Producing population estimates for Little Egret

A paper in the January 2026 issue of British Birds has described a new approach to estimating the breeding population of Little Egrets in the UK, using data from the BTO’s Heronries Census and the RBBP. In this blog Ian Woodward, Senior Research Ecologist in the Wetland and Marine Research team at the BTO, tells us more about this work and its implications.

The ongoing population growth and range expansion of Little Egret (and more recently other egrets and colonial bird species) has been welcome to UK birders in recent years, creating additional interest and excitement for those visiting wetland sites. As it has spread, Little Egret has also been noticed more frequently by the non-birding public, as they are sometimes happy to forage along apparently uninspiring concrete stretches of rivers and streams in urban areas, where their all-white plumage makes them highly noticeable.

The RBBP excels at monitoring colonising species when they first reach our shores, and rarer breeding species are particularly well-monitored. Although there are exceptions, initial breeding records of species arriving in the UK often occur at prime sites where habitat is managed to benefit wildlife, and consequently these sites are usually very well-watched by birdwatchers and reserve staff. In the case of Little Egret, the first record was at Brownsea Island, Poole Harbour in 1996. Whether colonising birds are nesting at these ‘top’ sites or elsewhere, they also have a novelty value: observers are very aware of the importance of recording nesting attempts by the rarest species and will make special effort to sensitively monitor nests.

As successful species become more widespread, they start to become less novel and observers will therefore be less likely to spend time carefully monitoring and counting nests. At the same time, they also begin to colonise sites across the wider countryside where they will be less likely to be detected by birdwatchers. Both these factors mean that breeding numbers are likely to be under-reported for many of the more widespread RBBP species. A paper by Stroud et al. (2023) highlighted this problem: the more abundant a species is (i.e. ‘scarce’ or ‘less scarce’), the more likely the coverage will be classed as either ‘Moderate’ or ‘Low’.

Little Egret, Colin Bradshaw

For Little Egret, we are fortunate to be able to report ‘High’ confidence in the numbers counted through the RBBP annually. This is in part because the problems noted above that lead to under-reporting are mitigated for Little Egret through a national survey, the British Trust for Ornithology’s (BTO) Heronries Census. The Census has been running for nearly a hundred years and provides an existing mechanism for counting and reporting Little Egret nests, especially when they are present within heronries.

Heronries Census data are routinely shared with the RBBP, increasing the coverage for Little Egret and helping to ensure that the RBBP coverage can be considered ‘High’. However, there are still known gaps in Heronries Census coverage and hence the RBBP coverage cannot be assessed as being at an even higher level (‘Near complete’).

The figure, taken from the British Birds paper, shows the Little Egret population trend as estimated using raw counts, and from modelled outputs using Heronries Census (HC) and RBBP data.

For Grey Heron, the known gaps in Heronries Census coverage are accounted for by using a statistical modelling approach to produce annual population estimates, and we have now developed a similar model for Little Egret. This allows us to produce population estimates and hence assess the extent to which breeding is under-reported within the RBBP report; a paper describing this work has just been published in the January issue of British Birds. Our model included both Heronries Census data and RBBP data (as there are a small proportion of Little Egret nests that are currently only reported to the RBBP and hence are not available to the Census). For 2022, this model produced an estimate of 2,236 pairs of Little Egret (85% confidence intervals: 2,100–2,484) using maximum counts from either the Census or the RBBP data. This estimate is for the UK and its Crown Dependencies (i.e. Channel Islands and the Isle of Man, although there are as yet no breeding records from the latter) and is around 36–40% higher than the raw count presented in the RBBP report for that year, suggesting that, even though the Heronries Census helps mitigate under-reporting, many nests still go unreported each year.

Although the population estimate is above the 2,000 threshold above which species are often dropped from the RBBP list, RBBP has decided to continue to include Little Egret in the report presently. At the same time, BTO will now update and publish the combined Census/RBBP population estimate annually in the Heronries Census Summary Report and this population estimate will be reported in the header bar for Little Egret in future RBBP reports.

This analytical approach might be used for other egret species, such as Cattle Egret, if their populations continue to increase in the way that Little Egrets did (photo Mark Eaton).

Although a combined Census/RBBP population estimate will be published in the Census Summary Report, it should be noted that any data from the RBBP that have not been formally submitted to the Heronries Census will only be used to produce the combined estimate and that these records will not be added to the Census database.  To ensure the robustness of future breeding population estimates, we at BTO would welcome the direct submission of Little Egret counts from any sites that are not currently reported to the Census. This will be essential to ensure estimates continue to be accurate if and when Little Egret is no longer included in the RBBP report, but would be valuable even whilst RBBP is still reporting Little Egret, as the RBBP reporting schedule is two years behind the Heronries Census report due to the processing time needed to review all the RBBP records from multiple species received.

Finally, even though the new population estimates indicate breeding numbers may be substantially higher than reported previously using raw counts, there are reasons to suggest that the new population estimates could possibly still be too low. Little Egrets tend to breed later than the year than Grey Herons (with breeding activity peaking in May and June, as opposed to March and April for Grey Herons) and Little Egret nests can be very difficult to detect and count accurately as they are often lower than Grey Heron nests in undergrowth. We would therefore encourage Heronries Census volunteers and other recorders watching heronries to ensure that they carry out later survey visits and, where appropriate, longer more careful observations, so that nesting attempts by Little Egret (and other scarce species) are not overlooked. This will help ensure that both the RBBP report and future population estimates are more complete.

Thank you to everyone who submits records to the Heronries Census or records via their county recorder which feed into RBBP. If you’d like to find out more about the Heronries Census, visit https://www.bto.org/get-involved/volunteer/projects/heronries-census or contact Gill Birtles at heronries@bto.org

Reference – Stroud, D.A., Eaton, M.A., Francis, I.S., Baker, H., Holling, M., Norman, D.R., King, A., Stanbury, A.J., Balmer, D.E. (2023). The Rare Breeding Birds Panel: five decades of monitoring the UK’s rare breeding birds. British Birds. 116: 191-209. https://rbbp.org.uk/wp-content/uploads/2023/10/Stroud-et-al.-2023-RBBP-paper-in-British-Birds-.pdf