Few life history traits attract more popular attention than longevity records, particularly those of the longest lived or extreme species. Wisdom, a Laysan Albatross on Midway Atoll, is the world’s current oldest known wild bird at 70 (link), but these metrics do have their uses beyond their ability to come up in your next pub quiz, as they are sometimes used to inform population models. Extreme longevity records, such as Wisdom’s, easily catch the eye but we may not be able to immediately contextualise how extreme these records actually are in isolation. Longevity varies widely between species groupings, and as a result far shorter absolute longevities in a small passerine may be equally as impressive as the Manx Shearwater longevity of 50 years 11 months and 21 days.
If you want to skip the math background, you can skip the next section
The maths of longevity
So, how can we spot longevity records that are truly surprising? Fundamentally, how long you live (at least at the population level) is a game of chance. An individual is born, and we roll a dice, roll a 1 and you survive to your first birthday. Then you roll the dice again, roll a 1 again and you live to your second birthday, and so on. Each year, your chance your streak of 1’s continuing goes down, until eventually you don’t roll a 1 and your days are over. The kicker is some species are rolling dice with more sides than others and the number of sides your dice has changes as you age (hopefully your dice doesn’t have many sides!).
This is the basis for survival rates: an adult survival rate of 0.5 says there is a 50% chance you live to your next birthday each year as an adult. We usually represent these as two numbers for birds, a juvenile survival rate, and an adult survival rate after maturity. We have the adult survival rate as a fixed number for simplicity, but we know this rate is likely to fall in old age due to senescence. These numbers can help inform us about fundamental characteristics of populations which can help with conservation actions, but we are going to use them for a different purpose. If we take species with a juvenile survival rate of 0.3 and an adult survival rate of 0.5 which starts in its second year, the probability of reaching a longevity of 1 year is 0.3 (or 1 in 3.33). The probability of reaching age 2 is the probability of reaching age 1 times the adult survival rate, so 0.3 x 0.5 = 0.15 (1 in 6.67). For age 3 it’s the probability of reaching age 2 times the adult survival rate: 0.15 x 0.5 = 0.075 (or 1 in 13.33). So, 1 in 13.33 individuals from that species would reach age 3.
Let’s expand this to all the species in which we have these survival rates for. The BTO have both the juvenile and adult survivals on the BTO Bird Facts (link) pages for 111 species (sadly, many of our favourites are missing. More RAS or targeted ringing projects will help fill these gaps, and we particularly need juvenile survival rates!). We have adult survival estimates for many more, but really, we need juvenile rates too for our aim. You can use this formula I wrote to quickly calculate the probabilities (the 1 in x chance way of saying it is just 1 divided by the probability):
Probability of longevity = (J*A)*A^(L-K-1)
J is juvenile survival to year K, A is adult survival, and L is the current longevity record (in years).
BTO longevity probabilities
This spits out a series of probabilities of reaching the current longevity record. The big caveat here is they are very sensitive to changes in the survival estimates, and those estimates being correct! These estimates are often based on single populations, which maybe don’t translate to all other populations. They may also be outdated, and survival rates in declining species or populations will tend to be lower. In some cases, they may be based on populations that aren’t even British (more on that later). Reassuringly, we get quite a steady baseline of probabilities with a handful of outliers. In the first plot, the labelled outliers are unlikely longevities, on the second plot the labelled outliers are the longevities that are more likely to be achievable.
How unlikely is too unlikely?
Extreme rare chance events can be difficult, if not impossible, to conclude that they could not have happened. It’s tempting to brush off these extreme longevity chances as incorrect based on either: 1) Inaccurate survival estimates or 2) Mistakes (e.g. ring misreads). Is that fair though? A third option is it was just genuinely a freak unlikely event. To put the longevity chances into context, odds of winning the UK lotto are 1 in 45,057,474 (a probability of ~2.22 x 10-8) which is far less likely than even the biggest outlier in the BTO longevity records (1 in 980,392 for Carrion Crow). Yet, we know that Lotto winners happen relatively frequently (20 jackpot wins in 2020) so raw odds can only tell us a limited amount. There were 1,889,331,342 lines of Lotto played in 2020, so the odds would tell you you’d actually have expected 42 jackpot wins that year (an unlucky year perhaps, or the numbers people choose aren’t actually completely random!).
BTO longevity probabilities - corrected for number ringed
Evidently, we need to account for the number of times we have ‘tried’ to set a longevity (i.e., the number of birds ringed). This isn’t perfect because some species will be reencountered at different rates than others (don’t forget reencounters aren’t just re-trapping birds though). So, we multiply the total number of individuals ringed within the BTO scheme between 1909–2019 by the longevity probability and take another look at our outliers.
Labelled outliers (left to right): Carrion Crow, Shoveler
Labelled outliers (left to right): Black-headed Gull, Blackcap, Blue Tit, Common Tern, Great Tit, Guillemot, Herring Gull, Siskin, Spotted Flycatcher
What can this tell us? Well, nothing concrete. Although it does flag species whereby we may want to question the survival estimates (have they changed, are they particularly variable between populations, etc.) or if you’re of a disbelieving nature, maybe you’ll be tempted to assume a mistake was made. Those species that are no longer outliers (compared to the previous plots) we can in part put down to number ringed, so species such as Capercaillie and Black Grouse had low longevities because so fe are ringed, so they might be ripe for a new longevity record in the near future for people ringing them (although finding a Capercaillie in my Norfolk mist nets would be slightly surprising!). Conversely, Sedge warbler no longer being an outlier might just be down to us ringing loads of them! We also see a few species creep in as outliers like Blue Tits and Great Tits (where huge numbers have been ringed), where the longevity record is quite likely to have been achieved with the extremely high numbers ringed.
Europe-wide longevity probabilties
OK, what about some European-wide longevities? With these we cannot control for number ringed, as that information isn’t publicly available for Europe as a whole. It also comes with an even stronger caveat of BTO survival estimates potentially not being applicable (survival rates are likely to differ between the UK and mainland Europe). So, let’s just look at those raw probabilities:
Labelled outliers (left to right): Common Sandpiper, Guillemot, White-tailed Eagle
The general baseline of probabilities Europe-wide is lower than for the UK alone. We would expect that because the number of ‘attempts’ (i.e. number ringed) at setting longevity will be considerably higher, so those rare longer longevities are more likely to have happened. Kingfisher is the run-away outlier, with an incredible 1 in 530,152,400,000 (that’s ~530 billion) chance of reaching that longevity (a bonkers 21 years compared to our 4.5-year record) based on the survival estimates (that are from a British population). When we exclude that individual, we still get a set of other outliers:
Labelled outliers (left to right): Dunnock, Fieldfare, Robin, Sedge Warbler, Tufted Duck
For Kingfisher, the second highest longevity is actually19 years with a 1 in 41,563,970,000 (~41.5 billion) chance. This further indicates that survival rates of Kingfisher must be higher on the continent (the two highest records came from Belgium and Sweden respectively). If our survival estimates were correct for the mainland, the chance of both of these two longevities happening (assuming they are independent events) would be 1 in 22,035,238,000,000,000,000,000 (that’s 1 in 22 Hextillion, you’re more than 1 million times more likely to win the Lotto jackpot… twice).
Finally, let’s take a look at the amazing longevity record for Common Whitethroat recently set in Italy of 19 years (the rest of our longevity records we have looked at are a couple of years out of date). If we plug that longevity in, we get a 1 in 75,843,876 chance. For reference, the British record is just 8 years (1 in 2,477 chance). What can we say about this record? If our survival estimates were correct for that Italian population, it was pretty unlikely but it’s no Kingfisher and there are a handful of other species in Europe with equally unlikely longevities based on our survival estimates. Maybe mainland Common Whitethroat survival is just higher than ours? Well, the Italian adult survival rate (the value that has the highest impact on long-term longevity probabilities) for Common Whitethroat would need to be 0.695 for that 19-year record to be equally likely as our longevity record in the UK. That would be a very high adult survival rate for a warbler, but our longevity record is reasonably easy to achieve based on our survival estimates. What am I trying to say here? I don’t think it’s my place to throw doubt on the record, but I do think it’s right to point out the chance of reaching that longevity was pretty slim. Questioning results is a cornerstone of science, but we shouldn't just write records off out of hand.
If nothing else, hopefully this all highlights the importance of accurate survival estimates when scientists require them and why collecting data so that we can measure these things (through RAS schemes) can be so valuable.
Table of data:
| Species | BTO Longevity | Juvenile survival | K | Adult survival | Longevity Probability | European longevity (nearest year) | EU Longevity Probability |
|---|---|---|---|---|---|---|---|
| Arctic Skua | 25 years 10 months 24 days | 0.68 | 1 | 0.886 | 0.032987694 | 31 | 0.018010284 |
| Avocet | 24 years 8 months 21 days | 0.41 | 1 | 0.78 | 0.001054519 | 28 | 0.000500424 |
| Barn Owl | 15 years 3 months 21 days | 0.37 | 1 | 0.72 | 0.003722688 | 18 | 0.001389486 |
| Bewick's Swan | 28 years 1 month 27 days | 0.66 | 2 | 0.822 | 0.004038417 | 28 | 0.004038417 |
| Black Grouse | 3 years 9 months 18 days | 0.46 | 1 | 0.72 | 0.17169408 | 12 | 0.012399817 |
| Black-headed Gull | 32 years 3 months 26 days | 0.447 | 2 | 0.9 | 0.018948848 | 33 | 0.017053963 |
| Blackbird | 15 years 2 months 5 days | 0.564 | 1 | 0.65 | 0.001355396 | 22 | 6.64x10^-05 |
| Blackcap | 10 years 8 months 15 days | 0.356 | 1 | 0.436 | 0.001355396 | 14 | 7.32x10^-06 |
| Blue Tit | 9 years 8 months 16 days | 0.379 | 1 | 0.532 | 0.001293731 | 12 | 0.000366157 |
| Bullfinch | 9 years 2 months 9 days | 0.334 | 1 | 0.419 | 0.000317292 | 13 | 9.78x10^-06 |
| Buzzard | 30 years 5 months 21 days | 0.63 | 3 | 0.9 | 0.036634334 | 30 | 0.036634334 |
| Canada Goose | 31 years 10 months 29 days | 0.759 | 1 | 0.724 | 3.41x10^-05 | 32 | 3.41x10^-05 |
| Capercaillie | 3 years 6 months 8 days | 0.5 | 1 | 0.72 | 0.186624 | 9 | 0.036110207 |
| Carrion Crow | 20 years 10 months 7 days | 0.49 | 1 | 0.52 | 1.02x10^-06 | 21 | 1.02x10^-06 |
| Chaffinch | 13 years 11 months 26 days | 0.531 | 1 | 0.589 | 0.000545247 | 16 | 0.000189158 |
| Chiffchaff | 7 years 7 months 24 days | 0.222 | 1 | 0.306 | 5.58x10^-05 | 8 | 5.58x10^-05 |
| Chough | 23 years 11 days | 0.43 | 1 | 0.8 | 0.00317284 | 23 | 0.00317284 |
| Common Gull | 27 years 10 months 22 days | 0.25 | 3 | 0.86 | 0.005759719 | 34 | 0.002330194 |
| Common Sandpiper | 15 years 1 month 5 days | 0.57 | 1 | 0.844 | 0.053048011 | 15 | 0.053048011 |
| Common Tern | 33 years 6 days | 0.47 | 2 | 0.9 | 0.01793146 | 33 | 0.002709527 |
| Coot | 15 years 3 months 13 days | 0.38 | 1 | 0.701 | 0.002629274 | 21 | 0.000311992 |
| Cormorant | 21 years 6 months 21 days | 0.58 | 1 | 0.88 | 0.03958805 | 32 | 0.011025311 |
| Curlew | 32 years 7 months | 0.47 | 1 | 0.899 | 0.015574281 | 33 | 0.015574281 |
| Dunlin | 19 years 3 months 26 days | 0.74 | 1 | 0.74 | 0.003276443 | 29 | 0.000161332 |
| Dunnock | 10 years 7 months 23 days | 0.347 | 1 | 0.473 | 0.00019451 | 21 | 1.09x10^-07 |
| Eider | 35 years 6 months 26 days | 0.33 | 1 | 0.916 | 0.015305782 | 37 | 0.014020096 |
| Fieldfare | 14 years 8 months 24 days | 0.31 | 1 | 0.41 | 1.18x10^-06 | 18 | 8.10x10^-08 |
| Gannet | 37 years 4 months 16 days | 0.3 | 4 | 0.919 | 0.018472817 | 37 | 0.018472817 |
| Garden Warbler | 10 years 1 month 6 days | 0.265 | 1 | 0.5 | 0.000517578 | 14 | 3.23x10^-05 |
| Golden Eagle | 16 years 1 month 9 days | 0.15 | 4 | 0.95 | 0.081054013 | 32 | 0.035674033 |
| Goldfinch | 10 years 2 days | 0.337 | 1 | 0.371 | 4.49x10^-05 | 14 | 8.50x10^-07 |
| Goshawk | 18 years 8 months 27 days | 0.4 | 2 | 0.83 | 0.016841763 | 22 | 0.009629901 |
| Great Skua | 38 years | 0.8 | 1 | 0.888 | 0.009871195 | 38 | 0.009871195 |
| Great Tit | 10 years 5 months 18 days | 0.38 | 1 | 0.542 | 0.00153383 | 15 | 7.17x10^-05 |
| Greenfinch | 11 years 3 months 24 days | 0.416 | 1 | 0.443 | 0.000121096 | 14 | 1.05x10^-05 |
| Grey Heron | 23 years 9 months 2 days | 0.26 | 2 | 0.732 | 0.016841763 | 38 | 3.45x10^-06 |
| Grey Partridge | 5 years 2 months 13 days | 0.22 | 1 | 0.55 | 0.020131375 | 5 | 0.020131375 |
| Grey Plover | 25 years 1 month 18 days | 0.63 | 1 | 0.86 | 0.016877316 | 26 | 0.014514492 |
| Guillemot | 40 years 11 months 23 days | 0.56 | 1 | 0.946 | 0.060790188 | 43 | 0.054402111 |
| Hen Harrier | 15 years 9 months 2 days | 0.22 | 2 | 0.81 | 0.011513648 | 17 | 0.009326055 |
| Herring Gull | 32 years 9 months 25 days | 0.63 | 4 | 0.88 | 0.015464577 | 35 | 0.011975768 |
| Hobby | 14 years 10 months 19 days | 0.47 | 1 | 0.745 | 0.007625805 | 15 | 0.007625805 |
| Honey-buzzard | 12 years 11 months 21 days | 0.419 | 2 | 0.86 | 0.079743811 | 29 | 0.007139572 |
| House Martin | 7 years 1 month 12 days | 0.145 | 1 | 0.41 | 0.000688765 | 15 | 5.50x10^-07 |
| House Sparrow | 12 years 12 days | 0.489 | 1 | 0.571 | 0.001028741 | 20 | 1.16x10^-05 |
| Jackdaw | 18 years 26 days | 0.397 | 2 | 0.694 | 0.001149588 | 20 | 0.000553683 |
| Jay | 16 years 9 months 19 days | 0.6 | 1 | 0.59 | 0.000129355 | 17 | 0.000129355 |
| Kestrel | 15 years 11 months 1 day | 0.32 | 1 | 0.69 | 0.001224296 | 20 | 0.000277513 |
| Kingfisher | 4 years 6 months 29 days | 0.215 | 1 | 0.28 | 0.00132151 | 21 | 1.89x10^-12 |
| Kittiwake | 28 years 6 months 5 days | 0.79 | 1 | 0.882 | 0.02348267 | 29 | 0.02348267 |
| Lapwing | 21 years 1 month 15 days | 0.595 | 1 | 0.705 | 0.000547393 | 25 | 0.000135225 |
| Lesser Whitethroat | 9 years 2 days | 0.355 | 1 | 0.329 | 4.87x10^-05 | 9 | 4.87x10^-05 |
| Linnet | 8 years 3 months 25 days | 0.34 | 1 | 0.371 | 0.000328925 | 9 | 0.000122031 |
| Little Egret | 13 years 6 months 12 days | 0.32 | 1 | 0.712 | 0.00386714 | 22 | 0.000255405 |
| Little Owl | 13 years 10 months 3 days | 0.3 | 1 | 0.65 | 0.001109162 | 14 | 0.001109162 |
| Little Tern | 25 years 8 days | 0.578 | 1 | 0.899 | 0.04489112 | 25 | 0.04489112 |
| Long-eared Owl | 12 years 10 months 12 days | 0.48 | 1 | 0.69 | 0.005590238 | 18 | 0.000874331 |
| Long-tailed Tit | 8 years 11 months | 0.251 | 1 | 0.443 | 0.000372309 | 11 | 7.31x10^-05 |
| Magpie | 21 years 8 months 23 days | 0.4 | 1 | 0.69 | 0.000165155 | 22 | 0.000165155 |
| Mallard | 20 years 5 months 17 days | 0.518 | 1 | 0.627 | 7.28x10^-05 | 23 | 1.80x10^-05 |
| Manx Shearwater | 50 years 11 months 21 days | 0.25 | 5 | 0.905 | 0.002533814 | 51 | 0.002533814 |
| Marsh Harrier | 13 years 3 months 8 days | 0.151 | 3 | 0.74 | 0.007435226 | 20 | 0.000903475 |
| Marsh Tit | 11 years 3 months | 0.19 | 1 | 0.47 | 9.99x10^-05 | 13 | 2.21x10^-05 |
| Merlin | 12 years 8 months 17 days | 0.23 | 1 | 0.62 | 0.000742041 | 13 | 0.000742041 |
| Mistle Thrush | 11 years 4 months 9 days | 0.57 | 1 | 0.621 | 0.00486173 | 21 | 4.15x10^-05 |
| Mute Swan | 29 years 1 month 11 days | 0.42 | 3 | 0.85 | 0.006139618 | 29 | 0.006139618 |
| Osprey | 20 years 11 months | 0.6 | 1 | 0.85 | 0.023255719 | 27 | 0.008770883 |
| Oystercatcher | 41 years 1 month 5 days | 0.39 | 5 | 0.88 | 0.003912378 | 43 | 0.003029745 |
| Peregrine | 21 years 10 months 24 days | 0.6 | 1 | 0.81 | 0.007183509 | 22 | 0.007183509 |
| Pied Flycatcher | 9 years 7 days | 0.37 | 1 | 0.47 | 0.000881018 | 11 | 0.000194617 |
| Pied Wagtail | 11 years 3 months 21 days | 0.386 | 1 | 0.485 | 0.000277974 | 14 | 3.17x10^-05 |
| Pink-footed Goose | 38 years 7 months 7 days | 0.775 | 3 | 0.829 | 0.000906285 | 41 | 0.000622836 |
| Pochard | 22 years 10 days | 0.55 | 1 | 0.65 | 6.48E-05 | 23 | 4.21x10^-05 |
| Razorbill | 41 years 11 months 23 days | 0.38 | 4 | 0.9 | 0.006934241 | 42 | 0.006934241 |
| Red Kite | 25 years 8 months 28 days | 0.5 | 1 | 0.61 | 2.15x10^-06 | 26 | 2.15x10^-06 |
| Red-throated Diver | 35 years 11 months 23 days | 0.37 | 2 | 0.84 | 0.000985593 | 36 | 0.000985593 |
| Redpoll | 8 years 5 months 22 days | 0.431 | 1 | 0.425 | 0.001079443 | 12 | 3.52x10^-05 |
| Redshank | 20 years 1 month 15 days | 0.43 | 1 | 0.74 | 0.001408871 | 27 | 0.000171196 |
| Reed Bunting | 9 years 11 months 18 days | 0.474 | 1 | 0.542 | 0.001913251 | 12 | 0.000562044 |
| Ring Ouzel | 9 years 13 days | 0.36 | 1 | 0.42 | 0.000348575 | 9 | 0.000348575 |
| Robin | 8 years 4 months 30 days | 0.41 | 1 | 0.419 | 0.00092957 | 19 | 6.50x10^-08 |
| Rock Dove | 7 years 8 months 25 days | 0.57 | 1 | 0.665 | 0.032781315 | 8 | 0.032781315 |
| Rook | 22 years 11 months | 0.25 | 1 | 0.79 | 0.001398737 | 24 | 0.001105002 |
| Sand Martin | 7 years 9 months 1 day | 0.215 | 1 | 0.3 | 4.70x10^-05 | 10 | 4.23x10^-06 |
| Sandwich Tern | 30 years 9 months 14 days | 0.358 | 1 | 0.898 | 0.014196233 | 31 | 0.014196233 |
| Sedge Warbler | 8 years 8 months 8 days | 0.25 | 1 | 0.224 | 1.58x10^-06 | 12 | 1.78x10^-08 |
| Shag | 29 years 10 months 25 days | 0.38 | 2 | 0.878 | 0.009945578 | 31 | 0.008732218 |
| Shelduck | 19 years 7 months 27 days | 0.166 | 2 | 0.886 | 0.018789494 | 25 | 0.010258496 |
| Shoveler | 22 years 7 months 24 days | 0.38 | 1 | 0.58 | 2.37x10^-06 | 23 | 2.37x10^-06 |
| Siskin | 8 years 6 months 10 days | 0.449 | 1 | 0.461 | 0.001986794 | 14 | 1.91x10^-05 |
| Snipe | 16 years 19 days | 0.48 | 1 | 0.481 | 8.19x10^-06 | 16 | 8.19x10^-06 |
| Song Thrush | 11 years 8 days | 0.463 | 1 | 0.563 | 0.001481375 | 18 | 2.66x10^-05 |
| Sparrowhawk | 17 years 1 month 11 days | 0.34 | 1 | 0.69 | 0.000897562 | 20 | 0.000294857 |
| Spotted Flycatcher | 8 years 3 days | 0.465 | 1 | 0.493 | 0.106462315 | 11 | 0.000394387 |
| Starling | 17 years 7 months 25 days | 0.518 | 1 | 0.687 | 0.000876182 | 23 | 0.000134084 |
| Stock Dove | 9 years 2 months 12 days | 0.4 | 1 | 0.55 | 0.003349358 | 13 | 0.000306487 |
| Stone-curlew | 22 years 4 months 1 day | 0.606 | 1 | 0.832 | 0.012736863 | 22 | 0.012736863 |
| Swallow | 11 years 1 month 11 days | 0.395 | 1 | 0.374 | 2.12x10^-05 | 11 | 2.12x10^-05 |
| Tawny Owl | 23 years 5 months 27 days | 0.301 | 1 | 0.738 | 0.000376536 | 23 | 0.000376536 |
| Tufted Duck | 24 years 3 months 13 days | 0.63 | 1 | 0.71 | 0.000238937 | 45 | 1.80x10^-07 |
| Turtle Dove | 11 years 2 months 15 days | 0.36 | 1 | 0.5 | 0.000351563 | 13 | 8.79x10^-05 |
| Whinchat | 6 years 1 month 13 days | 0.34 | 1 | 0.47 | 0.00779773 | 7 | 0.003664933 |
| White-fronted Goose | 18 years 9 months 22 days | 0.596 | 1 | 0.724 | 0.001780494 | 25 | 0.000256432 |
| White-tailed Eagle | 16 years 9 months 10 days | 0.395 | 3 | 0.936 | 0.156480051 | 33 | 0.054309094 |
| Whitethroat | 7 years 9 months 5 days | 0.289 | 1 | 0.391 | 0.000403771 | 9 | 0.000157874 |
| Willow Tit | 10 years 4 months 18 days | 0.59 | 1 | 0.63 | 0.00922395 | 11 | 0.005811089 |
| Willow Warbler | 10 years 11 months 18 days | 0.239 | 1 | 0.46 | 4.66x10^-05 | 11 | 0.000101386 |
| Woodcock | 15 years 5 months 12 days | 0.36 | 1 | 0.61 | 0.000355566 | 16 | 0.000216895 |
| Woodlark | 7 years 2 months 1 day | 0.22 | 1 | 0.6 | 0.01026432 | 7 | 0.01026432 |
| Woodpigeon | 17 years 8 months 19 days | 0.52 | 1 | 0.607 | 0.000107204 | 18 | 0.000107204 |
| Wren | 7 years 3 months 6 days | 0.263 | 1 | 0.319 | 0.00027714 | 7 | 0.00027714 |
| Yellow Wagtail | 7 years 1 month 14 days | 0.463 | 1 | 0.533 | 0.010615593 | 9 | 0.003015773 |
| Yellowhammer | 11 years 9 months 28 days | 0.529 | 1 | 0.536 | 0.000554969 | 13 | 0.000297463 |
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