31 May 2013

Marsh Tit population crash

The NERC Centre for Ecology & Hydrology (CEH) Marsh Tit project in west Cambridgeshire was outlined in a previous Demog Blog post in 2010, and is now in its 10th year. But after a decade of stability 2012-13 has seen a dramatic fall in the population, and ringing has enabled us to understand how and why this has happened.

Almost all of the Marsh Tits in the main study population are colour-ringed, and the number of territories in the four main study woods has typically been around 30-32 each spring, with about 15% of those being occupied by lone males. But in 2013 the number of occupied territories has fallen by a third to just 19, and more than a quarter of those did not have a female present.

Adult survival from the previous spring was only 44%, which was among the lowest recorded since the study began and some way short of the average 51%, but this alone could not explain the sudden drop in population. The main problem was juvenile recruitment, linked to the very poor weather during the breeding season in May 2012. In line with results for other tits shown by the Nest Record Scheme (click here for 2012 premilinary results), the Marsh Tits also seemed to struggle to rear young during 2012 as persistent rain hampered foraging. Things were so bad that we had the first ever record of adults abandoning a nest mid-way through incubation.

Reduced productivity, and probably also reduced fledgling survival, was apparent in the much reduced number of juveniles recruited into the population during our autumn trapping period, when I catch and colour-ring most of the new birds. During a typical year I expect to find 50-60 juveniles in the four woods between August to October, but in autumn 2012 the number was just 21. With so little recruitment on top of lower than average adult survival, there were not enough juveniles to replace adult mortality and maintain the breeding population in 2013. The gender imbalance in the surviving birds indicated that mortality was especially severe among females, which are at the bottom of the pecking order.

That extreme weather and a single poor breeding season could almost halve our breeding pairs was quite worrying. Marsh Tit is a Red-listed species that has declined nationally by 72% since the 1960s, and our study shows that random events such as the very wet spring 2012 can be a severe blow to fragmented populations of birds that may already be struggling. With 2013 shaping up to be another relatively poor breeding season for tits, the previously large and stable population of Marsh Tits in the Cambridgeshire study may take years to recover, if it ever does.

Thanks to Richard Broughton for writing this post and for the photos.

20 May 2013

Tawny having a tough time

The nests of Barn and Tawny Owl have been recorded in Lincolnshire and Nottinghamshire intensively since the mid 1980s. This has provided a wealth of information, which is used to look at many aspects of breeding success.

Adult Tawny Owl by Carol Greig

Adrian Blackburn and Jim Lennon, along with help from other ringers, checked 82 Tawny Owl nest boxes on 21-24 April 2013 in East Lincolnshire, and recorded an occupancy rate of just 17% (14 boxes). Squirrel, Jackdaw, Stock Dove and Great Tit were also recorded. In total in 2013, just 17 chicks were ringed from seven broods; half of these were provided by two surprisingly large broods of four chicks, with and one other containing three young while the remainder were the rest consisted of single nestlings or pairs of owlets. This is much lower than the average number of 36 ringed young recorded over the period 1996-2012 and far below the highest annual total ever, an impressive 62 chicks ringed in 2005.

There were still two nests with eggs on the last visit could potentially produce more chicks but are very unlikely too at this stage; the other five had failed either during incubation or soon after the chicks had hatched.  We posted previously about the hard time Barn Owls were having due to the cold spring, so finding that Tawny Owls were also having a hard time was not a great surprise.

Some reasonably healthy Tawny Owl chicks by Carol Greig
There is evidence of a lack of prey from the boxes with fewer voles and mice being found, these being replaced with more unusual items, including the hind leg of a hare, a Carrion Crow’s head and a few shrews. It is notable that the boxes in ‘prime’ deciduous woodland habitat, which usually produce the majority of offspring, fared worst, with those in suburban gardens achieving a much higher output. Could it be that small mammal populations in woods have become depleted during months of cold weather and poor vegetation growth, while garden rodent populations thrive on the artificial food provided for birds?  The situation is probably not helped by the breeding season for song birds being delayed by several weeks, potentially reducing the availability of young birds as prey.

Thanks to Adrian Blackburn and Jim Lennon for letting us know.

10 May 2013

Colourful Garganey

The Garganey is unique among British ducks, being a summer visitor to these islands from its wintering grounds in central Africa. It is estimated that we only have around 86 pairs in Britain so obviously the ringing totals for this species are very low. Last year, only three Garganey were ringed and the previous ones to that were in 2007. Hearing about a Garganey after it's been ringed is probably not going to be a happy story for the Garganey. 

A quick look at what we know about what happens to these birds on the ringing database show:
  • one bird controlled by a ringer 106km from the ringing site
  • five re-caught by a ringer at the place of ringing between 1962-74
  • two birds caught by an owl or raptor
  • three found dead with no obvious cause of death
  • seventy one shot or 'hunted'
  • eight completely unknown i.e. alive or dead?

 The current longevity record is 14 years 6 months and this bird's life was cut short in Mali (shot).

Drake Garganey - by Russel Slack

However, this is a happy story. Garganey EX75514 was ringed last year and migrated south to southern Europe or Africa and has just returned to where it was ringed at Wheldrake Ings, North Yorkshire.

To find out more information about these birds, colour ringing is being used to increase the amount of 'alive' reports and look into their survival and site fidelity. Hopefully this bird will be sighted for many years to come.

Thanks to Craig Ralston for letting us know and for more information about this bird, click here for the Lower Derwent Valley NNR blog.

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