24 July 2020

What makes for a long life?

Rob Robinson writes:

I am sometimes asked the question: “How long do birds live?” The answer is, as to many good
questions, “It depends”.

The oldest known wild bird is a Laysan Albatross called Wisdom who breeds on the very remote Midway Atoll in the Pacific Ocean and is around 70 years old. The oldest known bird in Britain is a Manx Shearwater from the island of Bardsey in north Wales that was over 50 years old when last seen back in 2008 (more on that in a bit). The reason we know this, of course, is because both of  these birds have been ringed, so we can follow them as individuals. In fact our shearwater outlasted both the original ring and its replacement, so was on its third ring when last caught!

Manx Shearwater. Photo by Lee Barber

Not all birds live so long, clearly; the longest-lived Blue Tit, by comparison, was last seen a few months short of his 10th hatchday. The graph below shows the longevity for each species ringed in Britain and Ireland. But measuring longevity in this way is tricky; to get an accurate figure we  generally) have to catch the bird to fit a ring in the year it was hatched, and then the individual has to be found shortly after it, sadly, dies (these are marked by squares in the figure). This happens rather rarely – Carrion Crow (at 21 years) is one example, but you can see there are relatively few other squares on the graph. More commonly, we catch the bird as an adult so don’t know when it was hatched (the downward-pointing triangles), or the individual is caught/seen before it dies (the upward-pointing triangles), or, sometimes, both (the diamonds on the graph). So the Manx Shearwater was first caught in 1957 as an adult (so at least five years old already, possibly more) and then seen every few years until its last capture in 2008, but we don’t know when it died; we only ‘knew’ it for part of its life.

The oldest ever Manx Shearwater, then, is (was or will be) certainly in their late 50s and possibly into their 60s. This highlights another issue with longevities, these individuals are, by their very nature, exceptional, and so few and far between (think how many people live beyond 100, for example). We also have to wait, sometimes for many years, before we can get a fair estimate. Goosander on the graph is a good example: we know the age of HW37685 fairly well, he was ringed as a nestling, and we know when he died (aged 9 ½), but only 10 or 20 goosanders are ringed each year, so the chances of someone ringing (and then finding) the oldest, or even close to the oldest, bird are pretty slim.

Fortunately, we have a better way of measuring longevity, through a measure known as annual survival, which is simply the proportion of individuals that make it from one year to the next; it is the flip-side of mortality. As the graph shows, and one should expect, there is a strong relationship between annual survival and overall longevity.

Species that tend to live longer have higher annual survival rates. Points are coloured by body size and the shape indicates whether the longevity of the individual is fully known. Squares represent a bird ringed as a nestling/juvenile and subsequently found dead. Pointed shapes indicate uncertainty in age, either at the beginning (downward-pointing) or end (upward-pointing) of the bird’s life.

While a few Blue Tits can live as long as 10 years, most do not. In fact, our ringing data show that the annual survival rate of an adult is 0.53. This means that for every 100 adult Blue Tits starting the year, 53 will make it to the end or, conversely, that 47 of them will die at some point during the year. It is worth just stopping for a second and reflecting on what this means out there in our gardens and woods. At last count, there were approximately 3.4 million breeding pairs of Blue Tits in Britain, so 7 million individuals, near enough. A survival rate of 0.53 implies that, in the normal course of things, around 9,000 Blue Tits die, on average, each day (7 million * 0.47 / 365); that’s about three in each and every 10 km square every day. Adding in young birds will, of course, make this figure much higher in the summer. The balance of productivity and survival is important then, since each of these birds needs to be ‘replaced’ if the population is to remain stable in size.

An alternative way of expressing survival is life-expectancy. In the Scottish play, Shakespeare reckoned on living “threescore and ten”, i.e. 70 years (a figure that, in fact, dates back to biblical times, although recently is more like 80 years in developed western countries but nearer 50 years in countries like Afghanistan and Somalia). Broadly, life expectancy in birds varies by size - heavier birds, marked by darker colours, fall more on the right side of the graph, lighter birds, in both colour and mass, to the left. Most small birds, like the Goldcrest, that fledge will not survive to breed. That is, their life-expectancy is less than one year; being a young bird is hazardous indeed! At the other end of the scale, Mute Swan cygnets have a reasonable expectation of making it into their early teens.

The graph shows how most birds cluster about an average line; as survival increases so does the recorded longevity. (The line curves sharply upwards on the right simply because, numerically, survival cannot be greater than 1.) Species that fall well below the line (like Lesser Spotted Woodpecker and Capercaillie) are ringed in such small numbers that we simply don’t have a good estimate of their total longevity yet; in future years these points should rise to become closer to the line. All the conservation effort that has been put into looking after Red Kites, probably means the longevity of that species is a bit higher than it might have otherwise have been. For Wigeon, on the other hand, the annual survival is much lower than we might expect from their longevity because they are hunted over much of their range, the point is well to the left of where it might otherwise be. This leads us on to the main reason why knowing about survival rates is interesting, and why ringing individuals is such a powerful tool.

By measuring how survival varies in different places or times we can understand how and why populations change. By and large, adult survival usually does not vary very much – we find it is pretty consistent between places and over time. When it does change though, the effects can be dramatic. The decline of the Lapwing (see figure) was caused, in large part, by a run of cold winters that made it hard for birds to find food. The population hasn’t recovered, though, not because survival rates have stayed low (they bounced back), but rather because the number of chicks that can be produced in Britain’s intensively managed landscapes was too low to replace those that succumbed. Similarly, the population of Oystercatchers on the Wash (and elsewhere) is sensitive to reduced survival caused by over-harvesting of cockles and mussels, a fact which is now taken into account in managing the shellfisheries there. More often though, it is the balance of survival and productivity that is key, as we see in the decline of Willow Warblers in England and understanding this balance is key to designing effective conservation measures.

Survival of lapwing (purple dots, with 95% confidence limits) was reduced during a period with cold winters (blue arrows) causing a decline in the breeding population (green line, shading indicates 95% limits) in the 1980s. Redrawn from Robinson et al. (2014).

So. What does make for a long life? Certainly if you are a bird being larger helps, and the ringing report shows that all the longest-lived birds are seabirds. But what I hope I have shown is that while the longest-lived individuals are interesting, ringing tells us more about how well the average individual does, and this is why the efforts of ringers are still important. Although tracking studies are producing fascinating new insights virtually every week, the number of birds tagged (or likely to be tagged in the near future) will generally not be sufficient to help us measure survival of birds across the population. Ringing remains the only way to do this, and will continue to be a vital tool in understanding and managing our bird populations for years to come.

If you find a dead bird with a ring on, even one that has hit a window or been brought in by your cat, then you too can contribute to our monitoring of survival. Please tell us by filling out the details at www.ring.ac. There is more information on where birds go, what happens to them and how many birds are ringed in each county on our online ringing and nest recording reports.

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