Ringing data suggests poor breeding season for passerines

Data collected by bird ringers operating Constant Effort Scheme (CES) Sites for the British Trust for Ornithology (BTO) has shown that 2020 was a poor breeding season for many passerines.

The scheme provides insights into the numbers, breeding success and survival rates of 24 widespread resident and migrant songbirds. The spring was one of the warmest on record but, perhaps counter-intuitively, this was not necessarily good news for all species.

Blue Tit, Keighley, West Yorkshire (Irene Harrison).

It had been suggested that birds may have benefited from the reduction in human activity across most of the British and Irish countryside in the spring and summer. While some birds that are particularly prone to disturbance, such as coastal waders and terns, prospered at some sites, the evidence generated by CES ringers suggests it was not the case for many passerines.    

Lee Barber, the BTO’s Demographic Surveys Officer, said: "In warm springs, caterpillars tend to hatch earlier and develop faster; birds also lay their eggs earlier, but not to the same extent, and so the availability of food peaks before the period when their young need it most, with fewer fledging as a result. The results from our ringers show that the average number of juvenile Blue Tits and Great Tits caught per adult was lower in 2020 than in any other year since the survey began almost 40 years ago."

Garden Birdwatch (GBW) Development Officer Rob Jaques commented: "Participants in the BTO's Garden BirdWatch survey reported Blue and Great Tits from fewer gardens than average during the summer. Data from CES ringers suggest adult numbers were higher than average in 2020, so the drop in GBW observations mostly likely reflects the poor breeding season."

On a more positive note, some of our summer visitors fared well in 2020, with the numbers of some returning migrant birds being well above average. The abundance of Blackcap and Common Chiffchaff was the highest since monitoring began in 1983, and numbers of the declining Willow Warbler were above average.