Monday, 22 August 2011

How long do Swifts live?

Contributed by Dick

This question is often asked, and one answer is that the oldest known Swift was 21 years old. We know this, because the bird was ringed as a chick in Switzerland and recovered still alive. Just how many Swifts get to 21 years old? For that we need an estimate of survival rate, which Perrins 1971 gives as 83%. In the chart below, you can see what percentage of Swifts there are in the population at each age. For example, ~8% are 5 years old. Swifts that get to 21 and beyond make up 2.4% of the population.
Swift population structure

How long could a Swift live? 
In many ways, a Swift's lifestyle is rather like certain pelagic seabirds, e.g petrels and shearwaters: they breed in a burrow, they lay typically 2 white eggs, they are normally single brooded, they take weeks, rather than days, to incubate their eggs and fledge their young, they only come back to land when they breed, they often fledge at night, they are perpetually in a moving environment and they undergo a long migration. Manx Shearwater is known to live for at least 50 years, so maybe Swifts are equally long-lived. If one ignores old age effects, such as arthritis, and with a UK population of the order of magnitude, say, 150,000, then one might speculate that there could still be a few Swifts around over 60 years old.

What is the average age of Swifts when they die?
The 83% survival rate is the average survival of birds after their first year. We have no direct measurement of 1st year survival.  However, of those birds that survive to their 1st summer (in their second calendar year), they will die somewhere between their 7th and 8th summers.

Of course, there is higher mortality in the first year, because the birds are inexperienced; the average age would be reduced if birds that died in their first year were included.

What is the average age of the Swift population?
Because we are assuming that annual survival is the same, regardless of the age of the Swift, the average age of Swifts in a flock, before any chicks have fledged, is also 7 years. (in contrast, the average age of the human population is a lot less than the average age when we die, because of old age effects)

What percentage of Swifts survive their first year?
To answer further questions, we need rather more than adult annual survival, such as average number of birds fledged each breeding season, age of first breeding and of course 1st year survival. If we know any two of these parameters, we can work out the third. A bird such as a Robin breeds in its first summer, it raises more than one brood of say 5 fledglings implying a  high first year mortality rate to maintain a stable population.

In the case of Swifts, it is thought that they breed in their 3rd or 4th summer (4th or 5th calendar year). They produce, on average, between 1 and 2 chicks. As each pair produces, on average, fewer young, annual survival should be a lot higher than for Robin. It works out that if the average age of 1st breeding is 3rd summer, with 1.5 chicks produced, then 1st year survival is 33%. If 1st breeding is 4th summer, then 1st year survival is 40% - roughly half adult annual survival.

For how many years does the average Swift breed?
If Swifts start breeding at, say, 3 or 4 years and they live on average until they are 7, then, maybe, they only breed 4 or 5 times. If Swifts are delayed in starting their breeding due to difficulties of finding a nest site, then the productive years of each Swift are reduced.

How many non breeders are there?
If Swifts don't start breeding until they are 3, then this implies that 31% are non-breeders. If they start at 4, then 43% are non-breeders.

[Of course, some of the assumptions made could be incorrect. For example the survival rates of breeders and non-breeders are probably different, as they have different life-styles and take different risks. A more complicated model could give more accurate answers. Perrins quotes another figure, 79%, for annual survival based upon ringing recoveries. With this figure only ~1% of birds would reach 21,  the theoretical maximum age would be between 40 and 50, and other figures given would be lower. Hopefully the information above will give readers a good idea of the lifespan of a Swift.]

Reference:
Perrins, C. 1971. Age of first breeding and adult survival rate in the
Swift
. Bird Study 18: 61-70.

UPDATE 8 Nov 2012
According to the BTO longevity page, the oldest British Swift was at least 19 years old when it was retrapped alive. The oldest known Swift anywhere reached 30 years see 30 year old Swift.

4 comments:

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  2. Just rescued a young swift (young but strong flyer) from a small courtyard in Portugal. I suspect that, through inexperience, that it didn't know the layout and flew into our windows above the yard, then didn't have enough room to get airborne.

    I was surprised it was not dead when, this morning we were able to get access through the neighbours to the yard. It let itself be captured with no struggle at all and flew off vigourously and pretty relieved, it seemed.

    I've always loved swifts but never seen one so close up. Even more beautiful than I thought. On its release, maybe I'm being too anthrpomorphic, but a great amount of shrieking, chattering broke out.

    Anyway, my girlfriend asked me (I'm a quite good ornithologist) how long swifts live and I found your site. Very interesting figures about life span. What I wanted to ask though is, given the great sppeds at which they fly and their amazing agility, is there any significant predation on swifts. For example, I can't even imagine a peregrine bothering to try to catch one. So do you know the main cause of mortality? And why there is such a big difference between the average age of death and the maximum. Thanks. Paul.

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  3. Hello Paul, I agree that Swifts are beautiful birds, especially a fully fledged chick. Yes, Swifts have predators including Hobbies, Peregrines Sparrowhawks and even Little Owls and many more raptors in Africa no doubt. This Saturday I saw a Sparrowhawk ambush a Swift, which it brought to ground but I succeeded in rescuing the Swift.

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  4. I just this week found a grounded swift and rescued him. he had a twisted wing tip so I think he hit the telegraph cables and crashed. I had him in a box for a few hours and on his release I can confirm the other swifts overhead celebrated his return, it was very touching, they sounded so excited to see him back.

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