Preliminary report on the 2023 breeding season

This report provides a preliminary assessment of the 2023 breeding season in terms of population sizes and breeding success, comparing this year’s results to the averages recorded over the previous five seasons.

The primary aim of BTO surveys is to monitor changes in the health of Britain’s birds, tracking declines and increases via the BTO/JNCC/RSPB Breeding Bird Survey (BBS) and exploring the factors driving them through bird ringing and nest recording. The long-term trends in abundance, survival and breeding success generated by these schemes are presented in the BTO’s Trends Explorer.

How do we monitor the breeding season?

All of the data presented here are collected by BTO volunteer ringers and we are extremely grateful for their efforts, both in collecting the data and submitting it promptly. Numbers of adult birds are monitored by qualified bird ringers running a network of approximately 120 Constant Effort Sites (CES) across Britain & Ireland between May and August. As their effort is standardised annually, the number of birds caught in each year provides an accurate measure of changes in abundance. Recaptures of birds ringed in previous years also allow survival rates to be calculated. The ratio of juvenile to adult birds caught on CESs provides a measure of breeding success that also takes into account the number of successful breeding attempts made per adult (as many species attempt to rear more than one brood per season) and the survival of young birds immediately after fledging.

CES covers 24 woodland, scrub and reedbed species. Not all of the CES data collected in 2023 have been received yet, so these results are based on a subset of sites for which we currently hold sufficient data to analyse.

What was the weather like in 2023?

After a slightly milder and drier than average winter, spring was, overall, warmer than average due to high temperatures in May (Figure 1a). March, although average in terms of temperature, saw high rainfall totals across most of the UK (Figures 1b-d), making it provisionally the sixth wettest March on record. The summer was warm, particularly in June which was the warmest on record. July again saw high rainfall totals, making it the sixth wettest July on record, while two named storms in August brought unseasonably wet and windy weather.

Migratory species

The 2023 breeding season proved to be a fairly average year for migratory species. Of the eight migrant warblers monitored through CES, only Chiffchaff recorded a significant increase in abundance compared to the previous five-year mean (2018–22) while only Blackcap recorded a significant decrease. Both species are short-distance migrants for whom population trends, as collected by BBS participants, show long-term increases. The increase in Chiffchaff abundance was driven by results in the north and east of the UK, while the decrease in Blackcap numbers was driven by results in the east and west. Chiffchaff is one of three migrant species to record a significant increase in adult overwinter survival from 2022 to 2023, which is likely to have been part of the reason for the high numbers seen on CE sites last year. Adult survival figures for Blackcap were not significant, suggesting this was not the mechanism for their decrease in 2023. The species recorded significantly high productivity in 2022, but the low abundance figures in 2023 suggest recruitment of those juveniles into the breeding population was not high.

Two migrant species recorded significant changes in productivity in 2023 compared to the five-year mean: Garden Warbler (+43%), driven by results in the north and west of the UK, and Sedge Warbler (-24). Neither species recorded significant changes in abundance or their overwinter survival rate so it is difficult to assess the driver behind these results without additional information from the Nest Record Scheme, which was not available at the time of writing. The other two migrant warblers to record increases in adult overwinter survival were Willow Warbler and Whitethroat. Both species recorded significantly lower-than-average abundance in the 2022 breeding season (but no significant changes in productivity), so it is possible that the smaller population numbers led to reduced competition for overwinter resources, which boosted overwinter survival.

Resident species

Only two resident species recorded a significant change in abundance in 2023 compared to the five-year mean (2018–22): Bullfinch and Goldfinch. Bullfinch abundance declined for the third consecutive year, despite high adult overwinter survival and high productivity in 2022, suggesting poor recruitment of juvenile birds might be driving this decrease. The BBS trend for Bullfinch also shows a recent population decline and the BirdTrack recording rate for 2023 was also well below average. The cause of the decline in Goldfinch numbers is less clear, although the BBS trend for this species also shows a slight decline in recent years.

Data collected on CE sites suggest that it was an average breeding season in terms of productivity for resident species, with none recording a significant increase or decrease compared to the five-year mean. In addition to Bullfinch, the adult overwinter survival rate for Blackbird and Chaffinch also increased in 2023; no species recorded a decline in survival. It is likely that the mild winter was instrumental in all three of these species surviving the winter in good numbers. None of the tit species recorded any significant changes in any of the parameters measured through CES in 2023.

Join us in monitoring birds in 2024

If you are a qualified ringer with access to an area of scrub, woodland or reedbed where there is the potential to catch at least 250 birds per season, why not register a CES? And if you aren’t able to start your own site, why not consider helping out at an existing one? Contact the CES Organiser for more information. We are currently undertaking a trial of CES in garden habitats (although the results don’t yet feed into these analyses). If you are interested in participating in the trial, the CES Organiser can provide further details.

Acknowledgements

We are extremely grateful to all Constant Effort Site ringers for their monitoring efforts and for the support of the BTO/JNCC partnership, which the JNCC undertakes on behalf of the Country Agencies. Additional funding for the BTO Ringing Scheme is provided by the National Parks and Wildlife Service (Ireland) and the ringers themselves. The Breeding Bird Survey is run by the BTO and jointly funded by the BTO, the JNCC and the RSPB.