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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 per nesting attempt (clutch size, brood size and failure rates at the egg and nestling stages) that can be combined to give an overall estimate of the number of Fledglings produced Per Breeding Attempt (FPBA) – see here 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 onwards. Previous reports have explored annual variation in clutch size, brood size and stage-specific nest failure rates, 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 figure representing the mean number of young leaving each nest in a given year, 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, 36 species exhibited significant trends in FPBA  over the past 20 years or more, of which six were negative, indicating that reproductive output has decreased over time. Birds exhibiting declines in productivity include one BoCC red-listed species (Nightjar), three amber-listed species (Willow Warbler, Bullfinch and Reed Bunting) and two green-listed species (Greenfinch and Chaffinch). While productivity of Nightjar, Willow Warbler and Reed Bunting has been falling consistently for the past 40 years, trends for the other three species are curvilinear, increasing between the mid 1960s and mid 1980s and decreasing thereafter.

There is increasing evidence that 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 (Both et al. 2009). Long-distance migrants are thought to be particularly susceptible, 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), and this mechanism could therefore underpin the productivity declines detected for Nightjar and Willow Warbler. In addition, recent declines in the number of aerial insects (Shortall et al. 2009), particularly moths (Conrad et al. 2006), have been reported across the UK and these may also impact on the productivity of nesting attempts of Nightjar and Willow Warbler by reducing food availability for both parents and offspring. Both species may also be experiencing negative impacts of climate change in their African wintering grounds, where reduced rainfall could lead 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).

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). Contrary to predictions, the NRS data set provides no evidence for any change in Pied Flycatcher productivity at a national scale. 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. Similarly, the results presented in this report provide no evidence that Great Tit productivity has changed, although that of Chaffinch, another woodland insectivore heavily reliant on moth larvae to provision its offspring, has decreased significantly. 

The reduction in Chaffinch breeding success is primarily due to increasing failure rates during incubation. Declining nest survival at either the egg or chick stage is also implicated in the productivity declines of Nightjar, Willow Warbler, Bullfinch and Reed Bunting, suggesting that increasing predation pressure might be responsible. However, while there is good evidence to suggest that corvids, Sparrowhawks and grey squirrels 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 1993Stoate & Szczur 20012006), previous studies have failed to find any evidence of a significant impact at a national scale (Gooch et al.1991Thomson et al. 1998Chamberlain et al. 2009Newson et al. 2009). 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. A recent investigation of Nightjar productivity suggested that nest failure is most likely in areas heavily frequented by walkers and dogs (Langston et al. 2007). 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 increasing rapidly in many areas of the UK (Newson et al. 2011), has been identified as a possible driver of population declines, the removal of the herb and shrub layer potentially reducing the availability of both food and well-concealed nesting sites (Fuller et al. 2005). This could have contributed to the observed declines in productivity of both Willow Warbler and Bullfinch. A recent study using BTO/JNCC/RSPB Breeding Bird Survey deer data indicated that declines in Willow Warbler were most pronounced in areas where Reeves’s muntjac had increased at the fastest rate (Newson et al. 2011), in agreement with a previous study looking at regional variation in Willow Warbler population trends (Morrison et al. 2010). While a similar negative relationship was identified for Bullfinch, it was not statistically significant. Causes of decline in the breeding success, and indeed the population decline, of this species are still unclear despite a significant number of demographic studies (Siriwardena et al. 1998a19992000b2001Proffitt et al. 2004).

Declining food availability may also be an issue for Reed Bunting. Reduced access to winter stubbles due to changes in farming practices has been linked to declines in survival rates of a range of farmland passerines, resulting in population declines (Siriwardena et al. 1998b, Peach et al. 1999, Siriwardena et al. 2000b). If adults are in poorer condition at the start of the breeding season, their investment in reproduction may be reduced. Investigations into Linnet declines using BTO data sets have indicated that population declines observed for this species were driven by a fall in productivity (Siriwardena et al. 1999, 2000b). 

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.

As Chaffinch and Greenfinch have both exhibited concurrent declines in productivity and increases in population size, we cannot currently exclude the possibility that increasing levels of intraspecific competition are reducing reproductive output. The recent outbreak of trichomonosis, which has significantly and rapidly reduced the abundance of Greenfinch at a national scale (Robinson et al. 2010b), may provide a good test of the hypothesis that Greenfinch productivity declines over the last 50 years represent a density-dependent response.

Productivity has increased significantly over time for 30 species (Table D1), with positive trends in FPBA recorded across a wide variety of taxonomic groups. Population trends can be calculated for 26 of these species, 14 of which have significantly increased in abundance over the same period, including several raptors (Sparrowhawk, Buzzard), the pigeons (Stock Dove, Woodpigeon, Collared Dove), the corvids (Magpie, Jackdaw, Carrion Crow), and some of the smaller passerines (Wren, Robin, Reed Warbler, Long-tailed Tit, Nuthatch). It is therefore possible that increasing productivity has contributed to the population growth exhibited by these species over the past 40 years.

Conversely, eight species, with the exception of Kestrel all passerines (Skylark, Dipper, Dunnock, Blackbird, Starling, House Sparrow, Tree Sparrow), have declined in number as productivity has increased, suggesting that density-dependent reduction in intraspecific competition may have exerted a positive influence on breeding success.

 

Table D1 Significant trends in fledglings per breeding attempt measured between 1968 and 2009

Species Period
(yrs)
Mean
annual
sample
Trend Predicted
in first year
Predicted
in last year
Change Comment
Nightjar 41 13 Linear decline 1.44 fledglings 0.71 fledglings -0.73 fledglings Small sample
Reed Bunting 41 28 Linear decline 2.76 fledglings 2.21 fledglings -0.55 fledglings Small sample
Willow Warbler 41 32 Linear decline 3.49 fledglings 3.01 fledglings -0.48 fledglings  
Chaffinch 41 56 Curvilinear 1.61 fledglings 1.45 fledglings -0.16 fledglings  
Greenfinch 41 59 Curvilinear 2.16 fledglings 2.04 fledglings -0.12 fledglings  
Bullfinch 41 18 Curvilinear 1.36 fledglings 1.3 fledglings -0.06 fledglings Small sample
Dunnock 41 57 Curvilinear 1.68 fledglings 1.72 fledglings 0.04 fledglings  
Collared Dove 41 27 Curvilinear 0.84 fledglings 0.95 fledglings 0.11 fledglings Small sample
Blackbird 41 107 Curvilinear 1.46 fledglings 1.61 fledglings 0.15 fledglings  
Woodpigeon 41 39 Curvilinear 0.56 fledglings 0.76 fledglings 0.2 fledglings  
Yellowhammer 41 28 Curvilinear 0.77 fledglings 1.07 fledglings 0.3 fledglings Small sample
Stock Dove 41 66 Linear increase 1 fledglings 1.43 fledglings 0.43 fledglings  
Reed Warbler 41 82 Linear increase 2.38 fledglings 2.83 fledglings 0.45 fledglings  
Sedge Warbler 41 24 Linear increase 3.13 fledglings 3.59 fledglings 0.46 fledglings Small sample
House Sparrow 41 57 Curvilinear 2.36 fledglings 2.84 fledglings 0.48 fledglings  
Wren 41 60 Linear increase 2.57 fledglings 3.06 fledglings 0.49 fledglings  
Buzzard 41 22 Curvilinear 1.17 fledglings 1.67 fledglings 0.5 fledglings Small sample
Skylark 41 23 Linear increase 1.05 fledglings 1.55 fledglings 0.5 fledglings Small sample
Robin 41 89 Linear increase 2.25 fledglings 2.81 fledglings 0.56 fledglings  
Tawny Owl 41 56 Linear increase 1.39 fledglings 1.96 fledglings 0.57 fledglings Nocturnal species
Merlin 41 21 Curvilinear 3.15 fledglings 3.75 fledglings 0.6 fledglings Small sample
Kestrel 41 38 Curvilinear 3.02 fledglings 3.63 fledglings 0.61 fledglings  
Peregrine 41 12 Linear increase 1.78 fledglings 2.39 fledglings 0.61 fledglings Small sample
Pied Wagtail 41 46 Linear increase 3.03 fledglings 3.76 fledglings 0.73 fledglings  
Grey Wagtail 41 26 Linear increase 2.61 fledglings 3.45 fledglings 0.84 fledglings Small sample
Jackdaw 41 36 Linear increase 1.9 fledglings 2.84 fledglings 0.94 fledglings  
Sparrowhawk 41 23 Linear increase 2.81 fledglings 3.83 fledglings 1.02 fledglings Small sample
Tree Sparrow 41 179 Linear increase 2.77 fledglings 3.8 fledglings 1.03 fledglings  
Barn Owl 41 24 Linear increase 2.43 fledglings 3.48 fledglings 1.05 fledglings Small sample
Long-tailed Tit 41 21 Linear increase 2.01 fledglings 3.12 fledglings 1.11 fledglings Small sample
Dipper 41 55 Linear increase 2.26 fledglings 3.39 fledglings 1.13 fledglings  
Nuthatch 41 22 Linear increase 4.4 fledglings 5.66 fledglings 1.26 fledglings Small sample
Starling 41 63 Linear increase 2.88 fledglings 4.22 fledglings 1.34 fledglings  
Carrion Crow 41 23 Linear increase 1.89 fledglings 3.31 fledglings 1.42 fledglings Includes Hooded Crow
Redstart 41 34 Linear increase 3.8 fledglings 5.42 fledglings 1.62 fledglings  
Magpie 41 31 Curvilinear 1.12 fledglings 3.03 fledglings 1.91 fledglings  

  See Key to species texts for help with interpretation

 

Changes in productivity from Constant Effort Sites ringing data

The CES started monitoring populations in 1983, so the changes in productivity shown in 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.

Overall, five species exhibit significant declines in the proportion of juveniles captured (Table D2). The apparent productivity of Sedge Warbler and Goldfinch has fallen by more than 50% over the last 25 years, while Song ThrushBlackbird and Blue Tit show reductions in relative productivity of between 30% 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, Baillie et al. 2009). For species such as Goldfinch and Blue Tit, where population increase has occurred, reductions in productivity may be driven by density-dependent processes, whereby increased competition for resources in an expanding population reduces the mean breeding success per pair.

Two species, Chaffinch and Reed Warbler, have shown a significant increase in productivity with, in the case of Chaffinch, reproductive output per adult more than doubling over the last 25 years. The marked difference between this trend and the decline in productivity identified by the NRS data set requires further investigation, but it may be that changes in post-juvenile survival over time are responsible. 

 

Table D2 Changes in productivity indices (percentage juveniles) for CES, 1984-2009, calculated from smoothed trend

Species Period
(yrs)
Plots
(n)
Change
(%)
Lower
limit
Upper
limit
Comment
Goldfinch 25 35 -68 -87 -7  
Sedge Warbler 25 71 -57 -72 -35  
Song Thrush 25 90 -43 -58 -22  
Blackbird 25 101 -39 -54 -23  
Blue Tit 25 102 -37 -51 -17  
Reed Warbler 25 61 46 7 106  
Chaffinch 25 84 116 23 309  

 See Key to species texts for help with interpretation

 

Changes in average laying dates from Nest Record Scheme data

Over the past 25 years, 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, since the mid 1960s, the majority of species exhibiting significant trends show an advancement of laying dates rather than a delay. Thus 44 species are laying between oneand 30 days earlier, on average, than they were 40 years ago. The efforts of volunteer inputters enabled long-term laying-date trends for Pied Flycatcher to be produced for the first time in 2009. It is interesting to note that, while 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), the magnitude of the laying-date shift in both Pied Flycatcher and Redstart (11 days and 13 days respectively), is greater than that displayed by many resident species. However, the mean laying date of the migrant species is still approximately a fortnight later than that of common residents such as Blue Tit and Great Tit. No taxonomic or ecological associations are apparent and a wide range of species demonstrate trends of a similar magnitude (Crick et al. 1997).

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 from 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). The conservation significance of such phenological disjunction remains an active research area with potentially important policy implications for conservation.

Only six species exhibit significant trends towards later laying. These include Woodpigeon, added to the suite of species for which long-term productivity trends are produced for the first time in this report thanks to the efforts of volunteer data inputters, . A recent collaboration between BTO and Aberdeen University, using NRS data, identified an increase in the frequency of repeat brooding in Yellowhammers (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 planting of autumn-sown cereals has restricted the potential for repeat nesting attempts (Chamberlain & Siriwardena 2000), but this species may also increasingly have moved to alternative habitats.

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 also related to weather, but that their weather-mediated cues do not show any trend over time (Crick & Sparks 1999).

 

Table D3 Significant trends in laying date measured between 1968 and 2009

Species Period
(yrs)
Mean
annual
sample
Trend Predicted
in first year
Predicted
in last year
Change Comment
Magpie 41 33 Linear decline Apr 24 Mar 25 -30 days  
Long-tailed Tit 41 50 Linear decline Apr 21 Apr 5 -16 days  
Greenfinch 41 93 Linear decline May 25 May 9 -16 days  
Corn Bunting 41 14 Linear decline Jun 28 Jun 13 -15 days Small sample
Lesser Redpoll 41 10 Curvilinear May 27 May 13 -14 days Small sample
Redstart 41 62 Curvilinear May 21 May 8 -13 days  
Chiffchaff 41 52 Linear decline May 16 May 3 -13 days  
Coal Tit 41 44 Linear decline May 3 Apr 20 -13 days  
Carrion Crow 41 30 Curvilinear Apr 17 Apr 4 -13 days Includes Hooded Crow
Nuthatch 41 28 Linear decline May 2 Apr 20 -12 days Small sample
Blackcap 41 39 Curvilinear May 21 May 10 -11 days  
Pied Flycatcher 41 416 Linear decline May 21 May 10 -11 days  
Blue Tit 41 417 Linear decline May 4 Apr 23 -11 days  
Great Tit 41 332 Linear decline May 5 Apr 24 -11 days  
Treecreeper 41 13 Linear decline May 7 Apr 26 -11 days Small sample
Goldfinch 41 23 Linear decline Jun 7 May 27 -11 days Small sample
Dipper 41 63 Linear decline Apr 18 Apr 8 -10 days  
Stonechat 41 40 Curvilinear May 3 Apr 23 -10 days  
Whitethroat 41 19 Curvilinear May 26 May 16 -10 days Small sample
Chaffinch 41 108 Curvilinear May 10 Apr 30 -10 days  
Sedge Warbler 41 47 Curvilinear May 29 May 20 -9 days  
Reed Warbler 41 173 Curvilinear Jun 17 Jun 8 -9 days  
Marsh Tit 41 14 Linear decline Apr 28 Apr 19 -9 days Small sample
Kestrel 41 23 Linear decline May 5 Apr 27 -8 days Small sample
Oystercatcher 41 48 Linear decline May 17 May 9 -8 days  
Swallow 41 191 Curvilinear Jun 19 Jun 11 -8 days  
Tree Pipit 41 19 Linear decline May 24 May 16 -8 days Small sample
Robin 41 132 Linear decline Apr 28 Apr 20 -8 days  
Ring Ouzel 41 21 Linear decline May 15 May 7 -8 days Small sample
House Sparrow 41 62 Linear decline May 25 May 17 -8 days  
Wren 41 88 Linear decline May 15 May 8 -7 days  
Willow Warbler 41 83 Linear decline May 20 May 13 -7 days  
Starling 41 84 Curvilinear Apr 27 Apr 20 -7 days  
Moorhen 41 70 Linear decline May 10 May 4 -6 days  
Grey Wagtail 41 60 Linear decline May 8 May 2 -6 days  
Wood Warbler 41 32 Curvilinear May 24 May 18 -6 days  
Jackdaw 41 26 Curvilinear Apr 23 Apr 17 -6 days Small sample
Whinchat 41 27 Linear decline May 30 May 25 -5 days Small sample
Garden Warbler 41 22 Linear decline May 27 May 22 -5 days Small sample
Peregrine 41 10 Curvilinear Apr 5 Apr 1 -4 days Small sample
Tree Sparrow 41 251 Linear decline May 28 May 24 -4 days  
Reed Bunting 41 47 Curvilinear May 19 May 15 -4 days  
Pied Wagtail 41 81 Curvilinear May 18 May 16 -2 days  
Nightjar 41 20 Curvilinear Jun 17 Jun 16 -1 days Small sample
Blackbird 41 215 Linear increase Apr 25 Apr 27 2 days  
Bullfinch 41 33 Linear increase May 26 Jun 1 6 days  
Yellowhammer 41 26 Linear increase May 31 Jun 6 6 days Small sample
Skylark 41 19 Curvilinear May 25 Jun 1 7 days Small sample
Stock Dove 41 23 Linear increase May 30 Jun 11 12 days Small sample
Woodpigeon 41 78 Linear increase Jun 1 Jun 21 20 days  

 See Key to species texts for help with interpretation

 

This report should be cited as: Baillie, S.R., Marchant, J.H., Leech, D.I., Renwick, A.R., Eglington, S.M., Joys, A.C., Noble, D.G., Barimore, C., Conway, G.J., Downie, I.S., Risely, K. & Robinson, R.A. (2012) BirdTrends 2011. BTO Research Report No. 609. BTO, Thetford. http://www.bto.org/birdtrends