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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, and may even be the main factor responsible for controlling the size of a population. 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 species abundance may result from range expansion into new, sub-optimal 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 sub-optimal habitats, leading to a subsequent increase in average productivity.

4.5.1 Changes in clutch and brood size

Productivity data are currently available for 72 species. Those species exhibiting statistically significant trends in clutch and brood size over the past 31 years are shown in Tables 4.5.1.1 and 4.5.1.2. Although the numbers of species showing increases and decreases in clutch size were approximately equal (14 vs. 13 species respectively) there were many more species showing increases in brood size than decreases over the same period (25 vs. 7).

Table 4.5.1.1 Significant trends in clutch size

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Two species (Great Tit and Long-tailed Tit) exhibited decreases in both clutch size and brood size over 31 years, whilst another 10 species (Swallow, Yellowhammer, Dunnock, Stonechat, Skylark, Grey Wagtail, Redstart, Tree Sparrow, Starling and Wren) exhibited increases in both clutch size and brood size. For two species, declines in clutch size were partially (Magpie) or fully (Moorhen) compensated for by increases in average brood size, suggesting that conditions for young had improved for these species whilst conditions for parent birds during egg formation may have deteriorated.

Table 4.5.1.2 Significant trends in brood size

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Variation in clutch or brood size may influence population sizes of the following species:

• Decreased clutch and population size: Whitethroat, although the change in clutch size is relatively small, as is the sample size.
• Decreased brood and population size: Yellow Wagtail – the decline in average brood size of nearly half a chick per nesting attempt may be potentially important in determining the population size. The BTO project on Yellow Wagtails, initiated in 2002, aims to investigate the influence of decreased brood sizes on the abundance of this species.
• Increased clutch and population size: Wren, although an increase in brood size would also be important.
• Increased brood and population size: Two corvid species are prominent here; Rook and Magpie have both exhibited increased average brood sizes, as has Sparrowhawk. For the latter, the return of the species into areas of the eastern 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 nearly 1.5 extra young per nesting attempt. It would seem likely that this has helped to drive the population increase of this species. Collared Doves have also experienced a slight increase in average brood size over the past 31 years. The population increase of this species is more likely to be due to range expansion, however.

Density dependent changes in average clutch or brood sizes are suggested for 15 and 16 species respectively, i.e. sizes have increased as populations decreased or vice versa. For a few species, long-term population data are not available and changes in clutch or brood size may be density dependent responses that are suggestive of population declines. This situation applies to Stonechat, Whinchat and Wheatear, species for which atlas data support the suggestion of population declines.

4.5.2 Changes in nest failure rates

Statistically significant trends in the daily nest failure rates at the egg and chick stage over the past 31 years are shown in Tables 4.5.2.1 and 4.5.2.2 (75 and 69 species, respectively, were analysed in total). The number of species exhibiting declines in failure rates at the chick stage was double the number exhibiting increases (20 vs. 10), while the number of species exhibiting declines in failure rates at the egg stage was more than three times the number exhibiting increases (35 vs. 10).  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