Capping off a busy breeding season

Over the last couple of weeks we have been hearing of large numbers of Blackcaps being ringed (www.bto.org/ringing), predominantly at sites in western and central parts of the country. Ringing migrants on passage provides a reliable indicator of the number of birds moving through an area and helps to improve our knowledge of migration routes and the rates at which birds move. British & Irish breeders are known to migrate south through the country between July and September whilst continental birds wintering here show north-westerly movements across Britain & Ireland during September and October (Wernham et al 2002). The stories below appear to show a southerly movement of Blackcap suggesting that the majority of these birds are UK breeders (migrant birds tend to travel down the eastern and southern coasts (Wernham et al 2002)). The large numbers of birds encountered may be an indicator of a successful breeding season, a welcome event for a species that experienced its worst breeding season on record in 2012. but may also result from a more synchronous movement, with a higher than average proportion of the population travelling simultaneously. The ratio of juvenile to adult birds caught could help us to determine which explanation is more likely.

Despite the poor breeding season in 2012, nationally, Blackcap numbers are increasing and their range is expanding northwards. Breeding Bird Survey data show that Blackcap has increased in abundance by 137% over the past 20 years (download the latest BBS report from www.bto.org/bbs). Data from another BTO survey, Garden BirdWatch (www.bto.org/gbw) show that our migrant Blackcaps are currently moving out of gardens. The data suggest they won’t return in any numbers until the wintering populations move into gardens in November. 

Female Blackcap. Photograph by Liz Cutting.

Peter Fearon rings in North Liverpool and writes:

We ring at a relatively urban site in North Liverpool - Rimrose Valley Country Park - the site is a long, thin country park separating different housing estates. This former rubbish dump has grown to be a diverse habitat for breeding and migrant birds, especially Blackcap. This summer, it wasn't until Sunday 31 August that I was able to get out with my reliable band of trainees to ring at one of our sites on Rimrose. We were rewarded with our biggest catch at the site of over 130 birds ringed, with 52 of these being Blackcap. We were able to set up another session the following Thursday and managed to catch a further 35 Blackcap, only one being retrapped from the previous session indicating a significant turnover. The following weekend (6 & 7 September) however, it was a different matter with two sessions completed but only 25 Blackcap ringed. It would seem that there was a significant movement of Blackcap in this week and it was a shame that the weather prevented us from surveying their movement through some of our other sites. I think that it is interesting to note that we had ripened blackberries in mid-July and most of our sites are now devoid of any soft fruit other than the elderberries.

Mick Townsend rings at Stanford Reservoir on the Leicestershire-Northamptonshire border and writes:

We are just about as far from the coast as you can get, yet we have just experienced the most extraordinary event in the 39 years of ringing at the site. On Monday 1 September the weather forecast was not promising at all with rain due about 10am so I decided to go for a 5am start. I erected six nets in the hope of a few warblers, Blackcaps and Chiffchaff being my target. The rain came early and by 8:30am I was taking the nets down with only 46 birds caught, not a great morning but I at least caught nine Blackcaps. Tuesday dawned and I was full of hope for a much better day and I was at the reservoir for 3:45am. As the time arrived for the first net round I could hear Blackcaps ticking away in the bushes so off we set. The first round total was 163 birds caught, 96 being Blackcaps. The rest of the day continued in the same vain with lots of Blackcaps caught in each round of the nets. We finished on 331 birds caught and of these 298 were new, 214 being Blackcaps. The following day saw another 198 birds caught, 118 of which were Blackcaps. This time we had a control and the BTO tell me the ring was issued to a ringer in Doncaster. The fourth day was a quieter day all round with only 96 birds caught, 47 of which were Blackcaps. The Blackcap rush didn’t stop there though and on the 6 September, we caught 119, on the 8 September we caught 150, on the 9 Sept we caught 101, on the 11 September we caught 111 and on the 16 September we caught 94. At this time of year we usually catch fewer than 30 a session! Blackcap numbers for the month so far are a staggering 994, we have only managed just over 700 for the year in the past.

Site locations - pink: Rimrose Valley Country Park, turquoise: Stanford
Reservoir, green: Swindon STW, orange: Salisbury Plain
Map tile courtesy of National Park Service.

Matt Prior rings on Salisbury Plain and writes:

The North Wilts RG has two teams led by Matt Prior and Graham Deacon. The Salisbury Plain is an area of grassland the size of the Isle of Wight but it has dotted patches of scrub that are very important for migrating warblers on their way back to their wintering grounds in southern Europe and Africa. We have ringed much higher numbers of Blackcaps than ever before suggesting that they have had a very good breeding season. Over the years the Blackcaps that we ring tend to get recovered along the south coast between Sussex and Devon. Blackcap totals are as follows: 24 August – 147, 31 August – 245, 6 September – 160, 7 September – 192, 8 September – 194, 10 September – 161, 13 September – 99 making a grand total of 1198! A further 309 Blackcaps have been ringed in just two ringing sessions at Swindon STW, a 9 hectare nature reserve owned by Thames Water. This compares with two sessions in the same period at the same site in 2013 when 104 Blackcaps were ringed. This year, we recorded Blackcaps breeding in a wide variety of locations where we wouldn’t normally find them such as in tiny areas of scrub on farmland.

To date, we have not heard any similar stories from the east of the country; feel free to leave a comment if you have anything further to add to the story!

Wernham, C.V., Toms, M.P., Marchant, J.H., Clark, J.A., Siriwardena, G.M. & Baillie, S.R. (eds) (2002) The Migration Atlas: Movements of the birds of Britain and Ireland. T. & A.D. Poyser, London.