Goose Ringing Project

Every year since 1982 the Dartford Ringing Group has carried out a roundup of the geese at the Sevenoaks Wildlife Reserve (formerly 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 2010.

  Original ringing year of birds recaptured in 2010

Retraps
The table below shows the year of ringing for all retraps from 1986 to 2010 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.

Greylag Goose retrap histories

As can be seen from Figure 1 below, 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 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 a ratio of 1.75. A slight falling away in 2007 and 2008 is followed by a small rise in 2009 and then a more significant fall in 2010..

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 no more so than over the last four years with 2010 having the highest percentage of juveniles in the catch. 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, 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

And finally, a big thank-you to all the visiting ringers, canoeists and helpers, who all contributed to the success of the 2010 roundup.