Jill Pakenham

Barn Owls and winter weather - the latest BOMP results Dave Leech, Humphrey Crick and Colin Shawyer discuss the findings of the first five years of the Barn Owl Monitoring Programme.

The 2004 breeding season saw BOMP participants heading out into the field for the fifth successive breeding season. Over the course of the year, 333 breeding sites were monitored by BOMP Network participants and a further 198 core and supplementary sites were monitored by the Wildlife Conservation Partnership (WCP), with breeding Barn Owls recorded at 322 sites in total (figure 1). The latest estimates of Barn Owl abundance, calculated from data collected between 1995 and 1997 by fieldworkers for the BTO/Hawk & Owl Trust Project Barn Owl, suggest that the size of the UK population currently stands at approximately 4,000 pairs. These figures suggest that BOMP participants are responsible for monitoring approximately 8% of the UK Barn Owl population – this is amazing coverage and we are extremely grateful to everyone who has taken part in BOMP thus far.

Figure 1 Distribution of BOMP sites monitored in 2004 Purple circles = BOMP network sites Yellow triangles = Wildlife Conservation Partnership sites

What causes breeding success to vary between years?

The proportion of sites at which Barn Owls were recorded as present at BOMP sites, whether breeding or roosting, has declined steadily over the last five years. However, the proportion of sites at which breeding Barn Owls were recorded has fluctuated annually, being relatively high in 2000, 2002 and 2004 and relatively low in 2001 and 2003 (figure 2). These results suggest that fewer pairs are attempting to breed in some years than in others, but why?

Figure 2 Annual variation in occupancy rates at BOMP sites

Thin line = presence of Barn Owls, whether breeding or roosting, thick line = presence of breeding Barn Owls

One factor previously reported in 1987 as affecting the propensity of Barn Owls to breed is the weather during the preceding winter, with low temperatures and high precipitation causing a decline in the numbers or availability of small mammals, whilst also making it physically more difficult for owls to hunt and simultaneously increasing the energetic costs of maintaining body condition. Analysis of BOMP data with respect to temperature and rainfall over the winter period did indeed indicate that the proportion of boxes in which breeding Barn Owls were recorded fell following cold, wet winters, although there was no effect on the proportion of boxes at which owls were recorded as present. These results suggest that poor winters are not leading to increased mortality, but rather to a decrease in body condition that results in some individuals electing to suspend breeding until the following year.

Weighing prey. Photograph by Colin Shawyer

Weather conditions were also found to influence laying dates, with owls initiating clutches later following cold, wet winters, and there was also some evidence to suggest that clutch sizes were reduced. Analysis of the BTO’s Nest Record Scheme dataset over the period 1980-2002 indicated that cold, wet weather also had a negative impact on brood sizes. These results all support the hypothesis that such weather negatively influences body condition, leading to a reduction in the amount of energy that parents are able to invest in producing eggs and rearing young. Recent predictions of climate change in the UK suggest that, while winters are likely to become slightly warmer over the next 75 years, they are also likely to become much wetter. What will the implications be for the UK Barn Owl population? The continued collection of data by BOMP participants is vital if the potential impact of climate change on this species is to be determined.

Does Barn Owl productivity vary across the country?

The proportion of BOMP sites occupied by breeding Barn Owls increased towards the west of the country. Westerly-biased occupancy rates of this species may be a response to milder winter weather, in particular higher temperatures, in the west of the country due to the proximity of the Gulf Stream, which may in turn influence survival rates and/or body condition. However, nest site densities are not necessarily standardised throughout the country and it is possible that the observed trends are driven by relative differences in nest site availability rather than by Barn Owl population sizes. Barn Owls were also significantly more likely to be present at WCP sites in areas of natural grassland and lowest in pastoral areas. Such a relationship might be predicted if less intensively managed natural grassland can support a higher density of prey species on which Barn Owl can feed, the closely cropped sward of grazing land providing less cover and less food for small mammals. However, similar habitat preferences were not observed at BOMP Network sites and further work is necessary to investigate the influence of habitat on occupancy rates. Neither geographical location nor habitat type were observed to have any effect on laying date, clutch size or brood size

Ageing and sexing. Photograph by Colin Shawyer

How can you help?

BOMP has successfully established a protocol for data collection that enables trends in population size and in breeding statistics to be calculated and is already providing valuable data for the conservation of the species. BOMP’s value is shown by the inclusion of its results in the annual and widely disseminated document The State of the UK’s Birds 2003 that reports the current status and trends of bird populations in the UK, as well as in the annual report of the Rare Breeding Birds Panel, published in the journal British Birds. While coverage of the UK is generally good , we would be very keen to encourage more Barn Owl enthusiasts to register sites for BOMP, particularly in Scotland and Wales where relatively few data are currently collected, so if you’re interested, please contact us at barnowls@bto.org (Tel. 01842 750050). In order to participate in BOMP you must have access to a potential Barn Owl nest site and have a Schedule 1 Licence for the species – for more detail about obtaining a Licence, contact Jez Blackburn, the Licensing Officer at the BTO.

Photograph by Jill Pakenham

Thank you

We are very grateful to the Barn Owl observers who have visited sites for BOMP and to all the landowners who have allowed them access. We are extremely grateful to the Sheepdrove Trust for providing funding to permit the development and operation of this urgently needed programme, and to the Wildlife Conservation Partnership for their major part in the project design, fieldwork and planning. The WCP expresses special thanks to Major Nigel Lewis for assisting them and for providing much of the data for the southwest region. Thanks to Carl Barimore and Mandy Andrews their help in collating and computerising the data.

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