Optimisation of Open-habitat Bird Surveys – a report to the Forestry Commission

In the process of offering financial grants to landowners seeking to create new areas of woodland in England, the Forestry Commission (FC) requires surveying of proposed woodland creation sites for breeding birds in order to assess the suitability of sites for woodland creation. FC commissioned a report to analyse existing breeding bird survey data in the interest of determining optimal timing and number of visits required for breeding bird surveys, specifically investigating the effect of reducing survey effort while aiming to maintain survey efficacy. We aimed to investigate the efficacy of a reduced number of surveys at prescribed timings in identifying species richness within a proposed woodland creation site and estimating the density of breeding bird territories, simulating the impact of reduced survey visits at specific timings as per new FC guidance. Reports of breeding bird surveys in England in 2021 and 2022 were initially assessed on their compliance with FC guidance for surveys at woodland proposal sites, quantified based on criteria met by surveys. Species richness analysis sought to compare the number of species detected after five or six survey visits spread across the breeding season (as per FC guidance for 2021 and 2022) with the simulated number of species detected after four randomised visits and four visits following new guidance survey date windows. The proportions of simulated species richness estimates after four randomised visits and four visits within new survey windows relative to species richness after six visits were compared to assess the efficacy of random spacing of surveys against surveys specifically spaced within new survey windows. Estimates of territory densities were determined by assessing mapped survey records to estimate territories within sites for seven bird species of conservation importance and with sufficient data for analysis, comparing estimates derived from six survey visits to mean estimates derived from comparing all four-visit combinations of the six survey visits. Species richness reduced by a mean of 10% with four randomised survey visits and 11% with four visits within the new survey windows, relative to species richness after six survey visits. There was no significant difference between the two methods of four-visit selection, with each method producing similar results. Territory density was found to decrease by a mean of 33% when reducing survey visits from six to four, approximately matching the reduction in survey effort of one third. The proportion of territories still detected after four visits varied widely between species (20-100%) but for the two wader species analysed, curlew Numenius arquata and lapwing Vanellus vanellus, the estimates only dropped just below key thresholds for additional site assessment on two occasions.Overall, our data suggest that the proposed change from six to four visits would have a relatively minor impact on species richness detected relative to the reduction in survey effort. The change in breeding territory density for species considered is predicted to be more substantial, but given the number of visits will still be higher than some widely used wader survey methods, and that in most cases the reduction would not have caused a change in decision-making, this is likely to remain a robust survey approach. The proposed use of defined consecutive survey windows is likely to provide a more even distribution of surveys across the season. Our analyses give a potential framework for future analysis to assess future changes in guidance for breeding bird surveys at woodland creation sites, and to simulate the effects of proposed changes to guidance prior to implementation using existing data