Goose Ringing Project

Every year since 1982 the Dartford Ringing Group has carried out a roundup of the geese at the Sevenoaks Wildlife Reserve (previously the Jeffery Harrison Reserve). This involves a large number of ringers, canoeists and other helpers and takes place during the period when the geese are flightless in mid June.

The tables below show the number of new birds and retraps processed, together with a retrap history showing the original ringing year of birds retrapped in 2007. This year’s total of 205 birds included 3 Mute Swan and a single Coot.

Retrap history

Some interesting observations on the long-term catch data
The group has a long running data series from the annual goose roundups and over the years we have published several short papers on various aspects of our study; the most important of which was Ron Terry's paper published in the 1997 group report. This looked in detail at the monthly and annual sight counts of Greylag Geese from 1961 to 1997 and the annual ringing totals from 1982 to 1997. Two of the most interesting extracts from the data were the number of first year birds in the annual catch and the relationship between birds that had not previously been caught and ringed and those already wearing a ring. As several years have passed since Ron's paper it seemed that a short update on some aspects of the study would be useful, pending a more detailed paper in the near future.

Retraps
The table below shows the year of ringing for all retraps from 1986 to 2007 and raises some interesting questions; why do the number of birds retrapped from the previous year fluctuate so much and why do some birds appear in the retrap data each year whilst others are absent for many years after their original ringing date, only to reappear for several years in a row before finally disappearing altogether; there is obviously a lot of scope for further study here. In 2007 a new longevity record was set by 5167153, originally ringed on 20th June 1993 some 14 years and 7 days before this years retrap date of 24th June. This bird has only been retrapped twice in the intervening years, in 2003 and 2005.

As can be seen from Table 1 the number of retraps caught each year fluctuates but of course so does the total catch. Figure 1 below plots the number of retraps as a percentage of the total catch for each year's roundup. As one would expect the early years show a gradual increase in the ratio as more ringed birds are added to the population. However, once we get to 1993 the ratio begins to fluctuate considerably, with a peak of three retraps to each new bird in 1994 and a low of one to one in 1997. This was followed by a further three years at almost the same ratio before rising again in 2001 and 2002. In 2004 the catch was very small and therefore the number of retraps from that year in the population as a whole was much reduced and therefore there were more un-ringed birds available to be caught in 2005. There was a return to the long-term average in 2006, which is beginning to flatten out at about 1.75%, or five retraps for every three new birds in the catch, but then a slight falling away from this in 2007.
Figure 1

Percentage of juvenile birds in the annual catch
Another interesting extract from the data is the number of juveniles in each year’s catch. As can be seen from Figure 2 below there have been large fluctuations in the percentage of juveniles and although in the early years of the study, the timing of the roundup and the capture methods meant that there were either a large number of juveniles or virtually none at all. However, since the late 1980’s the date of the roundup and the capture method has been relatively consistent. It may be that this graph does, to some extent, reflect the productivity of the Greylags at Sevenoaks

Figure 2

Contributions to other research projects
During the 2007 roundup, feather samples were collected under licence for Steffen Hahn of the Netherlands Institute of Ecology. The main objectives of his study are to show how parental habitat choice is reflected in diet for goslings at an individual or family level, and also how habitat use by Greylag families differs on a European scale. Samples are being taken in The Netherlands, Germany, Sweden, Norway, Estonia and the Czech Republic, with Sevenoaks being the main UK representative site.

And finally, a big thank-you to all the visiting ringers, canoeists and helpers, who all contributed to the success of the 2007 roundup.