Changes in breeding performance

Changes in a range of aspects of breeding performance can be measured under the Nest Record Scheme (NRS) and the Constant Effort Sites (CES) scheme. The NRS provides information on components of breeding performance (clutch size, brood size and failure rates at the egg and nestling stages) that can be combined to give an overall estimate of productivity per nesting attempt (FPBA) – see NRS page for further information. The CES scheme provides an index of breeding performance accrued over all nesting attempts in a particular year. CES results also take into account any changes in the survival rates of fledglings in the first few weeks after leaving the nest, a period when losses of young can be high.

Breeding performance may be influenced by a variety of factors, including food availability, predation pressure and weather conditions. Variation in breeding performance maycontribute to fluctuations in abundance and may even be the main demographic factor responsible for determining the size of the population. Conversely, the breeding performance of a population may be inversely related to its size, with productivity decreasing as the number of individuals increases, and vice versa. This relationship may be due to the action of density-dependent factors, such as competition for resources: as numbers increase, competition for resources is likely to increase, possibly resulting in poorer productivity. Alternatively, increases in abundance may be accompanied by range expansion into less suitable habitats or areas where breeding performance is poorer, thus reducing the average productivity of the population. The converse is also true, and where declines result from the loss of individuals from these suboptimal habitats, there may be a subsequent increase in average productivity recorded.

Changes in Fledglings Per Breeding Attempt from Nest Record Scheme data

The NRS started collating nest histories of individual breeding attempts in 1939 and sufficient data are available for trends to be produced from the mid 1960s onward. The data collected allow annual variation in clutch size, brood size and stage-specific nest failure rates to be assessed, and these breeding parameters are included in the Summary tables. While detailed exploration of annual variation in productivity is essential if the impacts of environmental factors on breeding success are to be fully understood, the combined effects of concurrent changes in the number of offspring and failure rates can be difficult to interpret. These measures are therefore integrated into a single annual figure representing the mean number of young leaving each nest, termed Fledglings Per Breeding Attempt (FPBA; Siriwardena et al. 2000b, Crick et al. 2003).

All species displaying significant temporal trends in mean FPBA are included in Table D1. In total, 42 species exhibited significant trends in FPBA over the past 48 years, of which 12 were negative, indicating that reproductive output has decreased over time. Birds exhibiting declines in productivity include two BoCC red-listed species (Tree Pipit and Linnet), four amber-listed species (Nightjar, Willow WarblerMeadow Pipit and Reed Bunting) and six green-listed species (Moorhen, Great TitGarden Warbler, Treecreeper, Chaffinch and Greenfinch). While productivity of MoorhenGreat TitWillow WarblerGarden WarblerLinnet and Reed Bunting has been falling consistently, trends for the other six species are curvilinear. For five species, FPBA increased between the mid 1960s and mid 1980s or mid 1990s and decreased thereafter; whereas in the case of Nightjar, productivity decreased from the mid 1960s until the mid 2000s but has increased slightly over the last ten years. Corn Bunting did exhibit a significant FPBA decline in BirdTrends 2016; however following recent productive seasons the trend is no longer significant and breeding success is now at a similar level to that displayed in the mid 1960s.

Table D1 Significant trends in fledglings per breeding attempt measured between 1967 and 2015

Species Period
Trend Predicted
in first year
in last year
Change Comment
Garden Warbler 48 20 Linear decline 3.08 fledglings 2.33 fledglings -0.75 fledglings Small sample
Moorhen 48 51 Linear decline 2.58 fledglings 1.96 fledglings -0.62 fledglings  
Great Tit 48 541 Linear decline 5.95 fledglings 5.36 fledglings -0.59 fledglings  
Reed Bunting 48 48 Linear decline 2.74 fledglings 2.16 fledglings -0.58 fledglings  
Nightjar 48 23 Curvilinear 1.55 fledglings 0.98 fledglings -0.57 fledglings Small sample
Willow Warbler 48 69 Linear decline 3.6 fledglings 3.13 fledglings -0.47 fledglings  
Linnet 48 126 Linear decline 2.72 fledglings 2.31 fledglings -0.41 fledglings  
Chaffinch 48 127 Curvilinear 1.6 fledglings 1.31 fledglings -0.29 fledglings  
Greenfinch 48 86 Curvilinear 2.14 fledglings 1.93 fledglings -0.21 fledglings  
Treecreeper 48 21 Curvilinear 2.68 fledglings 2.49 fledglings -0.19 fledglings Small sample
Meadow Pipit 48 52 Curvilinear 2 fledglings 1.93 fledglings -0.07 fledglings  
Tree Pipit 48 15 Curvilinear 1.65 fledglings 1.62 fledglings -0.03 fledglings Small sample
Dunnock 48 125 Curvilinear 1.66 fledglings 1.66 fledglings 0 fledglings  
Blackbird 48 270 Curvilinear 1.48 fledglings 1.48 fledglings 0 fledglings  
Collared Dove 48 55 Curvilinear 0.78 fledglings 0.79 fledglings 0.01 fledglings  
Sedge Warbler 48 38 Curvilinear 2.97 fledglings 3.05 fledglings 0.08 fledglings  
Woodpigeon 48 88 Curvilinear 0.51 fledglings 0.62 fledglings 0.11 fledglings  
House Sparrow 48 107 Curvilinear 2.31 fledglings 2.59 fledglings 0.28 fledglings  
Skylark 48 42 Curvilinear 0.89 fledglings 1.18 fledglings 0.29 fledglings  
Robin 48 212 Curvilinear 2.3 fledglings 2.62 fledglings 0.32 fledglings  
Stock Dove 48 80 Linear increase 1 fledglings 1.36 fledglings 0.36 fledglings  
Yellowhammer 48 48 Curvilinear 0.83 fledglings 1.26 fledglings 0.43 fledglings  
Carrion Crow 48 39 Curvilinear 1.65 fledglings 2.09 fledglings 0.44 fledglings Includes Hooded Crow
Little Owl 48 19 Linear increase 1.9 fledglings 2.37 fledglings 0.47 fledglings Small sample
Buzzard 48 30 Linear increase 1.54 fledglings 2.03 fledglings 0.49 fledglings Small sample
Peregrine 48 25 Linear increase 1.78 fledglings 2.28 fledglings 0.5 fledglings Small sample
Wren 48 98 Curvilinear 2.37 fledglings 2.92 fledglings 0.55 fledglings  
Pied Wagtail 48 90 Linear increase 3.01 fledglings 3.56 fledglings 0.55 fledglings  
Sparrowhawk 48 30 Curvilinear 2.62 fledglings 3.19 fledglings 0.57 fledglings  
Kestrel 48 44 Curvilinear 2.89 fledglings 3.5 fledglings 0.61 fledglings  
Tawny Owl 48 68 Linear increase 1.38 fledglings 1.99 fledglings 0.61 fledglings Nocturnal species
Grey Wagtail 48 55 Linear increase 2.6 fledglings 3.4 fledglings 0.8 fledglings  
Jackdaw 48 66 Curvilinear 1.52 fledglings 2.32 fledglings 0.8 fledglings  
Starling 48 115 Linear increase 2.57 fledglings 3.42 fledglings 0.85 fledglings  
Wheatear 48 15 Linear increase 3.53 fledglings 4.4 fledglings 0.87 fledglings Small sample
Barn Owl 48 35 Linear increase 2.34 fledglings 3.25 fledglings 0.91 fledglings  
Dipper 48 89 Curvilinear 2.01 fledglings 2.96 fledglings 0.95 fledglings  
Merlin 48 21 Linear increase 2.44 fledglings 3.44 fledglings 1 fledglings Small sample
Tree Sparrow 48 324 Linear increase 2.77 fledglings 3.84 fledglings 1.07 fledglings  
Redstart 48 61 Curvilinear 3.39 fledglings 4.75 fledglings 1.36 fledglings  
Magpie 48 41 Curvilinear 1.1 fledglings 2.49 fledglings 1.39 fledglings  
Nuthatch 48 64 Linear increase 3.69 fledglings 5.44 fledglings 1.75 fledglings  

See Key to species texts for help with interpretation

A recent review paper focusing on long-distance migrant declines (Vickery et al. 2014) highlighted the important role demographic data play in the identification of mechanisms. Work by Morrison et al. (2013b) using BBS data reported a consistent positive relationship between latitude and the trajectory of long-distance migrant population trends within the UK, suggesting that abundance is, at least in part, determined by breeding success. This conclusion was supported by a study focusing specifically on contrasting regional trends in Willow Warbler numbers (Morrison et al. 2016c), which identified reduced productivity at lower latitudes as the underlying driver. There is increasing evidence that organisms at lower trophic levels are responding to climatic change more rapidly than those towards the top of the food chain (Visser & Both 2005, Thackeray et al. 2010, 2016). Resulting mismatches in the timing of food availability and of offspring food demand, referred to as phenological disjunction, can have severe impacts on breeding success and ultimately on population trends of bird species (Both et al. 2009), although there is evidence that the magnitude of these impacts may vary with diet and breeding habitat (Dunn & Møller 2014).

Long-distance migrants are thought to be particularly susceptible to disjunction between birds and their prey due to their later arrival on the breeding grounds and the energetic demands of their journey northwards, which may constrain their ability to advance their laying dates (Rubolini et al. 2010, Ockendon et al. 2012, Gilroy et al. 2016 but see Goodenough et al. 2011, Winkler et al. 2014); the resultant negative impacts on breeding success may be exacerbated by increased competition with less disadvantaged residents (Wittwer et al. 2015). Recent studies have detected negative correlations between May temperatures and both the population trajectories (Pearce-Higgins et al. 2015) and the extincton risk (Mustin et al. 2014) in a range of migrant species, lending weight to this hypothesis and potentially explaining the productivity declines reported here for Nightjar, Tree Pipit, Willow Warbler and Garden Warbler. Alteration to some habitats by humans may increase competition further by causing a reduction in nest site availability (Higginson 2017).

Trans-Saharan migrants may also be experiencing negative impacts of climate change in their African wintering grounds or on passage, with reduced rainfall leading to a fall in insect abundance and a subsequent loss of condition, resulting in a lower reproductive output during the following spring (Saino et al. 2004, 2011, Schaub et al. 2011, Ockendon et al. 2013, Finch et al. 2014). A similar effect has been found for Common Tern, with warmer waters in winter in Senegal being linked to later laying in northern Germany (Dobson et al. 2016). The importance of conditions outside the breeding grounds was emphasised by Gilroy et al. (2016), who found that species inhabiting larger wintering ranges relative to the size of their breeding range were less likely to exhibit population declines, this increased migratory diversity potentially buffers the impacts of reduced quality within individual wintering regions or habitats.

Disjunction risk is predicted to vary spatially in relation to the duration of resource peaks and previous research has reported more marked migrant population declines in highly seasonal habitats (Both et al. 2010), of which woodlands are a prime example. Invertebrate food availability in the canopy increases rapidly during the brief period when larval Lepidoptera emerge to take advantage of the spring leaf burst, prior to the foliage toughening and developing chemical defences. As springs have become warmer, oak leafing dates have advanced, a shift matched by caterpillars (Buse et al. 1999), but apparently not by tits (Visser et al. 1998) or flycatchers (Both et al. 2009), despite the apparent plasticity of passerine laying dates in response to environmental drivers (Phillimore et al. 2016). The figures presented in this report indicate that Great Tit brood sizes have fallen and that Pied Flycatcher nestling stage failure rates have risen, as would be predicted under a mismatch scenario, although FPBA trends are not significant for Pied Flycatcher due to a concurrent drop in egg-stage failure rates. However, FPBA of Chaffinch, another woodland insectivore heavily reliant on moth larvae to provision its offspring, has decreased significantly. It should be noted that Chaffinch exhibits concurrent declines in productivity and increases in population size, so we cannot currently exclude the possibility that increasing levels of intraspecific competition are reducing reproductive output (Greenwood & Baillie 2008). The population level impacts of disjunction-related productivity declines are still unclear and there is some evidence that reduced productivity under warmer temperatures may be buffered by density-dependent increases in survival in some species, including Great Tit (Reed et al. 2012, 2013, 2015), and possibly also in clutch size (Saether et al. 2016).

Recent declines in the number of aerial insects (Shortall et al. 2009), particularly moths (Conrad et al. 2006, Fox 2013) and butterflies (Fox et al. 2015), have been reported across the UK. These invertebrate groups form a significant element of the diet of all the long-distance migrants identified as displaying productivity declines and a reduction in food availability may increase the incidence of whole brood failure due to starvation or desertion by under-nourished parents. The latitudinal variation in population trends identified by Morrison et al. (2013b) may reflect a more pronounced drop in invertebrate numbers in the south of the UK where conditions are generally drier. An alternative explanation may be a lower usage of neonicotinoid pesticides in the north, as it is becoming apparent that detrimental impacts on invertebrate numbers may not be limited to the agricultural areas to which they are applied (Hallmann et al. 2014).

Clearly, declining food availability due to changes in farming practices, including agrochemical usage, may also be an issue for farmland bird species displaying negative trends in FPBA. Brickle et al. 2000 observed that Corn Bunting nest failure rates increased as invertebrate availability around the breeding site decreased, largely due to increased predation. Nest destructon resulting from agricultural operations has also been identified as a potential factor contributing to declines by reducing the probability of double-brooding (Brickle et al. 2000, Brickle & Harper 2002, Perkins et al. 2013). Reduced access to winter stubbles due to changes in farming practices have been linked to declines in survival rates of species such as Reed Bunting, resulting in population declines (Siriwardena et al. 1998b, Peach et al. 1999, Siriwardena et al. 2000b). If adults of stubble-feeding species are in poorer condition at the start of the breeding season, their investment in reproduction may also be reduced, and the granivorous diet of Linnet nestlings means that they could be further susceptible to shortages of weed seed in the breeding season as a result of agricultural intensification. Investigations into declines using BTO demographic data sets have indicated that Linnet population declines have been primarily driven by a fall in productivity (Siriwardena et al. 1999, 2000b).

Egg-stage failure rates are implicated in the reduced productivity of eight of the 12 species exhibiting significant declines in FPBA (MoorhenNightjar, Willow WarblerGarden Warbler, Meadow PipitChaffinch, Linnet and Reed Bunting), with rates more than doubling for MoorhenWillow Warbler and Reed Bunting over the last 48 years. Although there is good evidence to suggest that potential nest predators such as corvids, Sparrowhawk and grey squirrel are all increasing in number and that these species may have a negative influence on avian abundance at a very localised scale (e.g. Groom 1993, Stoate & Szczur 2001, 2006, White et al. 2014), previous studies have failed to find any evidence of a significant impact at a national scale (Gooch et al.1991, Thomson et al. 1998, Chamberlain et al. 2009, Newson et al. 2009, Vögeli et al. 2011, reviewed by Madden et al. 2015). Ground nesting birds, in particular waders, may also be vulnerable to predation from mammals such as red fox and hedgehogs, and several studies have identified predation as a factor or partial factor causing low productivity and hence population declines (e.g. Teunissen et al. 2008, MacDonald & Bolton 2008b, Mason et al. 2017Calladine et al. 2017). Several recent studies have also suggested that predation pressure may increase in response to climatic warming. For example, Cox et al. (2013) found that the incidence of nest predation by birds and snakes, but not mammals, increased with temperature in the USA, although the mechanism is unknown, while Auer & Martin (2013) demonstrated an increase in the proportion of predated nests across a range of species due to climate-induced shifts in plant–herbivore interactions. Development of land can also alter predator type and number, with negative consequences for nest survival, as demonstrated by Hethcoat & Chalfoun (2015). Predation rates may therefore be increasing and further research into the impacts of nest predators on population trajectories, at a variety of spatial scales, is urgently required.

Increased grazing pressure by deer, numbers of which are rising rapidly in many areas of the UK (Newson et al. 2012), has been identified as a possible driver of population declines in the UK (Fuller et al. 2005) and the USA (Martin et al. 2011), the removal of the herb and shrub layers potentially reducing the availability of both food and well-concealed nesting sites. Mustin et al. (2014) demonstrated that Garden Warbler were less likely to colonise woodland sites with poorly developed undergrowth and experimental exclusion of deer has been shown to impact positively on this species. Similarly, Holt et al. 2010, 2011 showed that Nightingale territory density was much higher within deer exclosures, and Newson et al. 2012 identified a negative correlation between deer and Willow Warbler population trends, which may also have been driven by reduced productivity.

Increasing human activity in the countryside, resulting from a growing population, could increase disturbance levels, in turn influencing the rates of predation and desertion. An investigation of Nightjar productivity suggested that nest failure is most likely in areas heavily frequented by walkers and dogs (Langston et al. 2007) and a review of recreational disturbance impacts found breeding success to be adversely affected by human activity levels in 28 out of 33 papers cited (Steven et al. 2011). However, Lowe et al. (2014) observed that, while Nightjar territory selection was influenced by disturbance, there appeared to be no concurrent impact on breeding success.

The colonisation of urban habitats by Greenfinch may also have increased the proportion of data originating from gardens, which may represent a relatively resource-poor breeding environment when compared with their more traditional farmland habitats, resulting in the smaller brood and clutch sizes observed. Similar reductions in reproductive output across an urban gradient have been observed for tit species, although results from localised studies are conflicting (see Chamberlain et al. 2009 for review) and more research is needed to see whether these are representative at a national scale. The recent outbreak of trichomonosis, which has significantly and rapidly reduced the abundance of Greenfinch at a national scale (Robinson et al. 2010b), could have impacted on breeding success and may also provide a good test of the hypothesis that productivity declines over the last 50 years represent a density-dependent response. Chaffinch is also known to be susceptible to the disease, although there is evidence that resulting population declines are less marked (Lehikoinen et al. 2013).

FPBA has increased significantly over the last 48 years for 28 species, across a wide range of taxonomic groups (Table D1). Population trends are also upward for 16 of these species, including raptors (Sparrowhawk, Buzzard, Barn Owl, Merlin, Peregrine), pigeons (Stock Dove, Woodpigeon, Collared Dove), corvids (Magpie, Jackdaw, Carrion Crow), and some small passerines (Nuthatch, Wren, Robin, Redstart and Pied Wagtail). It is therefore possible that increasing productivity has contributed to the population growth exhibited by these species over recent decades. Conversely, 12 species (Little OwlTawny Owl, Kestrel, SkylarkSedge WarblerStarling, Dipper, Wheatear, House Sparrow, Tree Sparrow, Grey Wagtail and Yellowhammer), have declined in number as FPBA has increased, suggesting that a density-dependent reduction in intraspecific competition or change in habitat use may have enabled breeding success to rise.

Two species (Blackbird and Dunnock) showed significant productivity increases until the early 1990s, which have been reversed by subsequent declines so that FPBA is now the same as it was in the mid 1960s. For both species, recent population increases have followed previous declines, so density-dependent processes could be responsible for the observed changes in FPBA. 

Changes in productivity from Constant Effort Sites ringing data

The CES started monitoring populations in 1983, so the changes in productivity (Table D2) cover roughly half the period of the Nest Record Scheme results. The CES data set is unique in providing relative measures of adult abundance and productivity from the same set of sites in mostly wetland and scrub habitats. While the NRS data set monitors the productivity of individual nesting attempts, the proportion of juveniles in the CES catch provides a relative measure of annual variation in productivity that integrates the effects of the number of fledglings produced per attempt, number of nesting attempts and immediate post-fledging survival. Use of these two techniques in combination provides a powerful method of determining which factors are responsible for observed declines in recruitment of young birds into the breeding population.

Table D2 Changes in productivity indices (percentage juveniles) for CES, 1984-2015, calculated from smoothed trend

Species Period
Willow Tit 31 27 -79 -94 -22  
Sedge Warbler 31 73 -62 -76 -39  
Blue Tit 31 105 -61 -71 -48  
Reed Bunting 31 63 -55 -74 -6  
Great Tit 31 103 -44 -59 -23  
Song Thrush 31 92 -43 -61 -21  
Garden Warbler 31 78 -43 -62 -14  
Blackcap 31 100 -40 -56 -19  
Willow Warbler 31 100 -33 -48 -11  
Blackbird 31 103 -31 -47 -10  

See Key to species texts for help with interpretation

Overall, 10 species exhibit significant declines in the proportion of juveniles captured (Table D2). The apparent productivity of Blue Tit, Willow TitSedge Warbler and Reed Bunting has fallen by more than 50% over the last 25 years, while Great TitWillow Warbler, Blackcap, Garden Warbler, Song Thrush and Blackbird show reductions in relative productivity of between 25% and 50%. 

Although two of these species, Song Thrush and Sedge Warbler, have experienced significant population declines, either on CES sites or more widely (based on CBC/BBS figures), previous analyses suggest that falling survival rates are likely to have been a more important contributor to population changes than reduced productivity (Peach et al. 1991, 1995a, 1999, Robinson et al. 2004, 2010, 2014, Baillie et al. 2009). The potential susceptibility of long-distance migrants to climate-induced phenological disjunction is discussed above and it is interesting to note that the productivity declines of Willow Warbler and Garden Warbler detected by CES are now mirrored in the NRS trends; a recent study using BTO data sets suggests that reduced productivity may be responsible for the negative population trends for Willow Warbler detected in the south of England (Morrison et al. 2016c)

Reed Bunting numbers also fell in the 1970s and early 1980s due to declining survival rates, but these have since risen again; falling productivity in recent years may now be preventing full population recovery (Peach et al. 1999). For species such as Blue Tit, Great Tit and Blackcap, where a concurrent population increase has occurred, reductions in productivity may be driven by density-dependent processes, where increased competition for resources in an expanding population reduces the mean breeding success per pair. While NRS trends in per-attempt productivity for the two tit species are in the same direction as the CES per-season productivity trend, they indicate a slight increase in FPBA for Blackcap, suggesting that for this species, density dependence might be influencing the number of nesting attempts initiated per pair rather than the number of chicks reared per brood.  

None of the 23 species monitored show significant positive trends in CES productivity in BirdTrends 2017. Two species (Reed Warbler and Chaffinch) did show a significant positive trend in CES productivity in BirdTrends 2016, but the trend for both species is no longer significant following lower productivity in 2016. A positive trend might be predicted if climatic warming enabled multi-brooded species, such as Reed Warbler, to extend their breeding season, increasing the number of broods reared per adult (Dunn & Møller 2014). Eglington et al. (2015) found that, using CES data from across Europe, Reed Warbler was the one species experiencing temperature dependent increases in productivity, particularly in the north of its range. NRS results are also suggestive of an increase in per-attempt productivity for this species and results of a recent food supplementation study suggest that this is as predicted if climatic change has increased food availability (Vafidis et al. 2016).

Changes in average laying dates from Nest Record Scheme data

Since the mid 1970s, many species have exhibited a trend towards progressively earlier clutch initiation (Crick et al. 1997) with laying dates showing curvilinear responses over the past 50 years as spring temperatures have cooled and then warmed (Crick & Sparks 1999). Table D3 confirms that the majority of species exhibiting significant trends since the late 1960s have advanced laying. Thus 39 species are laying between three and 23 days earlier, on average, than they were 48 years ago.

The results of previous studies predict laying-date advancement to be more constrained in long-distance migrants (Both et al. 2009, Rubolini et al. 2010, Kluen et al. 2016), although the extent to which populations are able to adjust migratory strategies in response to environmental pressures and the predicted impact on population size is currently the focus of much discussion (James & Abbott 2014, Winkler et al. 2014Kristensen et al. 2016). It is interesting to note that the magnitude of the laying-date shift in both Pied Flycatcher and Redstart (10 days and 14 days respectively) is greater than that displayed by many resident species, although their mean laying date is still approximately a fortnight later than non-migratory species with similar nestling diets, such as Blue Tit and Great Tit. No taxonomic or ecological associations are apparent within the group of species displaying laying-date advancements and a wide range of taxa demonstrate trends of a similar magnitude (Crick et al. 1997).

Table D3 Significant trends in laying date measured between 1967 and 2015

Species Period
Trend Predicted
in first year
in last year
Change Comment
Magpie 48 32 Curvilinear Apr 27 Apr 4 -23 days  
Greenfinch 48 86 Linear decline May 26 May 6 -20 days  
Long-tailed Tit 48 57 Linear decline Apr 21 Apr 4 -17 days  
Goldfinch 48 26 Curvilinear Jun 5 May 19 -17 days Small sample
Redstart 48 70 Linear decline May 24 May 10 -14 days  
Blackcap 48 45 Linear decline May 24 May 11 -13 days  
Coal Tit 48 44 Linear decline May 3 Apr 20 -13 days  
Nuthatch 48 38 Linear decline May 2 Apr 19 -13 days  
Chiffchaff 48 64 Linear decline May 15 May 3 -12 days  
Swallow 48 238 Linear decline Jun 24 Jun 13 -11 days  
Dipper 48 75 Linear decline Apr 18 Apr 7 -11 days  
Chaffinch 48 119 Linear decline May 12 May 1 -11 days  
Stonechat 48 51 Linear decline May 7 Apr 27 -10 days  
Reed Warbler 48 240 Linear decline Jun 20 Jun 10 -10 days  
Whitethroat 48 21 Curvilinear May 27 May 17 -10 days Small sample
Pied Flycatcher 48 489 Linear decline May 20 May 10 -10 days  
Marsh Tit 48 14 Linear decline Apr 28 Apr 18 -10 days Small sample
Great Tit 48 490 Linear decline May 4 Apr 24 -10 days  
Treecreeper 48 13 Linear decline May 6 Apr 26 -10 days Small sample
Corn Bunting 48 17 Linear decline Jun 25 Jun 15 -10 days Small sample
Kestrel 48 26 Linear decline May 5 Apr 26 -9 days Small sample
Robin 48 151 Linear decline Apr 28 Apr 19 -9 days  
House Sparrow 48 67 Linear decline May 25 May 16 -9 days  
Grey Wagtail 48 62 Linear decline May 8 Apr 30 -8 days  
Ring Ouzel 48 24 Linear decline May 14 May 6 -8 days Small sample
Sedge Warbler 48 45 Curvilinear May 29 May 21 -8 days  
Garden Warbler 48 23 Linear decline May 28 May 20 -8 days Small sample
Blue Tit 48 741 Linear decline May 2 Apr 24 -8 days  
Carrion Crow 48 29 Linear decline Apr 17 Apr 9 -8 days Includes Hooded Crow
Cuckoo 48 18 Linear decline Jun 10 Jun 3 -7 days Small sample
Willow Warbler 48 89 Linear decline May 20 May 13 -7 days  
Jackdaw 48 33 Linear decline Apr 26 Apr 19 -7 days  
Tree Pipit 48 22 Curvilinear May 28 May 22 -6 days Small sample
Wren 48 90 Linear decline May 14 May 8 -6 days  
Starling 48 86 Linear decline Apr 28 Apr 23 -5 days  
Moorhen 48 82 Linear decline May 9 May 5 -4 days  
Wood Warbler 48 38 Linear decline May 25 May 21 -4 days  
Oystercatcher 48 72 Curvilinear May 19 May 16 -3 days  
Tree Sparrow 48 357 Linear decline May 27 May 24 -3 days  
Swift 48 13 Curvilinear May 26 May 26 0 days Small sample
Blackbird 48 268 Curvilinear Apr 23 Apr 25 2 days  
Raven 48 12 Curvilinear Mar 3 Mar 5 2 days Small sample
Barn Owl 48 22 Curvilinear May 16 May 22 6 days Small sample
Yellowhammer 48 25 Linear increase May 31 Jun 7 7 days Small sample
Turtle Dove 48 12 Linear increase Jun 14 Jun 24 10 days Small sample
Woodpigeon 48 100 Linear increase Jun 3 Jun 22 19 days  

See Key to species texts for help with interpretation

The population-level consequences of phenological change are the subject of many current scientific studies, including several ongoing projects at BTO. Advanced laying is typically beneficial as early-nesting parents have an increased chance of recruiting offspring into the next generation (Visser et al. 1998). Climate-induced advances in phenology have been observed across a wide range of taxa and are occuring most rapidly at lower trophic levels, so that the annual cycles of predators are increasingly mis-timed with those of their prey (Thackeray et al. 2016). A frequently used model system is that of woodland passerines, where the timing of leaf emergence is advanced and the speed of caterpillar development is increased at higher temperatures (Buse et al. 1999, Visser & Holleman 2001), resulting in a food peak advancement that nesting birds are unable to match and a subsequent reduction in breeding success (though see Phillimore et al. 2016).

Both et al. (2006) demonstrated that mismatches between periods of food availability and chick demand can affect abundance in Dutch Pied Flycatcher populations, with those exhibiting the largest disjunction between arrival in spring and peak caterpillar abundance experiencing the greatest declines. Another study by Both and his colleagues, also in the Netherlands, suggested that the magnitude of disjunction may be mediated by habitat type, with species in more seasonal habitats at greatest risk of negative impacts on productivity (Both et al. 2010). However, while Dutch Great Tits have provided the model system for much of the recent research into phenological disjunction, recent papers suggest that these study populations are currently buffered from decline by density-dependent increases in survival (Reed et al. 2012, 2013, 2015). The ability to switch to different food sources to provide for chicks, as demonstrated for Wood Warbler (Mallord et al. 2017), may provide another buffer for some species. Whether such compensations will persist as the climate warms further remains to be seen and the population-level significance of trophic mismatches remains an active research area with potentially important policy implications for conservation. Projections of climatic suitability in Great Britain under future climate scenarios suggest that climatic suitability could increase for 44% of species and reduce for 9% of species by 2080, with the largest gains in abundance expected to occur in northern and western areas; however many of the species which are expected to reduce are those that are already red listed following long-term population declines (Massimino et al. 2017).

Only six species exhibit significant trends towards later laying, of which five (Woodpigeon, Turtle Dove, Barn OwlBlackbird and Yellowhammer) produce multiple broods per season. A collaboration between BTO and Aberdeen University, using NRS data, identified an increase in the frequency of repeat brooding in Yellowhammer (Cornulier et al. 2009) which, as mean laying dates are calculated across all broods, would result in the observed shift. Increased production of repeat broods could be stimulated by climatic amelioration, with later nests being more productive in warmer conditions, or by movement of birds away from farmland and into habitats where they are released from constraints on multiple brooding. A recent study using data from North America and Europe identified a positive temporal trend in the length of the breeding season of multi-brooded, but not single-brooded, bird species, consistent with the hypothesis that climate change is extending the window of opportunity for nesting for species less reliant on peaks in seasonal resources (Dunn & Møller 2014). 

The only single-brooded bird displaying a significant trend towards later laying is Raven, a species that initiates laying in February, prior to the the early spring period that has witnessed the most significant rates of warming. It is likely that the laying dates of the majority of those species that do not show a significant trend in timing of breeding are similarly related to weather, but that their weather-mediated cues do not show any trend over time (Crick & Sparks 1999).