Fifty years ago, in the spring of 1962, I took part in the first year of the BTO Common Birds Census, my patch being in the grounds and adjacent woodland of the National College of Food Technology at Weybridge, in Surrey. One of the commonest birds was the Willow Warbler Phylloscopus trochilus and most were located by ear. If I repeated the census now, and assuming there had been no population change, my counts would be much lower, as I would be failing to pick up many singing birds. The ageing process!
Now jump from Weybridge’s exclusive St George’s Hill to the remote Arabian island of Socotra where, since 1999, I have been engaged in BirdLife International’s census to determine the population of the breeding landbirds. This has involved undertaking 400 km of line transects, where all species have been recorded (seen and heard) over a fixed band-width. In spring 2011, I decided to repeat some of the earlier transects in the low coastal habitat of the endemic Socotra Cisticola Cisticola haesitatus – a bird located mainly by its high-pitched song. In crude terms, I was recording half the numbers that I did some ten years previously, yet my Socotran companion, Ahmed Saeed Suleiman, with his ever-sharp ears, was recording similar numbers to those of a decade ago.
Back to Norfolk where, for the past three years, I have made TTVs for the forthcoming BTO Atlas. As most BB readers will know, this required counting all birds, seen and heard, during a two-hour walk through a designated tetrad, then using this as the basis for a stab at the bird population (breeding and wintering) in the tetrad. Given that I find it now difficult to hear many bird songs farther away than, say, 50 m, and the calls of a number of species impossible to detect, I wonder just how good my population estimates are?
With an ageing BTO membership and therefore, presumably, an ageing voluntary survey workforce, how does this affect our survey results? Certainly I can no longer trust my records if listening for songs and calls is required and I’d hate to think that some species are becoming rarer simply because we can’t hear them! It would be interesting to know whether the age ‘signal’ can be isolated from any long-term survey results.
Richard Porter, King’s Head Cottage, Cley-next-the-Sea, Norfolk NR25 7RX; e-mail firstname.lastname@example.org
Editorial comment Andy Musgrove, Head of Monitoring at the BTO, has commented as follows: ‘I’m sure most of us would be quite happy to have Richard’s field skills, declining or otherwise! However, these are interesting reflections on a subject that arises from time to time at the BTO, and this reply is the result of discussion with a number of colleagues. Is the average age of surveyors increasing and, if so, what are the implications for the monitoring of birds by volunteers? Unfortunately, it’s not easy to answer either of these two questions definitively.
‘Despite the wealth of data that the BTO holds on birds, we hold rather less about most of our surveyors. The best information about participant age is probably from the Ringing Scheme, where the median age of ringers hasn’t changed markedly since 1999. Of course, we’d really like to have such information for all our surveys, going back to the 1960s, but it just doesn’t exist, unfortunately. Of course, many people have a “gut feeling” that birdwatchers are getting older, but it’s quite difficult to tease out; to what extent is this simply that each one of us, along with our circle of friends, is getting older? There are still large numbers of younger people interested in birding. Yes, many are, of course, initially motivated by information-fuelled twitching, but they do represent a large cohort of potential recruits as they get older, get jobs, settle down, have kids, and later retire. Indeed, although there may be relatively low numbers of new surveyors in their teens and twenties, the newly retired generation is a superb source of help that can often contribute many times the numbers of hours of a younger contributor.
‘However, even if one assumes that there has been an increase in the average age of surveyors, can we pick out an “age signal” in our monitoring? This is difficult to isolate against the many other factors that affect bird populations. For example, the Breeding Bird Survey (BBS) does a good job of monitoring trends of the Goldcrest Regulus regulus , but might we be missing an increasing proportion of them each year, due to a lower detection of their high-pitched song by older surveyors? Grasshopper Warbler Locustella naevia is another species increasingly missed by older ears; the draft results of the Atlas suggest that this species might be disappearing from large parts of southern England but increasing its range somewhat in the north. Perhaps this in itself is evidence against the observed changes being strongly driven by age of observers; it seems unlikely that we have an ageing network in the south and a more youthful one in the north.
‘One way to be able to measure the age-signal would be to have an independent measure of abundance against which to compare. For example, one might compare trends from BBS (where detection is mostly by song) against numbers of adults trapped in mist-nets during Constant Effort Ringing. Again, though, other biases come into play, such as differences in habitat representativeness of the two schemes. A better approach might be to look at measures of detectability from within BBS (broadly, the ratio of birds detected in the 0–25 m band nearest the transect line, to those detected in the next band, between 25–100 m). If you could show a decline in this ratio over time, and link that decline to the age of individual observers, and perhaps also link it to the pitch of the song or call of the bird species involved, then this could be evidence for such an effect occurring. However, even if you could detect such an effect, it would not imply an impact on national population trends unless, as discussed previously, you knew that the age-structure of the participants as a whole was indeed changing. Not easy!
‘In short, I think that we recognise the potential importance of the issue, but don’t see too much evidence of effects yet. We’re always amazed by the longevity and resilience of some of our long-standing surveyors, while the incredible level of participation in the 2007–11 Atlas, including thousands of new volunteers, has showed us that volunteer bird surveying is not on its last legs yet. Keep up the good work everyone!’