For the last 16 years I have been a member of the BTO’s Garden Birdwatch Scheme. We keep a diary by the kitchen window and record every bird, mammal, amphibian, reptile and invertebrate that we see in our garden. I enter the data into the central database every other month. It is a great scheme and provides a fantastic volume of data for one of our least monitored, and most common, habitats. You might ask what this has to do with bird ringing?
On Tuesday morning we had a lot of activity in the garden: lots of Redwing and House Sparrow flying around, Goldfinch, Greenfinch, Blue, Great and Long-tailed Tit on the feeders. The thing is that the most we saw of any of the Tit species at any one time was three individuals. So I decided to set a few nets and see just how many individuals were really there.
Over the course of just 2.5 hours, from 11:30, I caught the following: Blue Tit 17(2); Great Tit 4; Long-tailed Tit 8(1); Dunnock (1); Chaffinch 1; Goldfinch 3. Totals: 33 birds ringed from 5 species, 4 birds recaptured from 3 species, making 37 birds processed from 6 species. I have had smaller catches in some of my largest sites.
The reason for this post is this: birding is a great tool for the qualitative analysis of birds but if you want any meaningful quantitative analysis then the best currently available tool is ringing. The standard method of calculating population sizes in biological systems is to capture a subset, uniquely mark them, re-release them into the general population then carry out another catching exercise. The proportion of the second catch that is already marked allows you to work out the approximate size of the population. This is exactly what ringing does. The recapture rate across my regular sites is over 30%, which gives a good indication of the size of the population of the resident bird species. With mortality rates of 0.1% and accidental damage rates of less than 0.05% (i.e. statistically insignificant to the point of non-existence) within our team’s ringing activities, plus the reports and management suggestions we make to the owners, who often act on our recommendations, I feel that is all the justification needed for continuing our work. Fortunately all of the landowners of my sites agree.