02 May 2013

CES Starts in 2013

Today, Thursday 2 May, is the start of the Constant Effort Site (CES) season.  Over the next 10 days hundreds of volunteer ringers across Britain and Ireland will be carrying out the first ringing visit of the CES year.  They will set their nets in exactly the same place and spend the same amount of time catching birds as they did last year.  At the end of the summer, after 12 visits, data will be submitted to the BTO. 

At the end of last year, CES data confirmed that 2012 had been the worst breeding season since CES began in 1983.  The number of birds present at the start of the season was very similar to recent years and adult survival, as indicated by the proportion of birds returning from 2011, was around average.  However, productivity, measured as the ratio of juveniles to adults caught, was significantly lower than average for the majority of species.  All eight migrant warblers covered by CES demonstrated significant declines with Blackcap dropping by 62%. Many resident species didn’t fare much better with Blue and Great Tits dropping by 31 and 34% respectively.

Current CES sites - blue dots were new in 2012.

What will 2013 bring? Most of our ringers are hoping for better weather if nothing else. It was
tough finding enough dry, not too windy, days to do CES last year. More importantly there are two
important questions to answer:

* How did the weather in 2012 affect the adults?  There were anecdotal reports of underweight adults last year as they worked hard to feed their young in difficult conditions.  Some also extended their breeding season in an attempt to successfully produce young, which means they may have ended up migrating or moulting later than normal or in poorer condition.  We suspect that this will affect adult survival, but we need this year’s data to prove it.

* How many birds will return in 2013?  Given that relatively few juveniles were produced in 2012, we might expect numbers to drop this year.  However, it’s possible that the drop in juvenile numbers reduced competition during autumn and winter, resulting in a higher than average proportion surviving to breed in 2013.

CES in 2013 should give us an insight into the answers to these questions.

Allison Kew
CES & RAS Organiser

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