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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 and the Constant Effort Sites 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 a reduction in productivity. Alternatively, increases in abundance may result from range expansion into new, suboptimal habitats where breeding performance is poorer and the average productivity of the population is thus lowered, whilst declines may result from the loss of individuals from these suboptimal habitats, leading to a subsequent increase in average productivity. 4.5.1 Changes in clutch and brood size Those species exhibiting statistically significant trends in clutch and brood size over the past 35 years (1968–2003) are shown in Tables 4.5.1.1 and 4.5.1.2. More species showed decreases than increases in clutch size (17 decreases, 12 increases) while the opposite was true for brood size (23 increases, 15 declines). Table 4.5.1.1 Significant trends in clutch size measured between 1968 and 2003
See Help for information on category meanings. Nine species (Pied Wagtail, Long-tailed Tit, Blue Tit, Great Tit, Magpie, Raven, Chaffinch, Greenfinch and Twite) exhibited decreases in both clutch size and brood size over the period, whilst another eight species (Barn Owl, Skylark, Grey Wagtail, Dunnock, Redstart, Starling, Tree Sparrow and Yellowhammer) exhibited increases in both clutch size and brood size. For Moorhen and Blackbird average brood sizes increased despite a reduction in clutch size, suggesting that conditions for young had improved for these species whilst conditions for parent birds during egg formation may have deteriorated. Mistle Thrush showed the opposite pattern with a reduction in average brood size despite an increase in clutch size of similar magnitude. Table 4.5.1.2 Significant trends in brood size measured between 1968 and 2003 See Help for information on category meanings. 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 brood size were recorded for Yellow Wagtail and House Sparrow. Both species show reductions of about half a chick per nesting attempt. The BTO project on Yellow Wagtails, initiated in 2002, aims to investigate the influence of decreased brood sizes on the abundance of this species. 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. Work by Kate Vincent at the University of Leicester has 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 may have been compensated for by reduced nest failure rates at the egg and chick stages. The reduction in average brood sizes of Mistle Thrushes in recent years may be contributing to the failure of this species to recover from its population decline and would merit further investigation. Several increasing species show increasing brood sizes, particularly Sparrowhawk, Collared Dove, Wren and Nuthatch. The return of Sparrowhawks into eastern areas of the UK, where populations of songbird prey are greater, may be a factor in this increase. The UK Nuthatch population, which has been expanding northwards and has increased considerably in size, has exhibited an increase in average brood size of more than one extra young per nesting attempt. It would seem likely that this has helped to drive the population increase of this species. Inverse associations between clutch or brood size and population trend are found in some 24 species. Such relationships may arise through density-dependent processes where increased competition leads to reduced clutch or brood sizes at higher population densities. Totals of ten increasing species and 14 decreasing ones show such associations. Notable examples amongst increasing species include Mute Swan (clutch size), Great Tit (clutch and brood size), Long-tailed Tit (clutch and brood size) and Magpie (clutch and brood size). Amongst declining species the examples include Barn Owl (clutch and brood size), Skylark (clutch and brood size), Tree Sparrow (clutch and brood size) and Corn Bunting (brood size). 4.5.2 Changes in nest failure rates Statistically significant trends in the daily nest failure rates at the egg and chick stages over the past 35 years (1968-2003) are shown in Tables 4.5.2.1 and 4.5.2.2. The number of species exhibiting declines in failure rates at the chick stage (24) was more than treble the number exhibiting increases (7), as was the number of species exhibiting declines in failure rates at the egg stage (37 vs. 12). Thus the general picture is one of improving nesting success. Table 4.5.2.1 Significant trends in egg-stage daily failure rate of nests measure between 1968 and 2003
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The changes in egg-stage and chick-stage failure rates were both positive for Mute Swan, Nightjar and Linnet. For a further 17 species (Merlin, Stock Dove, Barn Owl, Tawny Owl, Sand Martin, Robin, Redstart, Stonechat, Song Thrush, Reed Warbler, Magpie, Jackdaw, Carrion Crow, Starling, House Sparrow, Tree Sparrow and Yellowhammer), egg-stage and chick-stage failure rates both decreased. For a further two species (Whinchat and Long-tailed Tit), declines in the failure rate at one stage were partially 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 Significant trends in chick-stage daily failure rate of nests measured between 1968 and 2003 See Help for information on category meanings. 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 negative long-term trends in population size for seven species, and may have contributed to the observed population declines. These species were Nightjar (both stages), Skylark (egg stage), Dunnock (egg stage), Willow Warbler (egg stage), Spotted Flycatcher (nestling stage), Linnet (both stages) and Reed Bunting (egg stage). Although Nightjar is included in this list of declining species on account of its red-listed status, it should be noted that recent surveys show a population increase. Reductions in breeding performance at the egg stage have been implicated in a detailed analyses of the population declines of the Linnet (Siriwardena et al. 2000b). It has also been suggested that poor breeding performance may be preventing the recovery of Reed Bunting populations (Peach et al. 1999). However, the increasing trend in chick-stage failure rates of Spotted Flycatchers has only just become significant and previous work suggested that other demographic factors were more important in the decline of this species (Freeman & Crick 2003). Thirteen species showed clear associations between long-term increases in abundance and long-term reductions in nest failure rates. Sparrowhawk, Buzzard, Pied Wagtail, Blue Tit and Great Tit experienced reduced nest failure rates at the egg stage, while Grey Heron, Collared Dove and Great Spotted Woodpecker showed reduced failure rates at the chick stage. The remaining five species, Stock Dove, Robin, Magpie, Jackdaw and Carrion Crow, showed reduced failure rates at both the egg and chick stages. A reduction in the egg-stage failure rates of Long-tailed Tits may have contributed to their population increase, but this is partly offby an increase in check-stage failure rates. Corvids, such as Magpie, Carrion Crow and Jackdaw, appear to have benefited from improvements in nesting success at the egg stage, as have raptors such as 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 a major impact on the size of the population, given the high number of breeding attempts made by this species each year. 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, perhaps aided by the declining impact of organochlorine pesticides and improvements in water quality of riverine and standing water bodies. Decreased chick-stage failure rates of Collared Doves may have aided the rapid growth of the UK population over the last 35 years, particularly as this species makes a relatively large number of breeding attempts per year. Inverse associations between changes in egg- or chick-stage nest survival and population trend are found in some 18 species, while only Long-tailed Tit (above) shows such a relationship at one stage but a compensatory one at the other stage. Such relationships may arise through density-dependent processes where increased competition leads to increased failure rates at higher population densities. Two increasing species showed long-term increases in nest failure rates. Failure rates of Mute Swans increased at both the egg and chick stages while failure rates of Oystercatchers increased at the egg stage. Some 16 declining species showed evidence of improving nesting success. Snipe, Redshank, Woodlark, Tree Pipit, Wood Warbler and Marsh Tit showed decreased failure at the egg stage while Meadow Pipit, Bullfinch and Corn Bunting, showed decreased chick-stage failure rates. The remaining seven species, Merlin, Barn Owl, Song Thrush, Starling, House Sparrow, Tree Sparrow and Yellowhammer show decreased failure rates at both stages. 4.5.3 Changes in productivity from CES The CES results start in 1984, so the changes in productivity shown in Table 4.5.3 cover roughly half the time period of the Nest Record Scheme results. The proportion of juveniles in the CES catch provides a relative measure of annual variation in productivity that integrates the effects of fledglings produced per attempt, number of nesting attempts and immediate post-fledging survival. The CES is unique in providing relative measures of adult abundance and productivity from the same set of sites in wetland and scrub habitats. Overall, nine species exhibit significant declines in the proportion of juveniles while only Chaffinch shows an increase in this measure. Four species, Nightingale, Linnet, Lesser Redpoll and Sedge Warbler, all show greater than 50% declines in the proportion of juveniles captured over the last 19 years, although it should be noted that two of these occur on a relatively small number of plots. A further five species show reductions in relative productivity of between 25% and 50%. Four of the nine species showing these large productivity declines (Linnet, Lesser Redpoll, Song Thrush and Willow Warbler) have experienced significant population declines both on CES sites and 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 and Willow Warbler other work indicates that variation in survival rates is likely to have been a more important contributor to population changes (Peach et al. 1999, Robinson et al. 2004). 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 (Sedge Warbler, Blue Tit, Blackbird and Blackcap) with marked reductions in productivity on CES sites have not experienced related declines in abundance, either on CES sites or more widely. The causes and consequences of the productivity declines observed in these species are unclear and warrant further investigation. Three species with long-term declines in abundance of greater than 50% on CES sites, Willow Tit, Spotted Flycatcher and Lesser Whitethroat, all show stable or slightly increased productivity over the last 19 years. Taking the CES data set as a whole, 21 species show some decline in productivity over the last 19 years while only six show increases. The strong preponderance in trends towards lower productivity requires urgent and more detailed investigation. Table 4.5.3 Changes in productivity indices (percentage juveniles) for CES 1984-2003 (19 years) calculated from smoothed trend. Only those changes that are statistically significant are shown.
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4.5.4 Changes in average laying dates 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 34 years the majority of species exhibiting significant trends show an advancement of laying dates rather than a delay. Thus 33 species are laying between 29 days and 1 day earlier, on average, than they were 35 years ago. Four species, Garden Warbler, Turtle Dove, Stonechat and Lesser Redpoll, are added to the list of earlier layers published in the previous report in this series while Ringed Plover and Blackbird are removed from the list. 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 four species show significant changes towards later laying, all of which suffer from small sample sizes. 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 and productivity is currently unknown and needs to be investigated. Earlier average laying may be beneficial for birds because earlier fledging is often related to improved survival to the following year. However, several studies are beginning to show that birds are unable to advance their phenology sufficiently to match phenological changes in their food supply, such that later nesting birds are suffering from poorer productivity. Early nesting parents have an increased chance of having their offspring recruited into the next generation (Visser et al. 1998). The conservation significance of factors such as these needs to be assessed urgently. Table 4.5.4 Significant trends in laying date (Day 1 = 1 Jan) for 1968-2003
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