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 Michele Leveque-Shaw

Adrian Blackburn and Jim Lennon, along with help from other ringers, checked 82 Tawny Owl nest boxes on 21-22 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. This is very low compared to the average occupancy rate of Tawny Owl since 1996 (36 occupied boxes), and the maximum count of 62 occupied boxes in 2005.

Another nest check was done on 17 May, the number of chicks that had hatched and grown to a size that could be ringed was very small (17 owlets). There were still 10 nests with eggs that may, or more likely, may not hatch. 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 Michele Leveque-Shaw

There is evidence of a lack of prey from the boxes with fewer voles and mice being found, these being replaced with carrion e.g. the hind leg of a Hare, a Carrion Crows head and a few Shrews. Deciduous woodland seems to be particularly effected with more successful nests being in previously less optimum habitats. This is probably not helped by the breeding season for song birds being delayed by a few weeks - this would also reduce the availability of young birds as prey.

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

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.

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

Wintering Sand Martins benefit from fewer conspecifics

The theme of this Blog (Demography - "The study of the characteristics of populations, such as size, growth, density, distribution, and vital statistics") certainly applies to David Norman’s long-running study of Sand Martins breeding in mid-Cheshire. In more than 200 mist-netting sessions over 23 years (1981-2003), he ringed 6,053 adult Sand Martins and recaptured 1,179 of them in a subsequent year, many of them multiple times for a total of over 10,000 handlings of adult birds.

Mist netting - David Norman

This long run of data with large sample sizes, and annual assessments of the breeding population from counting nest-holes, allowed a detailed analysis, jointly with Will Peach, which has recently been published in Ibis. The details are in the paper (click here for a copy or e-mail david@davidnorman.org.uk) but in outline the work shows …

1. The annual survival of adult Sand Martins averages around 35%, varying from as low as 10% to a high over 60%, mostly determined by the rainfall in their Sahel wintering quarters. There are fewer insects in the dry years and more birds starve. This is already well-known for several trans-Saharan migrants but this study also showed that the effect is non-linear: above a certain level of African rainfall the birds’ survival flattens off, limited by mortality elsewhere in the life cycle.

2. In this study, there was no effect of summer weather (temperature or rainfall) on adult survival.

3. The size of the breeding population is mostly determined by the survival and return of adults, and much less by recruitment of new birds (one-year-old first-time breeders and immigrants from elsewhere).

Sand Martin - Lawrence G Baxter

 4.  This is the first study to show that overwinter survival in the Sahelian winter quarters is density-dependent. Thus, if the population is high, there is more competition for insect food and more Sand Martins die; if the population is low, even in a dry year there is more food to go round and more martins survive and return to the breeding grounds. 'Population' here means the winter population of all western European Sand Martins, which mix in the Sahel during winter.

5. The recruitment rate of first-time breeding adults was also density-dependent. More un ringed ‘new’ adults were captured during summers when local colony size in Cheshire was relatively small, and vice versa. Competition amongst breeding pairs for nesting sites or food might have caused this pattern.

Sand Martin - John Harding

David and Will comment that such density dependence was suspected from previous studies on other species but required a vast amount of fieldwork and statistical analysis to prove it. Such competition for insect prey may apply to other insectivorous migrants that rely on the seasonal flooding of wetlands across the Sahel zone. Better rains in recent years have allowed breeding populations of several Sahel-dependent species to increase in recent years, but the threat of drought continues to hang over the people and the wildlife of the region.

Thanks to David Norman for letting us know.

Tough times for sea birds

As you heard in a previous post, we have been receiving large numbers of reports of dead or dying Barn Owls. What we didn't mention is that we are also receiving reports of large numbers of dead seabirds on the east coast of Scotland and northern England but also as far south as East Anglia.

The majority of these birds have been found in the Firth of Forth, Scotland, and this seabird wreck has occurred at a time when these birds should be getting into prime condition for the breeding season, or even nesting now. High winds and 'uncomfortable' sea conditions are thought to be the prime cause, making finding food difficult.

Colour ringed Shag - Sarah Featherstone

The majority of reports of ringed birds have been Shag but have also included a wide variety of species, including Puffin, Razorbill, Guillemot, Kittiwake, Cormorant and even Little Auk. The graphs below show the 10 year average of reports (blue) against the number of reports so far this year.

 

You can clearly see the huge difference between early and late March. The number in early April is slightly less than late March and early indications show that late April will have many fewer reports of dead shags, as long as the trend continues.

 We receive very few recoveries of dead Puffins generally during the winter period but this year in late March we can see evidence of the wreck.

 

Strangely, Guillemots have fared very well, considering the previous two species, with a lower than average report rate. Although there have been large numbers of Guillemots washed up in Cornwall and Devon from a synthetic chemical called polyisobutene. Surprisingly only nine dead and two live Guillemots (which are yet to show in the graph above) have been found ringed out of a minimum estimate of 1500 dead birds.