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Wednesday, March 26, 2008

"Rassing"

My friends often point out that I am "hard to find" during March. This month, for me, represents the height of the "Rassing Season". So, "what is Rassing", you may ask? "Rassing" is the term I use to describe the undertaking of a 'RAS', and that is a type formal research investigation operated by the British Trust for Ornithology (BTO) and stands for "Retrapping Adults for Survival" (it used to be called "Adult Recapture for Survival Estimation but there were problems with the acronym).

In brief, the idea is to estimate the over-winter survival rate of adults of a chosen bird species. My species is the Marsh Tit, a "red listed" British woodland species that is declining alarmingly. I began the study in 2002 and expect t continue for as long as I am active. The longer the study continues, the greater is the precision of the survival estimate.

Marsh Tit, photo taken by bird-ringing friend Ken Hindmarch

The basis of the research is simple: to estimate the proportion of the adult birds that are alive in the study area during the breeding season (March - August) of one year that are still alive in the following year. This is the "survival rate", the proportion of the population that has survived the winter. In order to track the birds as individuals, they are caught in mist nets or traps and fitted with a metal ring issued by the BTO that carries a unique serial number. In order to recognise birds as individuals without having to recapture them, birds are additionally fitted with a unique combination of colour rings.

With my Marsh Tits, the "season" kicks off before dawn on 1st March when I am on location at my research site at Roudsea Woods and Mosses National Nature Reserve in Cumbria, having camped there overnight.

Dawn of the "Rassing Season" - breakfast under way in the woods

"Big Bertha" - birds may be lured into traps like this by baiting them with black sunflower seed

A Marsh Tit is fitted with a uniquely-numbered metal ring and an individual combination of colour rings








"Long Tom" - Marsh Tits will come to feeders like this to feed on black sunflower seed and their colour rings can be identified through a telescope trained on the feeder










A Marsh Tit visits Long Tom for a feed of black sunflower seed. It carries no rings - it will be targeted for capture! (Photo: Ken Hindmarch)

This Marsh Tit can be seen to carry colour rings "carmine above metal on the right; orange on the left". Without needing to catch this bird again, I know that the serial number on the metal ring is R681743 and it was caught for the first time on 5 November 2006. (Photo: Ken Hindmarch)

At the time of writing (26 March), I have this season accounted for 50 Marsh Tits, of which 31 are recaptures from previous years. This gives a provisional survival rate of about 46%, but this figure will increase as further birds are accounted for as the season advances: the average survival over the last five years is about 60%.

How long do they live?

This is a question I am often asked. So far this year I have recorded six birds that were first ringed in my first active season, in 2003. But since they were adults when ringed (having already survived their first winter) it means they are at least six years old.

However, we can use the survival data to estimate the potential longevity of a Marsh Tit. Imagine we have a population of 100 adults in Year One, we expect 60% (i.e. 60) to survive to Year Two. And then about 36 (60% of 60) to survive to Year Three and likewise 22 to Year Four; 13 to Year Five; 8 to Year Six; 5 to Year Seven, and so on. (But you can add a year to each because they were at least a year old when ringed as an adult).

It is not unreasonable to expect the occasional bird to live for 10 years or more. It is exactly this sort of information about population dynamics that helps us to understand the ecology of the birds. Once we know how to estimate survival rate, we can begin to investigate factors which affect survival, for example, forest management practices. Managing a forest in a way that maximises survival may help to promote the conservation of this attractive species.

24 Comments:

Blogger simon said...

Brilliant post. Ken photos are alway superb.

How many hours is spent in the woods each day?

9:43 pm  
Blogger Ellee Seymour said...

And how easy is it to actually ring the birds when they are held in a tight grip in your hand?

10:28 pm  
Blogger Magdalene said...

Fascinating! What a thoroughly interesting and worthwhile job.

Do you think the other birds take the piss out of the marsh tits' stripy socks?

10:55 pm  
Blogger TCA said...

I don't expect you got time for ANY leisure time this march.

W

10:56 pm  
Blogger Kiwi Nomad 2008 said...

oh dear oh dear, I will have to come back and read this properly later... I am laughing too hard over that ARSE acronym to concentrate!!!

11:41 pm  
Blogger Kiwi Nomad 2008 said...

OK I am sensible again now;-) I found the long tunnel with sunflower seeds very interesting where you could see the rings from a distance. I was also impressed by the dedication shown by eating breakfast outdoors in the dark at a campsite!

2:06 am  
Blogger Maalie said...

Simon, it varies; normally not less than a couple, often all day (8 hours). You can't catch birds in the rain.

Ellee, I would prefer to use the word secure than "tight"! With the appropriate experience, ringing a bird becomes second-nature.

Magdalene, over some decades of ringing there has been no evidence found for inter- or intra-species reaction to the presence of rings. If there was, the whole investigation would of course not be valid.

TCA, well maybe for very special occasions!

Kiwi, yes, choosing the correct feeder where you can see the birds' legs clearly is almost as much of an art as catching them in the first place!

8:02 am  
Blogger lorenzothellama said...

If you catch a bird without a ring, can you tell whether it is a baby from the previous year, or do all adults look alike?

Do you go into their nests to ring the babies?

5:17 pm  
Blogger Maalie said...

Lorenzo we don't call them 'babies', they are chicks or, more technically, pulli (singular, pullus).

Yes, we can tell a bird hatched last summer by the shape of the tail feathers (more pointy than an adult). But after the first post-nuptial moult (after the breeding season, about July) we lose the ability to age them and they are called "adult". However, for RAS study, any bird accounted for after the 1st March is treated as an 'adult' until the new fledglings appear.

We do ring nestlings if we can find them but they don't seem to like using conventional nest boxes. But we do ring a few broods as they sometimes nest in boxes put up for dormice.

5:40 pm  
Blogger simon said...

"cannot catch birds in the rain" why?

9:42 pm  
Blogger Maalie said...

Simon, there is a risk that water will get under the feathers and in contact with the skin and they could chill.

9:46 pm  
Blogger simon said...

I see! makes sense. 100%.

4:34 am  
Blogger simon said...

also- on age- we have a magpie that is about 8 years old here...( well its been here as long as we have) could be older

10:47 pm  
Blogger Martin Stickland said...

I am sending a big bird to stay with you called the landlords daughter ....good luck my friend and make sure you wear bicycle clips in bed!

10:54 pm  
Blogger Shrinky said...

Fascinating post Maalie, loved the photographs - but oh, it does look cold out there. (Brrrr)

10:39 am  
Blogger CrazyCath said...

Wow maalie this is the first time I've bothered to come over here and it is fascinating! I hadn't realised tits lived that long! And I am very envious of where you live. ;0)

What a wonderful job you do. It is a great write up and superb photos. Much as I love my cats, I love the birds in the garden too and feed them as best I can. They are smarter than the average cat and being able to stay above ground keeps them safe! :0)

Thank you for popping by my blog and commenting - there's a reply there for you too.

3:03 pm  
Blogger Merisi said...

Maalie, thank you for sharing this fascinating work with us. I did not know that such tiny birds would live that long.

Off topic:
Have you ever seen a blooming mistletoe?

5:06 pm  
Blogger Maalie said...

Crazycath, it's the feral cats that are not fed properly by their owners which I think are the biggest problem to wildlife.

Merisi, only a very small proportion will live that long. There is a huge mortality of juveniles during their first winter. If they can survive their first winter and set up a territory, the outlook is much better!

And that isn't off-topic as far as I am concerned, it is to do with Natural History! No, I don't think I ever have seen mistletoe in bloom!

7:14 pm  
Blogger Ellee Seymour said...

It's great to know a bird can live as long as 10. I would like to know which birds regularly return to my garden. I often spot a robin and wonder if it is the same one.

I hope the weather has been kind to you this week during your work.

7:40 pm  
Blogger NaNcY said...

a good read, jim.

1:40 am  
Blogger Plumpy said...

So, say, hypothetically, that someone had a collection of rings which they didn't make proper use of. Maybe because they look a bit garish, or smell strange, or aren't very tasty-AHEM! I mean, aren't very useful. Would the local RAS expert want them returned or should I just-AHEM! I mean should the collector just throw them out with their Mum's rubbish?

8:53 am  
Blogger NaNcY said...

http://www.photoblog.com/kangshung/2008/03/31/tidbinbilla-nature-reserve.html#comments


check it out, Dr.

5:06 pm  
Blogger Maalie said...

Superb pictures there, Nancy, thank you :-)

6:16 pm  
Blogger Ted M. Gossard said...

I really love the work you do, Maalie. It's so important, and I take it as stewardship God has mandated to humans. Many Christians wrongly assume it's unimportant, a gross misreading of Scripture. But a growing number see differently.

At any rate, your work is wonderful, and the pics. What lovely creatures. Keep at it, as long as you enjoy it, and evidently you do- and that's good to see.

4:47 am  

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