Abstract from BTO Research Report No 411:

Leech, D.I., Crick, H.Q.P., & Shawyer, C.R. (July 2005)

The BTO Barn Owl Monitoring Programme: fourth year 2003

Executive Summary

1. Barn Owl is a scarce breeding species that has undergone a substantial population decline in the UK during the 20th century. It is listed as of Amber conservation concern in the UK but has been poorly covered by the national, long-running population monitoring schemes operated by BTO. The BTO Barn Owl Monitoring Programme (BOMP) was set up in 2000 with the aim:

To monitor Barn Owl populations through standardised recording of nest occupancy rates, breeding performance and survival at a set of Barn Owl nest sites broadly representative of the distribution of the Barn Owl in Britain.

2. Fieldwork involves repeat visits to registered sites, particularly to paired nest boxes, over the Barn Owl nesting season between April and October, to assess occupancy, gather breeding statistics, and ring adults and chicks. The Wildlife Conservation Partnership (WCP) has undertaken the development of BOMP methodology and has carried out fieldwork since 2000 at a set of ‘BOMP Core’ sites, distributed throughout five regions of England and matched for nest box design. In 2002, a network of volunteer ornithologists began gathering additional information at ‘BOMP Network’ sites over a wider geographical area.

3. This is the fourth report of BOMP, covering the four seasons 2000–03. Rates of occupancy are investigated, along with breeding statistics, in relation to year, geographical location, main habitat type and weather conditions.

4. WCP visited 158 sites in 2000 and, despite Foot and Mouth Disease access restrictions, 171 in 2001. In 2002, 586 sites were visited in total, of which WCP covered 196 and BOMP Network volunteers 390. In 2003, 198 sites were monitored by WCP and a further 415 were visited by BOMP Network volunteers. WCP sites are located across the whole of England, although as a consequence of sampling methodology they tend to be concentrated in the southern and eastern regions and north-eastern. BOMP network sites are more broadly scattered across the UK, including several locations in Scotland and Wales.

5. The proportion of sites at which Barn Owls were recorded as present (whether breeding or not) has declined over the four years of the study, as has the proportion where breeding Barn Owls have been recorded. This may indicate a decline in Barn Owl populations over this period, but may also be an artefact of site selection of the BOMP Network sites, which may have originally been biased towards sites that were already known to have been occupied in previous years.

6. While the decline in presence is linear, that in breeding occupancy fluctuates between years, peaking in 2000 and 2002. Regular fluctuations in Short-tailed Vole numbers which peak and trough over a three to four year cycle are known to influence breeding success. Weather conditions may have an effect on both Barn Owl abundance and the proportion of pairs that are in sufficient condition to breed, with cold, wet weather during the winter reducing the availability of small mammal prey and the ability of Barn Owl to hunt successfully.

7. Analyses using national temperature (Central England Temperature) and rainfall (England & Wales Precipitation) datasets indicated that breeding occupancy, but not overall presence, was significantly lower following cold wet winters. This result suggests that, while inclement winter conditions may not have influenced survival rates, they may have resulted in a loss of body condition leading to suspension of breeding during the following season. Unfortunately the weather datasets are too highly correlated to permit us to distinguish the effects of temperature and rainfall.

8. Occupancy rates were also influenced by geographic location. Sites to the west of the UK were significantly more likely to contain breeding Barn Owls, suggesting that either abundance is higher or that a greater proportion of the population are in sufficient condition to breed, possibly due to the milder winters experienced in the west of the UK. Breeding occupancy was also higher in the north of the UK, possibly due to a general increase in the availability of natural grassland, and therefore increased rodent abundance, and/or nesting sites in less intensively farmed areas.

9. Habitat also influenced occupancy rates, with a higher proportion of sites overall occupied by breeding Barn Owl in natural grassland areas and fewer in arable and pastoral areas, possibly due to differences in prey availability. The apparent suitability of habitats did differ between WCP and BOMP Network sites, but this might simply reflect the geographical distributions of the two sets of sites.

10. Occupancy rates of three other species – Stock Dove, Jackdaw and Kestrel – were also analysed with respect to year, geographical location and habitat. Many of the significant relationships identified were in the opposite direction to those identified for Barn Owl, suggesting that there may be a degree of competitive exclusion occurring.

11. Sufficient data were collected over the four years of the study to permit analysis of laying dates, clutch sizes and brood sizes. None of the productivity parameters displayed any significant trends over the period 2000-2003, nor were they related to weather conditions. This suggests that individuals in poor condition suspend breeding rather than producing small clutches or reducing investment in incubation or provisioning. No significant relationships were identified between any measure of productivity and Northing, Easting or habitat type.

12. Additional funding from English Nature was secured to investigate the response of the UK Barn Owl population to the 2001 Foot & Mouth Disease (FMD) outbreak and the measures taken to prevent transmission of the disease using both BOMP and Nest Record Scheme data. Information about the location of infected premises, access restrictions and slaughtered livestock throughout the UK were obtained from the Central Science Laboratory and data concerning the use of rodenticides in the north of England was provided by the Rural Development Service. Neither FMD infection per se nor any of the procedures put in place to prevent disease transmission were found to have any major impacts on Barn Owl occupancy rates or productivity. However, it should be noted that these results should be interpreted with caution as it is possible that uncontrolled confounding factors may have affected the analyses.

13. BOMP's increasing value to conservationists is shown by the inclusion of its results in the annual and widely disseminated The State of the UK's Birds 2003 (Eaton et al. 2004) 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 (Ogilvie & RBBP 2003).

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