Simon Tucker
The age distribution of the Blue Tit, C. caeruleus, mist netted in Ravensroost Woods shows a distinctive pattern over the course of a year. Data collected between 2009 and 2013 shows that adult birds virtually disappear from the catch between April and November, returning during December and January with numbers tailing off between February and March. The proportion of adult birds older than one year and birds born in the previous year is 8:10. Birds born in the current year are almost the only Blue Tits caught between July and November.
The hypothesis suggested to explain this is a differential feeding strategy to prevent competition between the adults and young birds post-fledging.
Between July 2009 and October 2012 Ravensroost Woods was ringed on an ad hoc basis. Since October 2012 the woods have been ringed regularly. Birds are trapped using mist nets. During the winter months (November to March) the birds were fed at specific feeding stations with mixed seeds, peanuts and nyjer seed. Mist nets were set adjacent to the feeding stations. In the breeding season trapping took place in the established rides and in various coppice coupes in the general area of the same feeding stations. Nets have been set in the same places year on year. Outside of the breeding season playback lures were used for specific species. Although the only Paridae lure used was for Marsh Tit, Poecile palustris, the fact that Paridae flock together with many other species during the winter, such as Goldcrest, Regulus regulus, which were lured, may have had some impact.
All birds captured were ringed with unique identification rings and biometric data collected. Retrapped (i.e. previously ringed) birds also had biometric data collected. In the case of the Blue Tits ageing was based upon the moult state of the great and primary coverts: young birds have greyish coverts which moult through to the blue adult plumage. Birds of the year and adults are easy to differentiate, at the turn of the year birds of the previous year can still be identified by the same differentiation of the covert feathers until the post-breeding moult. After the post-breeding moult birds of the year can be differentiated from birds from previous breeding seasons but no differentiation is possible between birds of the previous year and older birds except by ring number. This was not done for this data set.
The age groups were defined as per the BTO ringing scheme classification:
Adults: birds older than one year old at 1st January
Adults from previous year: birds fledged during the previous calendar year, noted until May / June of the current year after which they are incorporated as Adults
Juveniles from this year: birds fledged during the current calendar year.
The number of birds of each age group were counted for each ringing session.
Table 1: Ringing Sessions per month by year:
Table 2: Number of Blue Tits caught per month per year by age
A = Adults
P = Adults from previous year
J - Juveniles
Table 3: Average numbers caught by month by age
Fig 1:
This review of the data collected for the Blue Tit in Ravensroost Woods came from a concern from the site warden, Robin Griffiths, who had not seen many Blue Tits in the summer of 2013 and thought that this might reflect a poor breeding season. It has also been said to me, admittedly by an opponent of the Ringing Scheme, that there is nothing new to be learned from ringing Blue Tits.
The Blue Tit is the most common species caught in Ravensroost Woods and so I analysed the historical data as well as the current data to establish what the status was. The early data set is a little patchy but since October 2012 the site has been ringed very regularly and the data will develop over the next few years and will enable us to see how these trends develop. One trend is clear: the number of adult birds trapped reduces significantly as the year progresses with the numbers building up through December, peaking in January and then decreasing gradually through February and March, declining massively thereafter.
Blue Tits are almost exclusively single brooded and the young usually appear in late May – June. Young birds appear in the catch in June onwards, currently peaking in September and then building up again into the winter months alongside the number of adults, with the numbers continuing to increase as the year turns and the birds move category from Juveniles to Fledged in the Previous Year. Birds fledged in the previous year become indistinguishable from older adult birds after undergoing their post-breeding moult in June and July.
The increase in adult numbers during the winter period entirely coincides with the provisioning of the feeding stations, whereas the juveniles are present throughout, from June onwards.
This then raises the question of where the adults are going between April and December. They are rearing young between April to June / July, busy feeding their young in the nest but are rarely trapped in mist nets. The obvious conclusion is that in Ravensroost they are either leaving the wood to forage elsewhere or else foraging in a part of the wood that makes it less likely that they will be trapped in a standard mist net. The Blue Tit is largely insectivorous, with the preferred food for their young being insect larvae, particularly butterfly and moth caterpillars. Ravensroost Woods and the associated meadows are home to large numbers of moths and butterflies from a wide range of species – a perfect source larder for Blue Tit feeding. It could be the answer to where the adult Blue Tits are going: into the canopy or out to the meadows to glean caterpillars for their young. Post-fledging of their single broods and with the onset of the autumn fruit period it will be interesting to find out whether the adults are taking advantage of this bounty before returning to the wood as that resource is used up. There is already evidence of movements around the Braydon Forest area, with Blue Tits ringed in Ravensroost being retrapped on Somerford Common and a site in the Wood Lane area. Conversely, a Blue Tit ringed at the Cotswold Water Park has been recovered at Ravensroost Woods. From my current ringing data these movements seem to involve as many adult birds as juveniles so it doesn’t look like post-fledging dispersal.
Given that the adults seem to forage in areas that prevent their being caught, why is it that the youngsters are readily caught once they have fledged? The suggestion is that the young must be searching for food in different areas from the adults. The simplest explanation for this is that it is a differential feeding strategy to reduce competition for food sources between the adults and young. It is difficult to envisage how this can be tested – however over the course of the next few years trapping of birds in the hedgerow could give an indication, i.e. if most of the birds netted are adults then this hypothesis holds, if juveniles outnumber adults then a different explanation needs to be found.
This analysis has thrown up questions that could form the basis of further projects:
1. To test whether the adult birds are foraging in the hedgerows around the wood, mist netting of the meadows around Ravensroost Woods can provide information from recaptured birds and noting where they were originally ringed. Ringing new birds and noting if they are recaptured within the wood as well.
2. To test the origin of the juvenile birds within the wood. We can already identify the source of retrapped adults within the wood. Are the juveniles coming in from outside of the wood or are they birds fledged within the wood? To investigate this, ringing of nestling birds will enable the source of retrapped juveniles to be established.
3. To test whether there is a differential feeding strategy between adults and fledged juveniles: using the data collected to cover point 1 above.
4. Comparison with a site which is fed all year round. The site is a large garden on the edge of woodland similar to that found at Ravensroost, although not coppiced. This data is already being collected but the data set is currently for eighteen months.
Thank go to:
Matt Prior for providing the data for the period 2009 until May 2012.
Dr Ian Grier for his ongoing encouragement, suggestions and support.
Wiltshire Wildlife Trust and Natural England for allowing me access to the Ravensroost complex and SSSI and supporting my projects.
Perrins, C.M. British Tits, Collins New Naturalist Series, 1979
Redfern, C.P.F & Clark, J.A. eds, Ringers’ Manual, 5th Edition, BTO, 2008
BWPi 2.0.1 Bird Guides and Oxford University Press, 2008