The primary aim of BTO surveys is to monitor long-term changes in the health of Britain’s birds, tracking declines and increases via the BTO/JNCC/RSPB Breeding Bird Survey and exploring the factors driving them through bird ringing and nest recording. All of this information is presented in the BirdTrends report.
So, how does the 2014 season compare with what’s gone before? This report provides a preliminary assessment of the number of birds that bred this year and the number of young they reared, based on initial analyses of data collected by bird ringers and nest recorders over the spring and summer.
How do we monitor the breeding season?
All of the data presented here are collected by BTO volunteers and we are extremely grateful for their efforts. Numbers of adult birds are determined by qualified bird ringers running a network of over 130 Constant Effort Sites (CES) across Britain & Ireland between May and August. As their effort is standardised annually, the number of birds caught in each year provides an accurate measure of changes in abundance. Recaptures of birds ringed in previous years also allow survival rates to be calculated. The timing of nesting and the number of fledglings reared during each breeding attempt is monitored by over 600 participants in the Nest Record Scheme (NRS), who locate nests and count the eggs and young at regular intervals. The ratio of juvenile to adult birds caught on CES sites provides a second measure of breeding success, which also takes into account the number of successful breeding attempts made per adult (many species attempt to rear more than one brood per season) and the survival of young birds after fledging.
CES covers 24 woodland, scrub and reedbed species, while NRS covers 150 species. Returns from both surveys are still arriving at the BTO, so the figures presented here are representative of a subset of sites and species for which we have sufficient data to analyse.
What was the weather like in 2014?
After two challenging seasons, the settled weather that most of Britain & Ireland experienced in 2014 was a welcome change for CES ringers and nest recorders alike. The year started badly, with flooding experienced across much of the UK thanks to winter rainfall totals that were more than double the average for the preceding five years (Figure 1). However, by March things had dried up and temperatures were a degree or two above the five-year average. The warm weather continued into late summer, and after a wet May, June and July rainfall totals fell well below mean values. A visit from ex-Hurricane Bertha livened things up towards the end of the season, but by this time the majority of species had finished breeding.
Migrant numbers down
While the weather may have improved, CES results indicate that 2014 was another poor year in terms of the number of birds returning to the UK to breed. The abundance of long-distance migrants, species that winter south of the Sahara, was generally lower than average over the previous five years, and this drop in numbers was statistically significant for four of the six species monitored (Willow Warbler, Sedge Warbler, Whitethroat and Reed Warbler, Table 1), with fewer Willow and Sedge Warblers captured than in any year since CES began in 1983. This could be a reflection of poor recruitment following a disastrous breeding season in 2012 and a mixed breeding season in 2013, or to conditions on the African wintering grounds. The Sahel Precipitation Index suggests that rainfall during the African growing season was below average, reducing vegetation growth and therefore food availability for migrants during the winter and at the start of their northward passage in the spring. CES data support this hypothesis for Willow Warbler, which experienced the lowest survival rates on record, and Garden Warbler, but adult mortality did not appear to be significantly higher for other long-distance migrants.
Blackcap and Chiffchaff, short-distance migrants that typically winter around the Mediterranean basin and into North Africa, displayed very different trends. While Blackcap mirrored the drop in numbers exhibited by long-distance migrants, Chiffchaff abundance was significantly higher than average (Table 1).
|Chiffchaff||13||(3.9, 23.8)||-5||(-14.5, 5.4)|
|Willow Warbler||-22||(-28.3, -14.4)||7||(-5, 19.5)|
|Blackcap||-10||(-16.2, -3.4)||62||(48.4, 77)|
|Garden Warbler||-12||(-23.2, 1.8)||32||(6.6, 63.1)|
|Lesser Whitethroat||-7||(-26.3, 16.8)||10||(-21.2, 53.8)|
|Whitethroat||-13||(-21.7, -3.4)||27||(11.4, 45.7)|
|Sedge Warbler||-20||(-26.4, -14)||52||(36.5, 68.7)|
|Reed Warbler||-8||(-12.6, -2.4)||38||(28.1, 48.6)|
|Cetti's Warbler||18||(-7.3, 50.5)||26||(-9.9, 76.3)|
|Treecreeper||6||(-20.7, 41.4)||8||(-26.1, 59.1)|
|Wren||40||(28.3, 53)||7||(-3.6, 19.4)|
|Blackbird||-17||(-24.1, -9.6)||112||(86, 141)|
|Song Thrush||-3||(-15.4, 12)||63||(31, 103)|
|Robin||22||(9.7, 36.4)||15||(1.4, 30.6)|
|Dunnock||3||(-6.5, 12.5)||16||(2.5, 31)|
|Chaffinch||-23||(-32.8, -11.5)||-8||(-25.4, 13.4)|
|Greenfinch||-36||(-48.7, -20.3)||9||(-22.2, 52.7)|
|Goldfinch||-37||(-49.5, -20.9)||54||(4.9, 126)|
|Bullfinch||7||(-2.9, 18.8)||56||(33.4, 81.3)|
|Reed Bunting||-7||(-17, 5.4)||19||(-2.5, 44.9)|
|Blue Tit||-8||(-15.5, 0.9)||7||(-3.9, 20.1)|
|Great Tit||-8||(-16.8, 2.8)||31||(15, 49.2)|
|Willow Tit||1||(-55.5, 127.9)||-63||(-89.5, 28.1)|
|Long-tailed Tit||-13||(-22.5, -1.2)||35||(15, 58.3)|
Good news for Robin and Wren
The winter of 2013-2014 was the stormiest in 20 years, rainfall totals reaching 165% of the five-year average, but temperatures stayed 1.5°C above the mean, with few frosts. These mild conditions undoubtedly helped some of our resident species to survive until the breeding season. In particular, Robin and Wren numbers in 2014 were significantly higher than the five-year average (Table 1), as were Wren survival rates.
Conversely, Blackbird, Chaffinch, Greenfinch, Goldfinch and Long-tailed Tit all experienced a significant drop in abundance in 2014, which may be related to the poor breeding success recorded in both 2012 and 2013, where seasons were wet and truncated respectively; Blackbird and Long-tailed Tit may have suffered particularly badly as both are early breeders and conditions have been worst at the start of the season. However, the relatively low survival rates for Chaffinch and Greenfinch suggest that other factors could be involved. One potential candidate is disease, with trichomonosis still widespread in the Greenfinch population and Chaffinch increasingly suffering from a number of conditions that cause leg and foot deformities, though national trends for the latter are still positive.
Early nesting all round
The exceptionally cold spring of 2013 saw laying dates for many species delayed by more than ten days, but this year early season temperatures were above average (Figure 1) and despite an exceptionally wet January and February, 2014 witnessed a return to more typical timing for residents and migrants alike. Average laying dates for early nesters, such as Blackbird, Wren and Dipper, advanced on average by around a week (Table 2) and later breeders, such as Kestrel, Meadow Pipit, Linnet and Nuthatch, were also well ahead of schedule.
After a noticeably late arrival in 2013, BirdTrack figures showed a return to more typical dates for short-distance migrants Blackcap and Chiffchaff in 2014, both of which laid earlier than in any year since 1966 (Figure 2). There was no sign of delay in long-distance migrant arrival times in 2014 either, and many, including Sand Martin, Pied Flycatcher and Willow Warbler, commenced breeding significantly earlier than the five-year mean (Table 2).
Significant delays were recorded for Barn Owl, Reed Warbler and Starling, but it should be noted that all these species are multi-brooded and the degree to which repeat nesting attempts are initiated may vary between years. If more second broods are produced, as was recorded by Barn Owl fieldworkers in 2014, the average value calculated across the season becomes later.
Other resident passerines
Owls and raptors
Good breeding season for songbirds
Both the CES (Table 1) and NRS (Table 2) figures indicate that it was a productive breeding season for some of our earliest breeders, including Long-tailed Tit, Song Thrush, Blackbird and Robin, with Blackbird displaying the highest productivity since CES began (Figure 3). Blue and Great Tit, which breed slightly later and specialise on caterpillars, also seemed to prosper, with Great Tit in particular exhibiting above average breeding success. The mild winter weather may have improved adult condition at the start of the season, allowing parents to invest more energy in breeding. The fact that the CES figures tend to show bigger increases than the NRS figures for multi-brooded species also suggests that more pairs were able to initiate repeat broods in 2014, possibly due to the early start, a stark contrast to the truncated season of 2013.
Finches also appeared to have a successful season (Table 1), although results for Chaffinch were mixed. Given that finch numbers were generally low in 2014 (Table 1), it is possible that increased productivity is the result of density dependence, whereby competition between pairs is reduced as numbers fall, allowing each to rear more offspring. However, this does not explain the results for Bullfinch, which experienced its most productive year on record.
Density dependence was also evident for migrants. CES ringers recorded above average productivity (Table 1) and the fact that Chiffchaff, the only species that increased in abundance, was also the only exception, strengthens the case. Reed Warbler experienced the most productive season on record at CES sites. Like other migrants, increased breeding success was more apparent in the CES dataset, suggesting that an increase in the number of broods attempted may have been the mechanism. The impressive figures for Blackcap recorded by both CES and NRS participants may explain the large catches reported by ringers in many parts of the country during autumn migration.
Owl and raptor recorders were reporting low Barn Owl box occupancy across the country by mid-season 2013, with similar reports for Tawny Owl. The 2013 NRS report suggested that the cold spring, together with low vole numbers, caused many birds to suspend breeding. In 2014, however, the year began with promisingly mild winter conditions and predictions of a good vole year, leading to reports of early laying Tawny Owls and Barn Owls. By October, many volunteers had ringed more Barn Owl chicks than ever before in a single year, leading to widespread reports of it having been one of the best raptor seasons in living memory. The NRS figures in Table 2 bear out this optimism, with both owl species and Kestrel all displaying levels of productivity of up to 40% above average, the highest on record for all three species (Figure 4).
The real loser in 2014 appears to have been Woodpigeon, which suffered a marked drop in productivity, primarily due to increase nest failure rates (Table 2). While they can start to breed in early spring, the peak Woodpigeon breeding season generally falls between July and August and it is possible that the rainfall and strong winds associated with ex-Hurricane Bertha lowered breeding success. Although hardly the UK’s most popular bird, the species does provide a food source for a wide variety of consumers and so any resulting drop in numbers may have direct implications for predators or indirect implications for alternative prey species.
Help us to monitor birds in 2015!
The records used to produce the NRS results are 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? nrs [at] bto.org (subject: Starter%20pack%20request) (Email us) for a Quickstart Guide or visit the NRS web pages to find out more.
If you are a qualified ringer with access to an area of scrub, woodland or reedbed where there is the potential to catch at least 250 birds per season, why not register a CES? And if you aren’t able to start your own site, why not consider helping out at an existing one? ces [at] bto.org (Contact the CES Organiser) for more information.
We are extremely grateful to all Constant Effort Site ringers and nest recorders for their monitoring efforts and for the support of the BTO/JNCC partnership, which the JNCC undertakes on behalf of the Country Agencies. Additional funding for the BTO Ringing Scheme is provided by The National Parks and Wildlife Service (Ireland) and the ringers themselves. The Breeding Bird Survey is run by the BTO and jointly funded by the BTO, the JNCC and the RSPB. BirdTrack is organised by the BTO for the BTO, RSPB, BirdWatch Ireland, SOC and WOS.