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 may contribute to fluctuations in abundance and 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. Such a relationship might occur 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 depending on how the driver of change affects the population.

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 over the full report period (50 years) are included in Table D1. In total, 42 species exhibited significant trends in productivity, of which 14 species now show lower FPBA: three red-listed species (Wood Warbler, Tree Pipit and Linnet), five amber-listed species (Nightjar, Willow Warbler, Dunnock, Meadow Pipit and Reed Bunting) and six green-listed species (Moorhen, Great Tit, Garden Warbler, Treecreeper, Blackbird and Chaffinch). While the trend for Moorhen, Great Tit, Willow Warbler, Garden Warbler, Linnet and Reed Bunting has been linear, i.e. falling consistently over the last 50 years, trends for the other eight species are curvilinear, and for some species in this latter group, FPBA is currently only marginally lower than in the 1960s. For seven of the species showing curvilinear trends, 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.

Two further species have recorded significant trends in FPBA but are not listed in Table D1 as the data do not cover the full 50-year period. The red-listed Song Thrush shows a curvilinear trend in productivity (an increase followed by a decrease) over 36 years (1981-2017), and the green-listed Coot shows a linear decline in FPBA over 26 years (1991-2017).

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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, 2012, Schaub et al. 2011, Finch et al. 2014); although for most species breeding ground climatic effects may be more important (Ockendon et al. 2013). Similar carry-over effects has been found for Common Tern, with warmer waters in winter in Senegal being linked to later laying in northern Germany (Dobson et al. 2017). 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. Climate change in the UK is also affecting bird populations, mainly through increased over-winter survival, but changes to rainfall and temperature during breeding and post-breeding may also affect productivity for some species, particularly in the longer term (Pearce-Higgins & Crick 2019).

Long-distance migrants are not alone in being at risk from changes to the timing of seasonal events, and short-distance migrants and residents may also be affected (Franks et al. 2018). The gap between the timing of seasonal events can also vary at different latitudes, and hence the effects of mismatch may differ across the UK (Burgess et al. 2018, Bell et al. 2019). 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 reduction in egg-stage failure rates. However, FPBA of Chaffinch, another woodland insectivore heavily reliant on moth larvae to provision its offspring, has decreased. It should be noted that, until recently Chaffinch exhibited 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). Although advances in laying dates do not necessarily match the shifts of food sources, the potential resultant declines may be offset by other benefits, e.g. increased fledgling development time is believed to have contributed to better first year survival for Pied Flycatchers in the Netherlands (Tomotani et al. 2018).

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) in the UK may reflect a more pronounced drop in invertebrate numbers in the south 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). Looking at trends in insectivorous bird species across Europe, Bowler et al. (2019) found that declines in these species were mostly associated with agricultural intensification and loss of grasslands.

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 nine of the 14 species exhibiting significant declines in FPBA (Moorhen, Nightjar, Willow Warbler, Garden Warbler, Blackbird, Meadow Pipit, Chaffinch, Linnet and Reed Bunting), with rates more than doubling for Moorhen, Willow Warbler and Reed Bunting over the last 50 years. Although there is good evidence to suggest that potential nest predators such as corvids, Sparrowhawk and grey squirrel have increased in number and that these species may have a negative influence on avian abundance, particularlly at a local scale (e.g. Groom 1993, Stoate & Szczur 2001, 2006, White et al. 2014), while 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 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. 2017, Calladine et al. 2017; see also review by Roos et al. 2018). Moreover, lower abundance may further worsen productivity for some wader species through density-dependent effects, as fewer breeding pairs may become less efficient at defending nests (Moller et al. 2018). 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; 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. Kubelka et al. (2018) linked shifts in patterns of wader nest predation to changes of climate though the robustness of these patterns is debated (Bulla et al. 2019, Kubelka et al. 2019). Development of land can also alter predator type and number, with negative consequences for nest survival, as demonstrated by Hethcoat & Chalfoun (2015). Large-scale releases of pheasants and red-legged partridges have also been linked to higher numbers of avian predators, as they provide additional food resources, enhancing over-winter survival and hence abundance during subsequent breeding seasons (Pringle et al. 2019). 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 the population trends of five woodland species, including Willow Warbler, 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. 2007a) 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 (Chamberlain et al. 2009a) and more research is needed to see whether these are representative at a national scale. Supplementary feeding in gardens is influencing the composition of bird communities across large spatial scales (Plummer et al. 2019), which may in turn affect productivity through density-dependent and interspecific effects. Whilst the effect of feeding on bird populations may be positive for some species, it may also increase risks of disease transmission (Lawson et al. 2018).  The recent outbreak of trichomonosis, which has significantly and rapidly reduced the abundance of Greenfinch at a national scale (Robinson et al. 2010b; Lawson et al. 2018), 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 from Finland that resulting population declines are less marked for this species (Lehikoinen et al. 2013), and it is unclear whether the more recent steep downturn in the UK since 2012 is linked to trichomonosis or other causes (Lawson et al. 2018).

FPBA has changed significantly and is currently higher than in the late 1960s for 28 species, across a wide range of taxonomic groups. This total includes ten species for which the change has been linear, i.e. consistent increases in productivity across the last 50 years, and 18 species which show curvilinear trends (i.e. early decreases in FPBA were followed by increases, or vice-versa). For some species in the latter group, FPBA is currently only slightly higher than in the late 1960s. Population trends are also positive for 18 of the 28 species, including raptors (Sparrowhawk, Buzzard, Barn Owl, Merlin, Peregrine), pigeons (Stock Dove, Woodpigeon, Collared Dove), corvids (Magpie, Jackdaw, Carrion Crow, Raven), and some small passerines (Reed Warbler, 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, 10 species (Little Owl, Tawny Owl, Kestrel, Skylark, Starling, Dipper, 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 a retreat into better quality habitat, may have enabled breeding success to rise.

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.

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Overall, six species exhibit significant declines in the proportion of juveniles captured (Table D2). The apparent productivity of Sedge Warbler and Reed Bunting has fallen by more than 50% over the last 25 years, while Blue Tit, Song Thrush and Blackbird show reductions in relative productivity of between 25% and 50%, and Blackcap productivity has declined by 24%. 

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).

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 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. Density-dependent effects may also work across more than one species. For example Gamelon et al. (2019) found that, whilst Great Tit density effects were driven mainly by intraspecific competition, Blue Tits were also be affected by competition with Great Tits: this could possibly explain the relatively greater decrease in breeding performance for Blue Tit following increases in the populations of both species.

Only two of the 23 species monitored shows significant positive trends in CES productivity (Chaffinch and Reed Warbler). The discrepancy between the positive Chaffinch CES trend and the decline in breeding success identified by the NRS warrants further study, but increased survival rates in post-fledging period could contribute to this, although data are sparse for this vital period.

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 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 40 species are laying between three and 21 days earlier, on average, than they were 50 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, Samplonius et al. 2018), 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. 2014, Kristensen et al. 2016). Species which have advanced their laying date least, whether migrants or residents, have generally experienced the biggest negative population trends (Franks et al. 2018). 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).

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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 four species exhibit significant trends towards later laying (Woodpigeon, Barn Owl, Blackbird and Yellowhammer), all of which 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). 

It is possible that the laying dates of the majority of those species that do not show a significant trend in timing of breeding, such as Raven, are related to weather, but that their weather-mediated cues do not show any trend over time (Crick & Sparks 1999).