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BBWC Home > Contents > Discussion > Changes in breeding performance 4.5 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 former provides information on components of breeding performance per nesting attempt. The latter provides an index of breeding performance accrued over all nesting attempts in a particular year, combined with the effect of changes in the survival of fledglings once they have left the nest but before they are caught as juveniles – 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 a population, and may even be the main demographic factor responsible for determining its size. Conversely, the breeding performance of a population may be negatively 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; conversely, where declines result from the loss of individuals from these suboptimal habitats, there may be a subsequent increase in average productivity. 4.5.1 Changes in clutch and brood size from Nest Record Scheme data Those species exhibiting statistically significant trends in clutch and brood size over the past 38 years (1968–2006) are shown in Tables 4.5.1.1 and 4.5.1.2. More species showed decreases than increases in clutch size (19 decreases, 15 increases) while the reverse was true for brood size (18 decreases, 22 increases). Table 4.5.1.1
See Help for help with interpretation Nine species (Nightjar, Pied Wagtail, Spotted Flycatcher, Long-tailed Tit, Blue Tit, Great Tit, Magpie, Chaffinch and Greenfinch) exhibited decreases in both clutch size and brood size over the period, whilst another eight species (Barn Owl, Skylark, Swallow, Dipper, Dunnock, Redstart, Tree Sparrow and Yellowhammer) exhibited increases in both clutch size and brood size. Moorhen, Grey Wagtail, Rook and Linnet all showed a decline in average clutch size and an increase in average brood size, although the magnitude of the change in Rook clutch sizes and in Grey Wagtail and Linnet brood sizes was small (<0.04 eggs/chicks). Jackdaw and Carrion Crow showed the opposite pattern, with clutch sizes increasing while brood sizes decreased, although again the magnitude of the change in Carrion Crow clutch size was very small (0.01 eggs); note that, for historical reasons, Carrion Crow figures include a small proportion of data from Hooded Crow. Table 4.5.1.2
See Help for help with interpretation Long-term changes in clutch or brood size are associated with long-term population trends in a number of species. Here we highlight those changes that are both statistically significant and likely to be of biological importance. Declines in population size and productivity were identified for Spotted Flycatcher (clutch and brood size), House Sparrow and Bullfinch (brood size). The mean number of eggs and chicks produced by Spotted Flycatcher began to fall in the mid 1980s, a worrying trend for a species that, by this point, had been in rapid decline for at least two decades. Population modelling work undertaken by Freeman & Crick (2003) suggests that reduced survival rates of first-year birds drove the initial population decline, but an additive effect of reduced reproductive output may now be a possibility. Declines in Bullfinch populations are also thought to have begun due to falling survival rates (Proffitt et al. 2004, Marquiss 2007), although the mechanism is not clear-cut (Siriwardena et al. 2001) and a reduction in brood size over the last 25 years may again have had detrimental effects at a population level. In the case of the House Sparrow, population modelling based on BTO data has shown that declines in rural areas were caused by reduced survival rates but that these declines were mainly halted due to improvements in breeding performance (Crick et al. 2002). The apparently accelerating reduction in brood size is therefore of some concern. Peach et al. (2008) suggested that insect food for the chicks may be limited in certain situations and recent brood size reductions may be a manifestation of this at a wider scale. However, it should be noted that, over the long term, some of the reduction in brood size might have been compensated for by reduced nest failure rates at the egg and chick stages. It is worth noting that Pied Wagtail and Grey Wagtail are also exhibiting significant declines in both clutch and brood sizes – data from WBS suggest that populations using rivers and canals declined significantly between the 1970s and the 1990s, although BBS results suggest that Pied Wagtail populations in the wider countryside are stable. Several increasing species exhibit concurrently increasing brood sizes, particularly Sparrowhawk, Collared Dove, Redstart and Nuthatch. Sparrowhawk has returned to eastern areas of the UK, where populations of songbird prey are greater, which may have impacted positively on breeding success, although both the population and the brood size trends began to level out in the early to mid 1990s. Collared Dove has spread rapidly since colonising the UK in the 1950s, and brood sizes have exhibited a steady increase over the past 35 years. Redstart is one of the only long-distance migrant passerines currently exhibiting significant population growth, and increasing clutch and brood sizes over the last three decades may have contributed to this. The UK Nuthatch population, which has been expanding northwards and has increased considerably in size since the 1970s, currently produces 0.75 more young per nesting attempt than it did on average during the late 1960s. It would seem likely that this has helped to drive the population increase of this species; brood sizes have started to fall in the last decade, however, possibly indicating the onset of density dependent-reductions in productivity (see below). Inverse associations between clutch or brood size and population trend are found in some 23 species (i.e. they show lower productivity with higher population size). Such relationships may arise through density-dependent processes, whereby increased competition leads to reduced clutch or brood sizes at higher population densities. Eleven increasing species and 12 decreasing species show such associations. Notable examples amongst increasing species include Magpie, Blue Tit, Great Tit, Long-tailed Tit, Chaffinch, Greenfinch (clutch and brood size) Robin and Chiffchaff (brood size). Amongst declining species examples include Skylark, Dunnock, Tree Sparrow (clutch and brood size), Lapwing, Starling and Mistle Thrush (clutch size). 4.5.2 Changes in nest failure rates from Nest Record Scheme data Statistically significant trends in the daily nest failure rates at the egg and chick stages over the past 38 years (1968–2006) are shown in Tables 4.5.2.1 and 4.5.2.2. The number of species exhibiting declines in failure rates at the egg stage (39) was treble the number displaying increases (13), and while 26 species exhibited declines in chick-stage failure rates, only 13 displayed increases. Thus, the general picture is one of improving nesting success. Table 4.5.2.1
See Help for help with interpretation Increases in both egg-stage and chick-stage failure rates were observed for four species: Nightjar, Spotted Flycatcher, Linnet and Bullfinch. Use of nest cameras to identify potential nest predators of Nightjar nests is currently under way in Thetford Forest as part of a collaborative project between BTO and the University of East Anglia, which is also monitoring Woodlark. Researchers have also used nest cameras to observe predation events at Spotted Flycatcher nests, identifying Jay as the main predator in their study population in SW England (Stevens et al. 2008). Predation is not necessarily the only cause of nest failure, but it is a major cause for open-nesting bird species. Eighteen species exhibited declines in both egg-stage and chick-stage
failure rates: Kestrel, Merlin,
Barn Owl, Tawny Owl, Stock
Dove, Sand Martin, Pied Wagtail,
Robin, Redstart, Stonechat,
Magpie, Jackdaw, Carrion
Crow, Raven, Starling,
House Sparrow, Tree Sparrow and
Yellowhammer. For a further six species (Tree
Pipit, Dipper, Whinchat,
Blackbird, Great Tit, Blue
Tit and Long-tailed Tit), better success
at one stage was partly cancelled out by increases in failure rates
at the other, suggesting that different factors may influence productivity
at egg and chick stages. Table 4.5.2.2
See Help for help with interpretation Long-term changes in egg-stage or chick-stage nest failure rates are associated with long-term population trends in a number of species. Here we highlight those changes that are both statistically significant and likely to be of biological importance. Increased nest failure rates were associated with long-term decreases in population size for ten species, and may have contributed to the observed population declines of Nightjar, Spotted Flycatcher, Linnet, Bullfinch (egg- and chick-stage failure rates increasing), Lapwing, Willow Warbler and Reed Bunting (egg-stage failure rates increasing). Although Nightjar has shown a large historical decline and is red-listed because of this, it should be noted that recent surveys show a population increase (Conway et al. 2007). Reductions in breeding performance at the egg stage have been implicated in a detailed analysis of the population declines of the Linnet (Siriwardena et al. 2000b), but the extent to which decreased productivity has influenced Bullfinch population trends is still not clearly understood (Siriwardena et al. 2001). It has also been suggested that poor breeding performance may be preventing the recovery of Reed Bunting populations (Peach et al. 1999). The increasing trend in egg- and chick-stage failure rates of Spotted Flycatcher has only recently become significant and previous work suggested that other demographic factors were more important in the decline of this species (Freeman & Crick 2003). Researchers have failed to find any trend in survival rates that might explain declines in Lapwing numbers (Peach et al. 1994, King et al. 2008), and a fall in productivity is thought to have been a major factor (Galbraith 1988), although a recent study failed to find any correlation between nest failure rates and changes in abundance at a regional scale (Sharpe et al. 2008). The population decline of Willow Warbler is much more pronounced in the south of the UK than in Scotland, and collaborative project between BTO and the University of East Anglia is currently investigating whether demographic parameters, including productivity, exhibit the similar patterns of spatial variation. Failure rates are also thought to be increasing at the egg and chick stages for the declining Hen Harrier and Twite, although sample sizes for both are extremely limited, and at the egg stage for Moorhen, a species for which the current population trajectory is unclear but which did decline in abundance during the 1980s. Sixteen species showed clear associations between long-term increases in abundance and long-term reductions in nest failure rates. Sparrowhawk, Buzzard, Wren and Greenfinch experienced reduced nest failure rates at the egg stage, while Grey Heron, Great Spotted Woodpecker and Collared Dove exhibited a reduction in failure rates at the chick stage. The remaining nine species (Barn Owl, Stock Dove, Robin, Redstart, Stonechat, Magpie, Jackdaw, Carrion Crow and Raven) displayed reduced failure rates at both the egg and chick stages. Corvids, such as Magpie, Jackdaw, Carrion Crow and Raven, appear to have benefited from improvements in nesting success at the egg stage, as have owls and raptors such as Barn Owl, Sparrowhawk and Buzzard. Decreased persecution and reduction in the use of pesticides are likely to have been important factors in the recovery of these species. The improvements in the nesting success of Stock Dove could have had a major impact on the size of the population, given the high number of breeding attempts made by this species each year, and the decreased chick-stage failure rates of Collared Dove may have aided the rapid growth of the UK population over the last 38 years. Grey Heron populations have increased over the last 70 years, and improvements in chick-stage nest survival may have played a part in this increase. Greenfinch has rapidly adapted to the provision of supplementary food in gardens and is now very much associated with human habitats. Such provisioning may lead to increased productivity because of its positive impact on adult body condition. Causes of the reduction in failure rates for Wren, Robin, Redstart and Stonechat are less clear, although all feed primarily on ground-dwelling invertebrates and changes in arthropod abundance or activity could be responsible. Inverse associations between changes in egg- or chick-stage nest survival and population trends are found in some 23 species. Such relationships may arise through density-dependent processes where increased competition leads to increased failure rates at higher population densities. Four increasing species (Mute Swan, Peregrine, Oystercatcher and Chaffinch) showed long-term increases in egg-stage failure rates, while 19 declining species showed evidence of improving nesting success. Ringed Plover, Snipe, Redshank, Woodlark, Dunnock, Wheatear, Sedge Warbler, Wood Warbler and Marsh Tit showed decreasing failure at the egg stage while decreasing chick-stage failure rates were identified for Meadow Pipit, Grey Wagtail and Corn Bunting. The remaining seven species, Kestrel, Merlin, Starling, House Sparrow, Tree Sparrow, Yellowhammer and Pied Wagtail (which is thought to be declining in waterway habitats), exhibited decreasing failure rates at both stages. Several species demonstrated a decrease in failure rates at one stage but a compensatory increase at the other, including Dipper, Great Tit, Blue Tit, Long-tailed Tit, Tree Pipit, Whinchat (declining egg failure, increasing chick failure) and Blackbird (increasing egg-stage failure, declining chick-stage failure). 4.5.3 Changes in productivity from Constant Effort Scheme ringing data The CES started monitoring populations in 1983, so the changes in productivity shown in Table 4.5.3 cover roughly half the time 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, ten species exhibit declines of greater than 25% in the proportion of juveniles captured, while only Chaffinch shows an increase of greater than 25% in the ratio of juveniles to adults. Six of these species, Nightingale, Sedge Warbler, Blue Tit, Linnet, Goldfinch and Reed Bunting, all exhibit declines in the proportion of juveniles captured over the last 20 years of greater than 50%, although it should be noted that Nightingale occurs on a relatively small number of plots. A further four species show reductions in relative productivity of between 25% and 50%: Song Thrush, Blackcap, Willow Warbler and Great Tit. Of the nine of these species for which sufficient Nest Record Scheme data are available for comparison (Nightingale is excluded), six have also been identified as exhibiting negative trends in either clutch size, brood size or nest survival (Sedge Warbler, Willow Warbler, Blue Tit, Great Tit, Linnet and Reed Bunting). Six of the ten species exhibiting productivity declines greater than 25% (Nightingale, Song Thrush, Sedge Warbler, Willow Warbler, Linnet and Reed Bunting) have experienced significant population declines, either on CES sites or more widely (based on CBC/BBS figures). For Linnet there is good evidence that variation in productivity has been important in driving the decline (Siriwardena et al. 2000b), but for Song Thrush, Willow Warbler and Reed Bunting other work indicates that variation in survival rates is likely to have been a more important contributor to population changes (Peach et al. 1995a, Peach et al. 1999, Robinson et al. 2004, Baillie et al. 2008). The large decline in Nightingale productivity may have contributed to the complex changes in its distribution shown by the 1999 survey, which identified decreases in abundance over large parts of the species' range. The four other species (Blackcap, Great Tit, Blue Tit, and Goldfinch) demonstrating marked reductions in productivity on CES sites have not experienced related declines in abundance, either on CES sites or more widely. These productivity declines may be driven by density-dependent processes, whereby increased competition for resources in an expanding population reduces the mean breeding success per pair. Taking the CES data set as a whole, 20 species show some decline in productivity over the last 22 years while only five show increases. The strong preponderance of trends towards lower productivity requires urgent and more detailed investigation. Table 4.5.3
See Help for help with interpretation 4.5.4 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 4.5.4 confirms that over the past 38 years the majority of species exhibiting significant trends show an advancement of laying dates rather than a delay. Thus 40 species are laying between 31 days and 1 day earlier, on average, than they were 38 years ago. Three species, Nightjar, Twite and Wood Warbler, are added to the list of earlier layers published in the previous report in this series. There are no taxonomic or ecological associations between the species showing such changes, and they seem to occur across a wide range of species (Crick et al. 1997). Only two species, Skylark and Yellowhammer, show significant changes towards later laying, and sample sizes are small. Both of these species are multi-brooded, however, and it may be that differences in the ratio of first to repeat broods initiated may be obscuring advances in laying date. Evidence of temporal and spatial variation in Yellowhammer repeat-brooding over time is currently being investigated in a collaborative research project between BTO and Aberdeen University. It is likely that the laying dates of the majority of those species that do not show a significant trend in timing of laying are related to some aspect of weather, but that those aspects do not show any trend over time (Crick & Sparks 1999). The significance of the changes in phenology for breeding performance
not well understood but is an active research area within several
research groups. Earlier average laying may be beneficial for birds
because earlier fledging is often related to improved survival to
the following year – 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 phenology
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).
The conservation significance of such phenological disjunction remains
an active research area with potentially important policy implications
for conservation. Table 4.5.4
See Help
for help with interpretation
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