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.

22 June 2020

Where will they tern up?

Rachel Taylor, Steve Dodd and Katharine Bowgen write:

Many of you will already have heard the news that the Skerries tern colonies have been almost completely abandoned this year; an event thought to have been triggered by predators colonising the islands; which in a normal year would likely have been managed by RSPB colony wardens. Sad though an event like this appears, it isn’t all that uncommon for colonial seabirds to move around; and entire colonies have been known to disappear and recover over time. What’s different about the Skerries is that recent colour-marking work provides an opportunity to understand the demographic processes behind this huge event – and birdwatchers can be a key part of the work.
The Skerries helipad. Photo by Rachel Taylor

The Skerries are a group of rocky islands in the Irish Sea, near Holyhead on Anglesey, owned by Trinity House. The significant seabird colonies are managed, protected and monitored by RSPB.

The Arctic Tern colony is the largest in the UK, with 3,816 pairs in 2016 - along with 290 pairs of Common Terns and an increasing probability of breeding attempts by Roseate Tern – in 2018 fledging two chicks for the first time in decades. In a normal year the terns arrive early in May, with hatching starting at the beginning of June and the first chicks fledging in early July.

Leg-flagged Arctic Tern on The Skerries image by Rachel Taylor

Since 2013, RSPB has supported local ringers Steve Dodd and Rachel Taylor in running a set of small demographic projects in the colonies on Skerries. A sample of 500 Arctic Tern chicks are ringed each year, and Retrapping Adults for Survival (RAS) studies run on both Arctic and Common Terns. The RAS studies use individually coded leg-flags to identify individuals. There are several good reasons why RSPB supported our work, but one major justification was an interest in understanding colony dynamics in terns.

Colonial seabirds in general have something of a dilemma in choosing a nest site. Colonies have advantages: for example in recognising and exploiting local resources like safe sites for nests, and good feeding sites. Individual nests run a lower risk of loss to predators, because there are many nests to choose from, and many eyes watching for those predators. But they also create problems – such as risk of disease transmission, increased competition for food resources, and vulnerability to local resource changes, random disturbance events and becoming a target for predators. Major colony collapses may be a result of several pressures operating at once. But terns are long-lived; several studies have identified the linked nature of adjacent colonies, with large colonies exporting chicks to smaller sites, and local populations remaining broadly stable despite rises and falls in breeding numbers at individual sites. Site abandonment tends to be more-or-less temporary, and if the pressure that caused the birds to leave is managed or removed entirely, the birds return.

Leg-flagged Common Tern 'CJ' in The Skerries. Photo by Rachel Taylor

When we set up the adult survival projects on the Skerries, we knew that most large colonies north  of Anglesey had at some time in the past, experienced significant population declines or abandonment – one of the most dramatic being the abandonment of the Shotton Steelworks Common Tern colony on the Dee in 2009. The reasons for that collapse, and the work underpinning the colony’s subsequent recovery, provide a fascinating example of the complex issues that long-lived seabirds experience. But we wanted to know some very specific things about the birds’ experience of a ‘colony collapse’ event. Of course, at the time the Skerries colony was going from strength to strength and we expected that to continue; but the national pattern was clear. We could learn from adult survival data in a healthy colony, but would also be prepared with a good  baseline in the event of something changing for the Skerries.

We had a specific set of ‘in-the-event’ questions. Firstly, since Arctic Terns are more site-faithful than Common or Sandwich terns, would a colony-breaking event cause them to simply take a year off and then return the following year, or would they immediately try somewhere else? Secondly, having attempted (and possibly been successful) elsewhere, would they return to the Skerries in subsequent years, or emigrate for life? Thirdly, terns are reluctant to change their breeding site for very good reasons; we wanted to understand the survival implications of the birds experiencing such a major upheaval as a colony collapse event. Survival studies take time, and like dispersal studies, depend on birds being individually recognisable without causing data-skewing and inappropriate disturbance – hence the common use of colour marking in RAS.

Lots of terns. Photo by Rachel Taylor

The collapse of the Skerries tern colonies this year is very sad, but certainly isn’t the end of the story. There is good evidence that such a collapse will be temporary if – as already seems likely - the cause is identified and can be managed. But more than that, the Skerries is a huge opportunity to learn about collapse events, and about how individual terns respond and recover from them. Birders are already beginning to look for birds from the Skerries, marked with coded orange (Arctic Tern) or yellow (Common Tern) leg-flags. In particular, the nearest colony on Anglesey (Caemlyn) has already seen an influx of Arctic Tern, and the wardens are on the lookout for marked adults. If you live or bird around the coasts of NW England, North and Mid Wales or the Isle of Man, or the Irish Sea coasts of the Republic of Ireland and Northern Ireland and if Government restrictions allow, please keep your eyes peeled and your bins poised – and help us find the scientific silver lining inside the dark cloud of this year’s Skerries colony abandonment.

Flag sightings to sg.dodd@yahoo.com

15 April 2020

The joy of garden nesting

BTO Scotland’s Steve Willis and BTO Northern Ireland’s Stephen Hewitt share their passion for monitoring garden nests and pass on some tips for how to get involved in this rewarding activity.

NOTE: If you are new to nest monitoring and want to record the progress of a few garden nests and nest boxes then Nesting Neighbours is the ideal interface - it’s simple to use and provides some really nice feedback on how other people’s nests are doing too. If you already use Demography Online (as a ringer or Nest Record Scheme participant) then please continue to do so, signing up to NRS if necessary. The data from both systems go into the same underlying database and contribute to the statistics. These schemes collect information on breeding success which helps us to understand the impacts of factors such as climate change and food availability on the number of young produced, and the influence this then has on population trends. Participation is therefore very valuable as well as rewarding. Scientific studies have shown that, as long as observers are careful and follow the BTO's Code of Conduct, making several visits to a nest to record the contents does not increase the probability of it failing. It is completely legal to look inside a nest in England, Scotland and Wales as long as you do not touch the contents and the species is not included on the list of scarce protected birds. If, like Stephen, you are in Northern Ireland or the Republic of Ireland, you must apply for a licence prior to monitoring a nest of any species. A separate licence is needed for photographing nests.

Steve: Since the Covid-19 lockdown, like many people I’ve been working from home. My home office has a view out over the garden, and I started noticing a female blackbird regularly visiting a particular evergreen shrub in the shade of a big holly tree. It looked like she was building a nest! I was aware of the Nesting Neighbours project and I wondered if I should give it a go, though I must admit I hadn’t done any nest monitoring before. A chat with Stephen (who is an old hand and infectiously enthusiastic) had me sold! I had a read of the guidance pages and then went out to take a look…

The Blackbird nest is in the coniferous shrub in the centre of the picture (Steve Willis).

The nest was about one metre off the ground in a dense coniferous shrub so to look down into it I needed to very carefully move a few small branches apart. Peering inside, I could see four beautiful speckled blue eggs! It felt like such a wonderful privilege to be getting such a close view. The female was off the nest at the time but I could hear the male singing somewhere nearby. I beat a hasty retreat and watched from the kitchen window. Reassuringly it wasn’t long before the female returned. I visited the nest again a few days ago - five days after my first visit - and this time the female was sitting tight, obviously incubating. I think I’m getting the hang of it!

I’m under strict orders from Stephen to follow the breeding attempt through, whatever happens. Nests fail naturally for a number of reasons, including predation, and this is a regular occurrence with early thrush nests. Information from all breeding attempts is important, not only the ones that succeed.

These Blackbird eggs are a beautiful speckled blue, but the colours can vary a bit (Steve Willis).

When I’m not doing various bird-related things, I enjoy a bit of woodwork, and although I already had one nest box up in the garden, I was inspired to make another out of some odds and ends. Here’s a short video of the build process. I might have put it up too late for anything to use it this year, but fingers crossed! If you want to have a go at building your own, the plans are here!

Stephen: Steve’s right, I am an old hand (though not that old, Steve!). I first got into nest recording about eight years ago when encouraged by the then BTO Northern Ireland Officer Shane Wolsey and Carl Barimore from BTO HQ. In a normal year I make regular visits to local patches in County Armagh. I absolutely love the process of searching for nests, tuning in to the subtleties of bird behaviour, song and call, and getting to know the types of places that different species favour. Every nest I find gives me a wee buzz and following a breeding attempt through to fledging is very satisfying! Of course it doesn’t have to be tricky at all - if you have tits nesting in your bird box then ‘nest finding’ is very straightforward!

Here’s a Song Thrush nest that I found. Note the hard, dry inner lining (like chipboard). The holly berries round the rim are a bit unusual, but apparently thrushes do occasionally decorate their nests (Stephen Hewitt).

I’d really encourage anyone who is considering garden nesting to give it a go. It is hugely enjoyable - absolutely - but remember you’ll also be collecting priceless information. By recording the number of eggs, fledglings and the outcome of nesting attempts, volunteers are helping to reveal how our wild birds are doing every year.

Many people start with a box, and monitoring these is straightforward if you have a nest box camera installed or if the lid can be removed. Just check the box every five days or so - easy! Before opening the box, give it a gentle tap to let anything inside know that you are coming, and then lift the lid just enough to see inside. Make a note of what you can see but take care never to touch eggs or chicks - if there is a bird incubating then simply replace the lid and come back later to count the contents. There are guidance notes here which give more detail.

If you enjoy nest monitoring (and I’m sure you will!) I’d really encourage you to ‘graduate’ to open nests, too. They are easier to find than you might imagine. You can sit quietly and watch the adults like Steve did, or carefully search trees, bushes and ivy. There are few things that compare with the thrill of pinpointing an open nest. Peering into a Dunnock’s nest at five recently-laid bright blue eggs takes some beating and provides vital information to help us understand how well this amber-listed species is breeding!

A Dunnock nest found by Stephen, near Lurgan, Co. Armagh in 2018.

Nest monitoring is a fantastic way to learn about bird behaviour, too. You’ll get to know the characteristics of each species, and you might notice differences between pairs. A particular male might have a characteristic song, with repeated phrases or mimicry. Some males sing throughout the breeding attempt, even whilst carrying food (male Blackcaps sometimes sing quietly while incubating!), whereas other males go very quiet once paired. Female behaviour can be interesting, too. They sometimes seem agitated when their eggs are about to hatch, giving repeated alarm-calls, even when seemingly undisturbed.

Nesting also throws up interesting questions; for instance why do female Song and Mistle Thrushes sometimes decorate the rims of their nests with berries and petals (see photo above)? And why would a female Whitethroat choose one of the decorated nests built by her mate, only to remove the decoration before laying?

Typically I monitor around 100 nests each year (my record is 117!) and for that reason I submit my data via the Demography Online interface. If you’re monitoring more than about twenty nests per year then this is probably the way to go, but for a smaller number of nests the Nesting Neighbours platform is ideal.

So there you have it - garden nest monitoring is really rewarding, hugely valuable, and you’ll learn a lot about ‘familiar’ birds through doing it. Go on - give it a go!

20 February 2020

It's National Nest Box Week!

It is National Nest Box Week (NNBW) this week and our wild birds are starting to think about the coming breeding season. This year is the 20th year of NNBW, which aims to encourage everyone to put up nest boxes for birds and to raise awareness of their needs ahead of the breeding season. While our migratory birds are preparing for their journeys back to Britain and Ireland for the summer, lots of our resident species are already singing to affirm their territory, finding a mate and choosing a nest site.

Tiny Blue Tit chicks thinking food has just arrived. Photo by Lee Barber

Around half of all households feed birds, so while food can often be plentiful, the lack of suitable nesting sites can be a problem. Putting up a nest box can therefore provide a welcome boost for some species. Blue and Great Tit are the most-frequent users of nest boxes but there is an amazing variety of species that use boxes, and by providing a suitable box, you could encourage them to set up home in your garden. House Sparrow, Nuthatch, Starling, Swift, Stock Dove, Tawny Owl and Kestrel are among the species that need suitable nesting sites and providing a box could really help them and some of our other declining red or amber listed species. For more information on what boxes are suitable for which species, click here.

Tawny Owl chicks waiting for the next delivery of food. Photo by Lee Barber

When buying a nest box, one of the most important features that I look for is a removable lid. This aids cleaning the box at the end of the breeding season (cleaning can be done between 1st September - 31st January) but most importantly it means that the contents of the nest can be recorded. BTO volunteer Nest Recorders around Britain and Ireland record the number of eggs, fledglings and the outcome of nesting attempts, providing a wealth of information on how our wild birds are doing every year. Nest recorders are provided with a code of conduct to make sure that the nest is checked in the safest possible way, so valuable information is gathered without affecting the birds.

BTO nest recorder checking on a Blackbird nest. Photo by Lee Barber

Nest cameras are a very popular choice and this also means that you can record the contents without even leaving your living room. They are particularly useful for boxes which are usually placed quite high or are difficult to get to, like Tawny Owl and Swift.

So far this year we've only seen a few birds inspecting our boxes at BTO HQ in Norfolk. Nest recording isn't just about those species that use boxes though; there are already a few Collared Doves sitting on eggs and we've also seen a brood of Egyptian Goose goslings paddling up the river. It's only a matter of time before we find our first Blackbird nest of the year and then the tit species will start building. It's a very exciting time!

Egyptian Geese are one of the earliest breeders. Photo by Lee Barber

For more information on becoming a nest recorder, check out our Nest Record Scheme website.