The BTO Tawny Owl Survey
Although Tawny Owls are common and widespread across Britain, their populations are difficult to monitor through national schemes like the BTO/RSPB/JNCC Breeding Bird Survey. This is because such surveys take place during the daytime and the Tawny Owl is a largely nocturnal species. However, the information that is available from these surveys does suggest that the Tawny Owl population may have been undergoing something of a decline over recent years. In order to derive a definitive answer, and to establish what has happened since the last national Tawny Owl survey in 1989, the BTO has just completed a major piece of new work on the species.
The BTO Tawny Owl Appeal
Through their contributions to the BTO Owl Appeal, the generosity of BTO and Garden BirdWatchers members has enabled the BTO to carry out some much-needed work on a number of our owl species. A large chunk of the money raised was directed towards work on Tawny Owls and we were able to carry out three distinct pieces of research to improve our understanding of these birds. One aspect of the work used professional fieldworkers to test the responses of Tawny Owls to the tape playback of Tawny Owl calls. It had been suggested previously that tape playback might be a more efficient way to solicit responses from breeding birds. Our work has enabled us to reassess how tape playback can contribute to Tawny Owl censusing. The second piece of work was a repeat of the 1989 Tawny Owl survey and this suggested that the Tawny Owl population has not changed by as much as was feared over the intervening 16 years. The final piece of work, centred around Garden BirdWatch and reported on here, looked at Tawny Owl calling behaviour throughout the winter. We have been able to examine how calling behaviour varied with time, weather conditions and habitat. While this article outlines the initial findings of this work, we hope to carry out further analyses over the coming weeks before publishing a more detailed report in the form of a scientific paper.
Those long winter nights
Some 1,883 Garden BirdWatchers and 1,719 other observers took part in the survey, which ran from October 2005 to April 2006. Participants were asked to spend 20 minutes on one night each week listening out for calling owls. We treated the two sets of observers (GBW and Non-GBW) separately because Non-GBW participants were less likely to send results in if they did not hear any owls. This is reflected in the fact that while 79% of GBW participants noted calling owls at least once during the survey, the corresponding figure for the non-GBW was 93%. Looking at the results on a week by week basis revealed that calling behaviour was more pronounced earlier in the winter. This is the period during which we would have predicted birds to be more vocal, since they are establishing territories in readiness for breeding. As the breeding season progresses territorial behaviour settles down and individual pairs get on with the business of rearing their young.
The degree of cloud cover is widely believed to influence calling behaviour and so we used the information that had been collected to examine this. The results showed very clearly that the owls were heard significantly less frequently as the degree of cloud cover increased. Research carried out elsewhere has already demonstrated that Tawny Owls dislike calling when it is raining and it has been suggested that this is because the background noise of falling rain reduces the distance over which a calling owl may be heard. If your neighbours cannot hear you then there is little point in wasting energy by calling to them.
What next?
The BTO Tawny Owl survey has not only revealed how our Tawny Owl population has fared over the last two decades but it has also enabled us to refine our survey methods for this nocturnal species. Knowing how calling behaviour varies throughout the breeding season, with weather and with location (analyses of the latter topic are still to be undertaken) will enable us to improve the ways in which we count Tawny Owls. There is a wealth of information about Tawny Owls contained within the data generated by the BTO Tawny Owl Survey. Further analyses was carried out looking specifically at habitat interactions. For example, we hope to determine how the presence of woodland locally will influence your chances of hearing calling Tawny Owls. We also want to find out if the pattern of calling behaviour is different in northern Britain to what happens in the south. Finally, the data are also being used to test the effectiveness of new mathematical models that allow us to take account of owls that may be present in an area but, by chance, are not heard during a given survey period. This should also prove very valuable when we come to look at other nocturnal or elusive species.
Steve Freeman (BTO Ecological Statistician) and Mike Toms ( BTO Garden BirdWatch Organiser)