1. Numbers of birds marked
While I had ringed a few birds in the garden the year before, 7 April 2007 saw the start of the Holt Blackbird Project proper as this was when I colour-marked the first bird, male Yellow over Green, Orange over Metal (YGOM), a full adult male. Initially I was using mist nets to catch the birds, but Blackbirds spend a lot of their time on the ground so this wasn’t hugely effective. I graduated to Potter traps, cages with a trap door triggered when the bird steps on a platform; these were more successful, but I still wasn’t able to actively select the birds I caught and many became ‘trap-happy’, accepting regular capture as a small price to pay for the food inside. The current drop trap was a revelation – I could easily select which birds to capture and extract and process them extremely quickly.
In total, including the birds marked during the 2014 breeding season, I’ve now colour-ringed 457 adult Blackbirds – not bad for a small suburban garden! And this total doesn’t include all the juveniles and wintering adults, which bring the overall total to over 750. Overall, I’ve caught slightly more males (Fig 1a) than females (Fig 1b); while this could reflect a true bias in the population, the more likely explanation is that the males are more easily caught being dominant at the feeders and spending less time at the nest.
It’s possible to distinguish first-year birds from full adults by their plumage as they retain their juvenile flight feathers until the end of the first breeding season. Figs 1a and 1b show that, while the number of adult birds caught is fairly constant through time, the abundance of first-years varies more markedly; the latter have been particularly thin on the ground in 2014, which agrees with data from Constant Effort Site ringing showing that breeding success in 2013 was poor.
2. Number and distribution of resightings
The real success of the project isn’t the number of birds ringed, however, it’s the number of times that the marked individuals have been resighted – and the HoBOs have done the project proud. Over the past eight years, an incredible 45,000 resightings of have been recorded, 12,000 of these originating from outside my folks’ garden, though none further than 550m from the ringing site (Fig 2). The most commonly recorded bird was male Orange over Red, White over Metal (ORWM), ringed on 4 May 2008 and seen on over 1,400 occasions subsequently – two other birds have now topped the 1000-sighting mark.
One of the really incredible outcomes of the project is an understanding of the sheer number of birds that use the garden during the breeding season. The greatest number of individuals recorded on one day was 74, and even then there were some unringed birds still present. So, next time you see ‘your’ pair of Blackbirds in the garden, remember that they may not be exactly who you think they are!
One of the other great things about the HoBO network is the opportunity it gives us to start exploring the movements made by individual birds. We know, for example, that Red over Orange, Black over Metal (RONM) was resident in Maggie Wilcox’s garden for most of this summer until she went on holiday for a few days, at which point he flew 500m to Mum’s garden and settled there, return to Maggie the day that she came back and started feeding the birds in her garden again. This may seem like a nice anecdote, but it does start to tell us something about the ways in which birds sample their environment and remember where the reliable food sources are.
3. Annual return rates
Just one of the original cohort of marked birds is still visiting the garden. Male Green over White, Yellow over Metal (GWYM) was ringed as a first-year male on 13 May 2007, so we know that he hatched in 2006 and is still going strong eight years later. National ringing data show that a typical Blackbird will survive for three breeding seasons, so he’s doing well; he’s got a way to go to beat the oldest on record, though, a 14-year-old ringed in Northamptonshire in 1937!
The initial aim of the study was to generate information on the survival rates of Blackbirds in gardens by monitoring which birds return each year. I was particularly interested to see how garden-breeding Blackbirds fared compared with the woodland and scrub birds caught by CES ringers. Rough calculations thus far suggest that c. 60% of individuals return each year, which is very similar to the figure generated by CES over the same period (see the results graph at the bottom of this page). This may indicate that the any benefits accrued due to the provision of supplementary food are offset by novel hazards such as cars, cats and windows, but more rigorous analysis will be needed before I can draw firmer conclusions.
4. Seasonal patterns – the mystery of the autumn leavers
One of the big mysteries of the Holt Blackbird Project is where they go in the autumn. Sightings typically peak in the breeding season, from mid-April to mid-July, when the demand for food to produce eggs and feed chicks is high (Fig 3). Numbers then drop markedly between August and October before increasing again between November and February, when natural food is scarce and energy demand under low temperatures is high. This pattern is repeated at all sites in the Holt Blackbird Project where birds are recorded regularly, and is evident in the national results produced by the BTO’s Garden BirdWatch survey.
So, where do they go? Some colleagues have suggested that they may simply be keeping a low profile while they moult, but given the intensity at which the gardens are watched and the magnitude of the difference in numbers, I’m not convinced; would birds spending lots of energy on feather replacement not be more likely to take the food on offer? My theory is that individuals are moving out of town to find nutrients not provided in gardens, but this remains to be tested and we’re currently trialing some tracking work. In the meantime, you can help by keeping an eye out for colour-ringed birds during this period, not just in Holt but in the surrounding countryside.