Citation
Overview
Hedgerows form an important semi-natural habitat for birds and other wildlife in English farmland landscapes, in addition to providing other benefits to farming. Hedgerows are currently maintained through annual or multi-annual cutting cycles, the timing of which could have consequences for hedgerow-breeding birds.
The aim of this report is to assess the impacts on nesting birds should the duration of the management period be changed, by quantifying the length of the current breeding season for 15 species of songbird likely to nest in farmland hedges.
These species are Blackbird, Blackcap, Bullfinch, Chaffinch, Dunnock, Garden Warbler, Goldfinch, Greenfinch, Linnet, Long-tailed Tit, Robin, Song Thrush, Whitethroat, Wren and Yellowhammer.
In more detail
Two BTO long-term volunteer monitoring datasets provide information on breeding phenology: the Nest Record Scheme (NRS), for which participants record nest histories, and the Ringing Scheme, where laying dates can be inferred from the timing of ringing of nestlings. Comparison of estimates generated using data from the two schemes indicated that the NRS dataset was likely to provide more robust results for this analysis.
Laying dates were back-calculated for individual breeding attempts monitored between 2011 and 2021 (omitting 2020) utilising standard NRS analytical methodology. Published information about the length of different stages of the breeding cycle was then used to project the timing of the clutch, brood and dependent fledgling periods for each individual nest.
These individual projections were then used to build a composite distribution of nesting phenology for each species across all years; the degree to which this distribution overlaps with four hedgerow management scenarios, specifically i) management permitted from 1st August, ii) from 15th August, iii) as currently, from 1st September and iv) from 15th September, could then be assessed.
Nests of 14/15 species still contained eggs or nestlings after 31st July, with the proportion of the annual nest total active at this time highest for Yellowhammer (26%), Bullfinch (10%), Goldfinch (9.9%) and Linnet (9.5%). If active attempts (i.e. those with active nests or dependent fledglings) were considered, these figures rose to 38%, 26%, 16% and 16% respectively.
Nests of 11/15 species were still active after 14th August, with the proportion of the annual nest total active at this time highest for Yellowhammer (12%); the respective figure for all other species fell below 10%. The figure for Yellowhammer rose to 28% when attempts with fledged but still dependent young were considered, and exceeded 10% for two other species: Bullfinch (15%) and Linnet (10%).
Nests of 6/15 species were still active after 31st August, and therefore potentially at risk under the current regulatory scenario, but the proportion of the annual nest total active in this period did not exceed 1.5% for any species. While 12/15 species were still feeding dependent fledglings at this stage of the season, the proportion of annual attempts that were active in this period did not exceed 10% for any species.
Over the 10 years of data analysed, annual variation exhibited in the 95th percentile of the laying date distribution for each species was no greater than +/- one week, indicating limited annual variability in the end of the season over the medium term.
Latitudinal and altitudinal variation in the 95th percentile of the laying distribution was of similar magnitude (1—2 weeks) and directionally inconsistent, noting that the power to detect impacts was limited by the relatively small sample sizes at more northerly and more elevated sites.
Variability between dominant farmland land-use types was apparently greater, with a two-week difference between 95th percentile estimates generated from subsets of data originating from arable- and grassland-dominated landscapes, but a more robust multivariate analysis would be needed to determine whether habitat type was the driver or whether there were confounding variables at play.
Estimates of the end of the season presented here should be treated as conservative due to a combination of observer fatigue and seasonal decreases in nest detectability.
Staff author(s)
This project was funded by Defra and we are grateful to Leanne Joyce, Daria Dadam, Christopher Thornton, Alex Gribben, Antonia, Hodgeson, Sam Turnock and Rebecca Pringle for helpful comments and suggestions on earlier drafts of this report.
We are especially grateful to the thousands of volunteer nest recorders and ringersnwho, over the years, have contributed their records and to all the landowners and managers who allowed ringing and nest recording to take place on their land. The NRS is funded under the Joint Nature Conservation Committee (JNCC)/BTO partnership that the JNCC undertakes on behalf of Natural England, NatureScot, Natural Resources Wales and the Northern Ireland Environment Agency. The BTO Ringing Scheme is funded by a partnership of the British Trust for Ornithology, the Joint Nature Conservation Committee (on behalf of Natural England, NatureScot, Natural Resources Wales, and the Northern Ireland Environment Agency), The National Parks and Wildlife Service (Ireland) and the ringers themselves. We thank Carl Barimore and Bridget Griffin for the initial extraction and processing of the NRS and ringing data respectively.
