Tit research
Wytham Wytham Woods Woods is a ‘ living lab, lab’ home to the best-known bird population on Earth. Jo Wimpenny discovers how research here has transformed what we know about the lives of great and blue tits.
A long-term study at Wytham Woods
Wytham Woods must be the most studied woodland in the world. “Harvard would argue with that, but we’re definitely in the top three,” laughs Nigel Fisher, conservator of Wytham Woods since 2000. “In any case, it’s the long-term datasets where we stand out – these records just don’t exist anywhere else.” It’s hard not to feel a little awed by these, as he reels off a list of continuous long-term studies on small mammals (officially since 1962, potentially earlier).
But it’s the great and blue tit research that tops the list. Since 1947 more than 100,000 great tits have been individually marked here. Dozens of generations of tits have been followed – the longest continual stretch over which a single great tit lineage has been tracked is an amazing 38 generations. More is known about this population of tits than any other bird population on the planet.
Wytham Woods were bequeathed to the University of Oxford by Raymond and Hope ffennell in memory of their daughter, under the condition that the place would be used for education, research and recreation. It was 1942, and the timing was good: the study of animal ecology was in full swing at Oxford and the Edward Grey Institute of Field Ornithology (EGI) had just been established.
NEW HOMES
In 1945, David Lack moved from a teaching position at Dartington School to become director of the EGI. He had just published his superb monograph The Life of the Robin and was keen to set up a population study of the species. Instead, inspired by a Dutch study, Lack switched to great tits, which are natural hole nesters but will readily use nestboxes. Two hundred nestboxes were erected in 1947, marking the birth of the Wytham tit study.
Massive expansion came in the late 1950s after Chris Perrins, now emeritus Prof of zoology, joined the EGI as a research assistant and starting erecting hundreds more boxes. By 1963, he’d increased the number to about 1,000. Ben Sheldon, who took over from Chris as EGI director in 2004, added 200 more boxes for blue tits (which have a 26mm diameter hole, as opposed to about 32mm for great tits). So today’s fieldwork team has the unenviable task of checking 1,207 boxes every breeding season.
BREEDING SEASON
Fieldwork starts in early Aprilpril and continues until mid-midJune, when all the chicks haveh fledged. It’s pretty intense, as Ella Cole, senior research her and research coordinator explains. “A typical day at th he peak of the field season can start before dawn, so ar round 5am, and finish at 5–6pm. We have to work k out which birds are breeding in each nestbox, w which generally means catching the birds a and reading their leg rings. Given that each o of us might be in charge of 150 nests, that is a lot of parent birds to catch!”
In addition to recording va arious types of data, all of the chicks mus st also be weighed and tagged. It’s bee en a few years since I handled small passe erines, but I well remember the initial p panic about how tight to hold a tiny, 15g g bundle of feathers. Ella agrees. “I’ll never forget the first time I held a wild great tit. It was so exhilarating… yes, I was a bit scared I might crush it, but the birds’ robustness and ability to withstand being trapped and handled is one of the reasons we use tits in our research.”
Great tits have been given numbered metal leg rings since the study began, but today each bird is also fitted with a plastic ring containing a RFID tag (similar to the microchips attached to cats and dogs). ThesThese tiny tags contain identity codes about each individua al, and can be read remotely, meaning fewer birds need to o be caught to be identified. In a good year, when there is ple enty of food and high chick survival, each fieldworker may e end up tagging upwards of 500 chicks: a phenomenal task. .
SURROUNDED BY SPRING
It all sounds a bit exhausting, and I wonderr how hard it is to remain motivated for the whole periodd. “It’s pretty wonderful watching spring happening all aaround you,” says Ella, “and we get to see so much of thee woodland wildlife. While going about my work I hhave – in some cases literally – bumped into courting hhares, tawny owl chicks, deer, foxes and badger cubs. Nott bad for a day in the office.” The flipside is when it rains,r as the work still needs to be done. “Sometimes you could spennd day after day
BY TRAINING CERTAIN GREAT TITS TO SOLVE PUZZLES IN DIFFERENT WAYS, IT’S POSSIBLE TO TRACK HOW THEIR BEHAVIOUR SPREADS.
sloshing around in the rain, trying to catch birds and keep everything dry, and thinking ‘I just want to go home!’,” admits Andy Gosler, a lecturer in ornithology and conservation who conducted fieldwork on Wytham tits for 30 years. What’s more, the birds didn’t always make it easy. “Certain experienced birds got to know me and became extremely difficult to trap,” Andy says. “There was a little battle of wills because I had to redesign my traps to make them more effective.”
Great tits can be rapid learners and are known to innovate novel feeding solutions, and while they are often thought of as the dominant birds around a feeder, individuals differ quite a lot in their personalities. When great tits are temporarily taken from the field and placed in a new environment some birds immediately explore every part of it, whereas others stay rooted to the spot. The bolder explorers also tend to be more aggressive and more likely to take risks than their ‘shy’ counterparts.
How these differences relate to their survival and breeding success is intriguing: bold birds tend to take a ‘live-fast die-young’ approach, prioritising their current broods over their own survival, whereas shy birds are more likely to abandon nests that are threatened and prioritise survival. There are other differences, too. Shy males avoid nesting near bold males, possibly to avoid confrontation with the more aggressive individuals. Bold males are more likely to cheat on their partner. However they suffer a cost for their dalliances, as their partners are more likely to cheat on them, too.
PASSING ON INFORMATION
A current focus is on social behaviour, for example, and how social interactions influence the spread of information. By training certain individuals how to solve simple boxopening puzzles in different ways, and then releasing them and the puzzles into the woods, it was possible to track how the behaviour spread. Birds readily learned from others: those that associated with knowledgeable birds were considerably more likely to pick up the technique.
Surprisingly for the team, tits were influenced by the specific technique most prevalent in their population, even though it had no advantage over the other in terms of accessing the food. “It’s the bird equivalent of ‘when in Rome, do as the Romans do’,” says Ben. “They are being conformists in the way that they learn.”
Decades-worth of data are also being used to answer more applied questions. The Wytham tit study was the first to show the impact of climate change on great tit breeding, something that Lack and his students could hardly have expected in the 1940s. Just as Britain’s army of citizen scientists is reporting wildflowers blooming earlier and the dates when we notice trees blossoming and coming into leaf have moved forwards, so the tits have adjusted when they lay their eggs.
Today, great tits lay two to three weeks earlier on average than in the 1960s. “When I began working at Wytham Woods, I would start checking the nestboxes on 20 April,” says Andy. “Now, if I started checking then all the birds would be incubating already. I’d have completely missed the laying period.”
GROWING UP FAST
Deciding when to start breeding every year is crucial for the tits. They rely on the superabundance of winter moth caterpillars, Operophtera brumata, which are available for a couple of weeks each spring when the oak trees are budding. Ella tells me that in the space of a fortnight parent tits could bring about 10,000 caterpillars back to their naked and blind chicks to transform them into fully formed birds capable of flight. “This is the equivalent of a human couple raising 10 newborn babies into 60kg teenagers in two weeks – it’s hard not to be in awe.”
It seems that the tits can ‘predict’ caterpillar availability, since larger clutches are laid in years where there will be lots of moth caterpillars. Just how they estimate this, and then adjust the timing of their broods, is a question that puzzled Lack. But while the precise mechanisms are still a mystery, it now appears that the Wytham tits are using a couple of cues.
First, the tits pay attention to the developmental stage of oaks near their nestbox. Females nesting near early budding trees start laying earlier in the season than females nesting near late-budding trees. Second, the birds can control when and how much they incubate their eggs, helping the hatch date coincide with caterpillar availability. On warm days, when caterpillars are growing more quickly, females spend more time on the eggs to speed up hatching. The opposite is seen on cooler days. It’s a web of connection between oaks, caterpillars and tits, and remarkable to realise that the birds are responding to climate change.
DEDICATED TEAM
“So what’s next?” I ask Ben. “How long have you got?” he replies. He’s got a point. An incredible amount of influential research has been done in 70 years, but that has no bearing on the ultimate potential of this system. In fact, the longer the Wytham tit study continues the greater its value to science, because the evolving insights from decades of observations enable researchers to formulate new and exciting questions that wouldn’t otherwise be possible.
I’m struck by the sheer enthusiasm of the researchers and their love of being in these woods. Andy tells me that he came to feel so connected to ‘his’ patch of woodland that it felt like a bereavement when he stopped working there. But surely you get fed up of hearing the great tit’s repetitive ‘teacher, teacher’ song? Ella laughs. “Never! Although I also don’t think I’ll ever be able to hear that song without thinking about work. In 50 years’ time, when I hear a great tit singing in spring, I’ll probably still think I need to get up early to catch birds…”
THE BIRDS CAN CONTROL HOW THEY INCUBUATE THEIR EGGS TO COINCIDE WITH CATERPILLAR AVAILABILITY.