Abstract from BTO Research Report No 294:

L.P. Beaven, D.I. Leech, C.R. Shawyer, D.E. Balmer & H.Q.P. Crick

The BTO Barn Owl Monitoring Programme: 2nd Report: Second Year 2001 ISBN 1-902576-55-1

Executive summary

1. This is the second report of the BTO Barn Owl Monitoring Programme, set up 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. The Wildlife Conservation Partnership (WCP) has undertaken the development of the methodology to be used in the Programme and has carried out fieldwork at a set of core sites, distributed equally throughout five regions of England and matched for nest box design. A network of volunteer ornithologists has started to gather additional information over a wider geographical area using carefully designed protocols.

3. The general approach to fieldwork involves repeat visits to registered sites, particularly to paired nest boxes, over the Barn Owl nesting season between April and October.

4. The Monitoring Programme is being carried out at three levels:

i) Primary information gathering carried out with minimal disturbance to Barn Owls, which includes the recording of site details, site occupancy, fledging success and any second breeding attempts.
ii) Breeding performance recording, involving visits to the nest to record clutch size, hatching success, brood size, age of young, losses of young, prey stored in the nest and laying dates.
iii) Qualified ringers are being encouraged to ring and measure young and adult birds at the nest to provide information on condition, survival and movements.

WCP is carrying out a protocol of egg measurements to allow the estimation of laying dates, to establish the value of this method for determining laying dates.

5. Access restrictions, caused by the Foot and Mouth crisis, meant that it was not possible to visit a number of sites during 2001. However, the WCP managed to record information from 122 of the set of 125 core nest sites (designated for priority annual monitoring). Data was also collected at some additional sites, bringing the total number of sites monitored during 2001 to 168, 16 more than were visited during the 2000 breeding season. Approximately half of the sites were on tilled agricultural land and a third on mixed grass/tilled land. The majority of the remainder were on unimproved pastoral land or in rural areas. The proportion of survey sites being monitored in pastoral areas was reduced in 2001 due to the Foot and Mouth Disease outbreak.

6. Breeding Barn Owls occupied 61% of sites, a decrease of 22% from 2000, possibly due to the influence of flooding during autumn 2000 on small mammal abundance. Occupancy rates were greatest in arable areas, possibly due to the relatively high quality foraging habitat provided by the vegetated ditches and field margins found in this habitat. Other species utilising Barn Owl nest boxes included Kestrel, Stock Dove and Jackdaw. The number of these species breeding in Barn Owl boxes increased markedly in 2001, possibly due to the decrease in Barn Owl occupancy rates.

7. Mean clutch size at all WCP sites was 4.6 eggs and the mean brood size near fledging (excluding total nest failures) was 2.9 chicks. There was some evidence that hatching success decreased during the 2001 breeding season, although this decrease was not statistically significant. A reduction in the abundance of small mammals may have led to a decrease in parental condition, or a reduction in the mean quality of individuals attempting to breed due to over-winter mortality.

8. No evidence was found to suggest that Barn Owl breeding success differed significantly between arable and pastoral areas. There was some indication that productivity was related to nest box design, but closer inspection of the data revealed that this result may well have been due to a regional bias in the distribution of ‘square’ design boxes at WCP non-core sites.

9. More than 70 volunteer ringers or nest recorders expressed an interest in taking part in the Monitoring Programme. Unfortunately Foot and Mouth Disease severely restricted access to fieldwork sites during 2001, and the data supplied by volunteers were very limited because of this.

10. Fieldwork by WCP in 2001 also looked into methods for detecting second breeding attempts and this will continue in 2002.

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