Abstract from BTO Research Report No. 454:

Leech, D.I., Barimore, C.J., Crick, H.Q.P. & Shawyer, C.R. (2006)

The BTO Barn Owl Monitoring Programme: sixth year 2005
ISBN: 1-904870-89-9

Executive Summary

1.1 The 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 the 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.

1.2 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 ‘core’ sites, distributed throughout five regions of England. In 2002, a network of volunteer ornithologists began gathering additional information at ‘BOMP Network’ sites over a wider geographical area.

1.3 This is the sixth report of BOMP, covering the seasons 2000–05. Rates of occupancy are investigated, along with breeding statistics, in relation to year, geographical location, main habitat type and weather conditions.

1.4 In 2005, 197 sites were monitored by WCP and a further 353 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, eastern and northern regions. BOMP Network sites are more broadly scattered across the UK, including several locations in Scotland and Wales.

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

1.6 The proportion of WCP sites occupied annually by breeding Barn Owl is lower than that at BOMP Network sites. This probably reflects the original sampling method used by WCP which were stratified for sites where Barn Owls had bred previously, roosted previously or where they had never been used before. The two types of site have also been monitored over different lengths of time. Assuming original occupancy rates were inflated, WCP sites have been monitored for six years whereas monitoring only began at BOMP Network sites four years ago and there has been less time for a more natural level of occupancy to be attained.

1.7 Weather conditions have previously been reported to affect 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. Analyses using national temperature (Central England Temperature) and rainfall (England & Wales Precipitation) datasets indicated that the proportion of sites at which owls were present was reduced following wet winters. The proportion of occupied sites at which owls attempted to breed was also lower during breeding seasons preceded by cold, wet winters. These results suggest that birds lose body condition during inclement winter conditions, leading to suspension of breeding during the following breeding season and possibly even to increasing rates of mortality.

1.8 Occupancy rates were also influenced by geographic location. Sites to the west of the UK were significantly more likely to contain breeding Barn Owls, than to the south and east. Possible causes behind these patterns are discussed, including the potential influence of climate, habitat and nest site availability. No significant effect of habitat type on occupancy rates was identified.

1.9 Sufficient data were collected over the five years of the study to permit analysis of laying dates, clutch sizes and brood sizes, although sample sizes for clutch size were relatively small. Evidence for temporal trends in productivity was weak but habitat type did have a significant effect on clutch and brood sizes, which were higher in rough grassland areas where small mammal, particularly Field Vole, population densities are likely to be greater.

1.10 Weather conditions during the winter months also had a significant effect on Barn Owl productivity, with females laying later after cold, wet winters and producing larger broods after drier winters. These relationships are as predicted if inclement weather increases thermoregulatory costs, reduces hunting efficiency or opportunity or reduces the abundance of prey species, thereby negatively influencing body condition. Adults in poorer condition may reduce their investment in reproduction during the following season.

1.11 Conversely, there was little evidence to suggest that weather conditions during the breeding season significantly influenced productivity, with significant relationships limited to a counter-intuitive negative correlation between clutch size and temperature.

1.14 NRS data for Barn Owl over the period 1980-2005 were also used to calculate laying date, clutch size and brood size at individual nests. The results of subsequent analyses supported the findings of the BOMP productivity analysis, demonstrating that brood sizes were larger in rough grassland habitats and during those breeding seasons preceded by drier winters.

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

1.16 The increasing value of BOMP to conservationists is shown by the inclusion of its results in the annual and widely disseminated The State of the UK's Birds 2004 (Eaton et al. 2005) 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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