NRS Preliminary Results 2013
The Nest Record Scheme is the primary source of information on the nesting success of Britain and Ireland’s birds. Each year, 600 volunteer nest recorders locate and monitor 35 - 40,000 nests of over 150 species, visiting regularly to record the numbers of eggs and nestlings. Click here for more information about the survey methods.
It would not be possible to produce the results presented here were it not for the fantastic efforts of our network of nest recorders and their speedy submission of data at the end of the season. The figures below use the NRS data submitted by the end of October to compare productivity of 26 species in 2013 with the average values over the preceding five years (2008-12). A full analysis of long-term trends in breeding success featuring a larger range of species will be published in the BirdTrends Report once the complete dataset has been received and analysed.
Exceptionally cold start to 2013 season
Thanks in part to persistent easterly and north-easterly winds, temperatures between March and June 2013 were up to four degrees below average (Figure 1), resulting in a very late spring. By the end of April, many trees were still bare, Blackthorn blossom was nowhere to be seen and, as reported earlier in the year , nest recorders were waiting for birds to start nesting. In contrast to the challenging conditions experienced during the spring, summer temperatures were well above the mean and rainfall totals were well below it.
Figure 1. a) Central England Temperature and b) England & Wales Precipitation indices show that a cold start to the 2013
season was followed by a warm, dry summer
Timing of breeding normal…for the 1960s
Song Thrush mean laying date was the latest recorded since the first figures in 1966
Unseasonably mild March weather significantly advanced the 2012 breeding season . In 2013, however, early-nesting species, including Blackbird, Song Thrush, Robin, Dunnock and Dipper, layed 8-12 days later than the five-year average (Table 1); first egg dates for both of the thrushes were the latest since NRS trends began in 1966 (Figure 2). This trend continued throughout the spring as cold weather carried on into May; laying dates of Jackdaw, Chaffinch and Linnet were 5-11 days later than average and Blue and Great Tit both delayed egg production by 12 days. For some species, mean first egg dates in 2013 were comparable with those exhibited in the 1960s (Figure 2).
BirdTrack data indicate that short-distance migrants Chiffchaff and Blackcap arrived in the UK several weeks behind schedule and laying dates of the former showed a comparable shift (the Blackcap sample size was small due to a decline in abundance, possibly related to the extremely poor breeding season in 2012). While the arrival times of long-distance migrants wintering south of the Sahara were generally close to the mean, the majority still delayed laying, with dates of clutch initiation 4-11 days later than average (Table 1).
Other resident passerines
Owls and raptors
A truncated season for early breeders?
In contrast to the results generated using Constant Effort Site (CES) ringing data, which provide an independent measure of breeding success using the ratio of juvenile to adult birds captured, NRS data indicated that the number of chicks fledged per nest was similar to the average for both Blackbird and Song Thrush (Table 1). This discrepancy suggests that a fall in the number of breeding attempts per pair, rather than the output per attempt, may have been responsible for reduced fledgling numbers recorded at CES sites in 2013; a truncated season might be predicted following the late start and the hot, dry conditions at the end of the season, which are likely to negatively affect food availability for species dependent on soil and leaf-litter invertebrates.
Conversely, the increase in Chiffchaff, Willow Warbler and Whitethroat breeding success identified on CES sites was not apparent in the NRS dataset (Table 1); it is possible that the warm weather in the latter half of the season may have encouraged a greater number of these facultatively double-brooded species to produce repeat clutches. The decline in Reed Warbler fledgling production identified by NRS was not reflected in the CES results; this may represent a regional difference as NRS data were all from the south of England, where ringing data suggest that productivity was lowest.
Figure 2. Annual average laying dates from 1966 to 2013 for a) Chiffchaff, b) Blue Tit, c) Song thrush and d) Blackbird; laying for all
species was very significantly delayed in 2013.
Caterpillar specialists struggle
Breeding success of Blue Tit, Great Tit and the migrant Pied Flycatcher all fledged significantly fewer chicks in 2013, with numbers per nest falling by 7%, 12% and 8% respectively relative to the five-year mean (Table 1). Low spring temperatures are generally beneficial for these species as they delay caterpillar emergence and development, increasing food availability during the nestling period; in recent warm years, there has been a tendency for invertebrate numbers to peak while birds are still on eggs. The poor breeding performance in 2013 was largely due to a reduction in clutch sizes, however, which suggests that the cold weather prior to laying may have had a negative impact on female condition and therefore on the average number of eggs laid (Figure 3).
Tree Sparrow clutch sizes were also lower than average in 2013, again suggesting that female condition may have been affected by the cold weather. The increase in mortality at the nestling stage is suggestive of issues with food availability, and the net result of these two factors was a significant decline in the number of fledglings produced per attempt (Table 1).
Figure 3. Clutch sizes of a) Great Tit and b) Blue Tit were significantly lower than average in 2013, possibly due to a decline in
female condition as a result of the low spring temperatures
Owls notable by their absence
Barn Owl is one of the species most frequently recorded by NRS participants, with around 2000 nests monitored per annum. The number of submissions in 2013 currently stands at about 25% of this total and a reduction of similar magnitude is apparent for Tawny Owl. Volunteers specialising in the recording of box nesting owls and raptors alerted BTO to sharp declines in occupancy rates at the start of the year. Longer-lived species may suspend breeding for a year if females are unable to attain sufficient body condition, and the cold conditions and the start of the season, coupled with low vole numbers, may mean that many pairs elected not to breed. Adult survival may also have been negatively affected and there is some evidence to suggest that the recent run of cold winters and poor breeding seasons may have led to a drop in Barn Owl numbers following a long period of population growth.
Help us to monitor breeding success in 2014!
The records used to produce these results our generated by over 600 NRS volunteers, who monitor nests ranging from Blue Tit boxes in gardens to seabird colonies on cliffs. If you haven't tried nest recording before, why not give it a go? Email us (nrs [at] bto.org) for a Quickstart Guide or visit the NRS web pages to find out more.
We’re extremely grateful to all Nest Record Scheme participants and Constant Effort Site ringers, and for the support of the JNCC/BTO partnership that the JNCC undertakes on behalf of the Country Agencies. The BTO Ringing Scheme is funded by the BTO/JNCC Partnership, The National Parks and Wildlife Service (Ireland) and the ringers themselves. BirdTrack is organised by the BTO for the BTO, RSPB, BirdWatch Ireland, SOC and WOS.