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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 months 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 help to influence 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 new, suboptimal habitats 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.

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, 39 species exhibited significant trends in FPBA over the past 46 years, of which 10 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), two amber-listed species (Nightjar and Reed Bunting) and six green-listed species (Moorhen, Garden Warbler, Sedge Warbler, Treecreeper, Chaffinch and Greenfinch). While productivity of Moorhen, Nightjar, Garden Warbler, Linnet and Reed Bunting has been falling consistently, trends for the other five species are curvilinear, increasing up to the mid 1980s and decreasing thereafter. Three additional amber-listed species, Willow Warbler, Dunnock and Bullfinch, were identified as exhibiting significant productivity declines in the previous report. The shape of the trend for Willow Warbler and Dunnock remains fundamentally unchanged but there is no longer a significant difference between the start and end values; Bullfinch productivity, however, has increased markedly over the past 15 years following a period of decline, with a turning point mirrored by the population trend.

Table D1 Significant trends in fledglings per breeding attempt measured between 1967 and 2013

Species Period
(yrs)
Mean
annual
sample
Trend Predicted
in first year
Predicted
in last year
Change Comment
Garden Warbler 46 19 Linear decline 3.06 fledglings 2.39 fledglings -0.67 fledglings Small sample
Nightjar 46 22 Linear decline 1.4 fledglings 0.75 fledglings -0.65 fledglings Small sample
Reed Bunting 46 47 Linear decline 2.72 fledglings 2.21 fledglings -0.51 fledglings  
Moorhen 46 52 Linear decline 2.54 fledglings 2.06 fledglings -0.48 fledglings  
Linnet 46 124 Linear decline 2.72 fledglings 2.31 fledglings -0.41 fledglings  
Chaffinch 46 128 Curvilinear 1.57 fledglings 1.27 fledglings -0.3 fledglings  
Tree Pipit 46 14 Curvilinear 1.49 fledglings 1.25 fledglings -0.24 fledglings Small sample
Greenfinch 46 89 Curvilinear 2.12 fledglings 1.93 fledglings -0.19 fledglings  
Treecreeper 46 20 Curvilinear 2.64 fledglings 2.55 fledglings -0.09 fledglings Small sample
Sedge Warbler 46 38 Curvilinear 2.89 fledglings 2.84 fledglings -0.05 fledglings  
Dunnock 46 122 Curvilinear 1.65 fledglings 1.65 fledglings 0 fledglings  
Blackbird 46 255 Curvilinear 1.47 fledglings 1.49 fledglings 0.02 fledglings  
Collared Dove 46 55 Curvilinear 0.78 fledglings 0.82 fledglings 0.04 fledglings  
Meadow Pipit 46 50 Curvilinear 2.01 fledglings 2.08 fledglings 0.07 fledglings  
Corn Bunting 46 10 Curvilinear 1.54 fledglings 1.66 fledglings 0.12 fledglings Small sample
Woodpigeon 46 84 Curvilinear 0.51 fledglings 0.67 fledglings 0.16 fledglings  
House Sparrow 46 102 Curvilinear 2.28 fledglings 2.57 fledglings 0.29 fledglings  
Robin 46 208 Curvilinear 2.29 fledglings 2.61 fledglings 0.32 fledglings  
Stock Dove 46 77 Linear increase 0.99 fledglings 1.38 fledglings 0.39 fledglings  
Yellowhammer 46 49 Curvilinear 0.8 fledglings 1.22 fledglings 0.42 fledglings  
Peregrine 46 23 Linear increase 1.77 fledglings 2.26 fledglings 0.49 fledglings Small sample
Buzzard 46 29 Linear increase 1.52 fledglings 2.05 fledglings 0.53 fledglings Small sample
Carrion Crow 46 40 Curvilinear 1.64 fledglings 2.18 fledglings 0.54 fledglings Includes Hooded Crow
Pied Wagtail 46 89 Linear increase 3 fledglings 3.56 fledglings 0.56 fledglings  
Sparrowhawk 46 31 Curvilinear 2.6 fledglings 3.17 fledglings 0.57 fledglings  
Tawny Owl 46 68 Linear increase 1.38 fledglings 1.95 fledglings 0.57 fledglings Nocturnal species
Kestrel 46 41 Curvilinear 2.83 fledglings 3.41 fledglings 0.58 fledglings  
Wren 46 96 Linear increase 2.53 fledglings 3.13 fledglings 0.6 fledglings  
Jackdaw 46 62 Curvilinear 1.5 fledglings 2.27 fledglings 0.77 fledglings  
Starling 46 115 Linear increase 2.58 fledglings 3.38 fledglings 0.8 fledglings  
Barn Owl 46 32 Curvilinear 2.04 fledglings 2.85 fledglings 0.81 fledglings  
Grey Wagtail 46 54 Linear increase 2.58 fledglings 3.41 fledglings 0.83 fledglings  
Merlin 46 21 Linear increase 2.45 fledglings 3.39 fledglings 0.94 fledglings Small sample
Dipper 46 85 Curvilinear 2.02 fledglings 3.03 fledglings 1.01 fledglings  
Wheatear 46 16 Linear increase 3.47 fledglings 4.51 fledglings 1.04 fledglings Small sample
Tree Sparrow 46 280 Linear increase 2.75 fledglings 3.83 fledglings 1.08 fledglings  
Magpie 46 42 Curvilinear 1.03 fledglings 2.37 fledglings 1.34 fledglings  
Redstart 46 57 Curvilinear 3.44 fledglings 5 fledglings 1.56 fledglings  
Nuthatch 46 57 Linear increase 3.69 fledglings 5.38 fledglings 1.69 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. 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). 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 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, but see Goodenough et al. 2011, Winkler et al. 2014); resultant negative impacts on breeding success may be exacerbated by increased competition with less disadvantaged residents (Wittwer et al. 2015). A recent study identified increased local extinction risk of Garden Warbler in areas where May temperature, and therefore the risk of mismatch, was greatest (Mustin et al. 2014) and a similar mechanism could have contributed to the increasing nestling failure rates underpinning the productivity declines detected for Nightjar, Tree Pipit and Sedge Warbler. 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 in the following spring (Saino et al. 2004, 2011, Schaub et al. 2011, Ockendon et al. 2013, Finch et al. 2014).

Woodland passerines that depend on short-lived peaks in the availability of larval Lepidoptera to provide food for their nestlings may also suffer reduced productivity as a result of climate-induced changes in phenology. As springs have become warmer, oak leafing dates have advanced, a shift matched by caterpillars (Buse et al. 1999) but not by tits (Visser et al. 1998) or flycatchers (Both et al. 2009). A recent study in the Netherlands found that responses to disjunction may vary spatially, with the negative effects exacerbated in more seasonal habitats, where the window of prey availability is smaller (Both et al. 2010), and regional variation in breeding success at sites across the UK is currently being investigated. While 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, FPBA trends are not significant 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; again, increasing nestling failure rates have contributed to this productivity decline. The population level impacts of disjunction-related productivity declines are still unclear, however, and there is some evidence that reduced productivity under warmer temperatures may be buffered by a density-dependent increase in survival in some species, including Great Tit (Reed et al. 2012, 2013, 2015). It should also 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).

Recent declines in the number of aerial insects (Shortall et al. 2009), particularly moths (Conrad et al. 2006, Fox 2013), 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. There have also been increasing concerns about the detrimental impacts of neonicotinoid pesticides, which have been implicated in declines of some avian populations due to detrimental impacts on invertebrate numbers that may not be limited to the agricultural areas where they are applied (Hallmann et al. 2014).

Declining food availability may also be an issue for farmland bird species displaying negative trends in FPBA. 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 and Yellowhammer, 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 seven of the 10 species exhibiting significant declines in FPBA (Moorhen. Nightjar, Garden Warbler, Sedge Warbler, Chaffinch, Linnet and Reed Bunting), with rates more than doubling for Moorhen. Nightjar and Reed Bunting over the last 46 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). However, several recent studies have suggested that predation pressure may increase in response to climatic warming. 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. 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. This process may have contributed to the observed declines in productivity of Garden Warbler and Bullfinch. 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. A negative relationship between Reeves's Muntjac increases and Bullfinch counts on BBS squares was also identified by Newson et al. 2012 but was not statistically significant. The causes of decline in the breeding success of this species, and indeed the drivers of its population decline, are still unclear despite it having been the focus of several demographic studies (Siriwardena et al. 1998a, 1999, 2000b, 2001, Proffitt et al. 2004).

Increasing human activity in the countryside, resulting from a growing population, could increase disturbance levels, which could in turn influence 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 recent review of impacts of recreational disturbance 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 was 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 broods 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 need 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 46 years for 29 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, 13 species (Tawny Owl, Kestrel, Starling, Dipper, Blackbird, Wheatear, Dunnock, House Sparrow, Tree Sparrow, Grey Wagtail, Meadow Pipit, Yellowhammer and Corn Bunting), have declined in number as FPBA has increased, suggesting that a density-dependent reduction in intraspecific competition 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 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, 1984-2013, calculated from smoothed trend

Species Period
(yrs)
Plots
(n)
Change
(%)
Lower
limit
Upper
limit
Comment
Willow Tit 29 28 -76 -92 -31  
Sedge Warbler 29 73 -57 -76 -29  
Blue Tit 29 104 -53 -65 -36  
Reed Bunting 29 63 -49 -71 -6  
Garden Warbler 29 78 -46 -64 -14  
Willow Warbler 29 100 -33 -50 -10  
Blackcap 29 100 -33 -48 -16  
Song Thrush 29 91 -33 -50 -6  
Reed Warbler 29 64 38 1 107  
Chaffinch 29 84 104 25 325  

See Key to species texts for help with interpretation

Overall, 8 species exhibit significant declines in the proportion of juveniles captured (Table D2). The apparent productivity of Blue Tit, Willow Tit and Sedge Warbler has fallen by more than 50% over the last 25 years, while Willow Warbler, Blackcap, Garden Warbler, Song Thrush and Reed Bunting 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 Garden Warbler and Sedge Warbler detected by CES are now mirrored in the NRS trends. Results of the two surveys are also similar for Willow Warbler, although NRS declines are currently non-significant, and a recent study using BTO data sets suggests that reduced productivity may be responsible for the negative population trends detected in the south of England (Morrison et al. 2010).

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 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, with increased competition for resources in an expanding population reduces the mean breeding success per pair.

Only Chaffinch and Reed Warbler display a significant increase in productivity at CE sites. Such a result 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). The marked difference between the CES trend and the decline in productivity identified by the NRS data set for the single-brooded Chaffinch requires further investigation, but it may be that changes in post-juvenile survival over time are responsible.

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 36 species are laying between one and 27 days earlier, on average, than they were 46 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), 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). 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 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 2013

Species Period
(yrs)
Mean
annual
sample
Trend Predicted
in first year
Predicted
in last year
Change Comment
Magpie 46 33 Linear decline Apr 22 Mar 26 -27 days  
Greenfinch 46 89 Linear decline May 26 May 7 -19 days  
Long-tailed Tit 46 54 Linear decline Apr 21 Apr 5 -16 days  
Goldfinch 46 25 Curvilinear Jun 5 May 20 -16 days Small sample
Redstart 46 67 Linear decline May 24 May 10 -14 days  
Coal Tit 46 45 Linear decline May 3 Apr 20 -13 days  
Blackcap 46 43 Linear decline May 24 May 12 -12 days  
Chiffchaff 46 61 Linear decline May 15 May 3 -12 days  
Swallow 46 222 Linear decline Jun 24 Jun 13 -11 days  
Dipper 46 71 Linear decline Apr 18 Apr 7 -11 days  
Nuthatch 46 34 Linear decline May 1 Apr 20 -11 days  
Chaffinch 46 121 Linear decline May 12 May 1 -11 days  
Pied Flycatcher 46 452 Linear decline May 20 May 10 -10 days  
Marsh Tit 46 14 Linear decline Apr 28 Apr 18 -10 days Small sample
Great Tit 46 426 Linear decline May 4 Apr 24 -10 days  
Treecreeper 46 13 Linear decline May 7 Apr 27 -10 days Small sample
Carrion Crow 46 30 Linear decline Apr 18 Apr 8 -10 days Includes Hooded Crow
Corn Bunting 46 17 Linear decline Jun 25 Jun 15 -10 days Small sample
Reed Warbler 46 225 Linear decline Jun 20 Jun 11 -9 days  
Whitethroat 46 21 Curvilinear May 27 May 18 -9 days Small sample
House Sparrow 46 65 Linear decline May 25 May 16 -9 days  
Robin 46 149 Linear decline Apr 28 Apr 20 -8 days  
Garden Warbler 46 22 Linear decline May 28 May 20 -8 days Small sample
Kestrel 46 24 Linear decline May 4 Apr 27 -7 days Small sample
Tree Pipit 46 21 Curvilinear May 28 May 21 -7 days Small sample
Grey Wagtail 46 61 Linear decline May 8 May 1 -7 days  
Ring Ouzel 46 24 Linear decline May 14 May 7 -7 days Small sample
Sedge Warbler 46 46 Curvilinear May 29 May 22 -7 days  
Willow Warbler 46 89 Linear decline May 20 May 13 -7 days  
Jackdaw 46 32 Linear decline Apr 26 Apr 19 -7 days  
Blue Tit 46 609 Linear decline May 1 Apr 25 -6 days  
Starling 46 84 Linear decline Apr 29 Apr 23 -6 days  
Wren 46 88 Linear decline May 14 May 9 -5 days  
Wood Warbler 46 37 Linear decline May 25 May 20 -5 days  
Oystercatcher 46 69 Curvilinear May 20 May 17 -3 days  
Meadow Pipit 46 43 Curvilinear May 19 May 18 -1 days  
Raven 46 12 Curvilinear Mar 3 Mar 5 2 days Small sample
Barn Owl 46 20 Curvilinear May 18 May 22 4 days Small sample
Bullfinch 46 34 Linear increase May 26 Jun 1 6 days  
Skylark 46 19 Curvilinear May 25 Jun 2 8 days Small sample
Yellowhammer 46 26 Linear increase May 31 Jun 8 8 days Small sample
Turtle Dove 46 12 Linear increase Jun 13 Jun 24 11 days Small sample
Woodpigeon 46 96 Linear increase Jun 1 Jun 23 22 days  

See Key to species texts for help with interpretation

The significance of the changes in phenology for breeding performance is poorly understood but has stimulated a large number of scientific studies, including several ongoing projects at BTO. Earlier average laying may be beneficial for birds because earlier fledging is often related to improved survival to the following year – thus early-nesting parents have an increased chance of having their offspring recruited into the next generation (Visser et al. 1998). However, the timing of leaf emergence and the speed of caterpillar development is also changing under increased temperatures (Buse et al. 1999, Visser & Holleman 2001) and the results of several recent studies have suggested that some birds may be unable to advance their breeding sufficiently to match phenological changes in their food supply, such that later-nesting birds are suffering poorer productivity. 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 demonstrating the largest mismatches between arrival in spring and peak caterpillar abundance exhibiting the greatest declines. As a consequence of climate change there may be an increasing mismatch between predator activities and the availability of their food supplies at different trophic levels within ecosystems (Both et al. 2009). Recent studies in the Netherlands have 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 Great Tits in the Netherlands 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). Whether such compensation 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.

Only seven species exhibit significant trends towards later laying, of which six produce multiple broods per season. A recent 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. Previous research into multiple brooding in Skylark populations has demonstrated that increased autumn sowing of cereals has restricted the potential for repeat nesting attempts (Chamberlain & Siriwardena 2000), but this species may also increasingly have moved to alternative habitats. 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).

 

This report should be cited as: Robinson, R.A., Marchant, J.H., Leech, D.I., Massimino, D., Sullivan, M.J.P., Eglington, S.M., Barimore, C., Dadam, D., Downie, I.S., Hammond, M.J., Harris, S.J., Noble, D.G., Walker, R.H. & Baillie, S.R. (2015) BirdTrends 2015: trends in numbers, breeding success and survival for UK breeding birds. Research Report 678. BTO, Thetford. www.bto.org/birdtrends