exploring and observing wildlife. I wonder if there were, and hopefully still are, any Marsh Tits (Sumpfmeise).

5:36 pm  

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