4.2 Survival rates

Survival rates are unlikely to remain constant throughout the year and may vary with season.

• Inclement weather and the decreased availability of food during the winter months may result in increased mortality during this period, particularly towards the end of winter when available resources have become depleted and birds are establishing breeding territories.

• Body condition, and therefore survival rates, may be reduced immediately after the breeding season if individuals have invested heavily in producing and raising offspring. The risk of predation might also increase during this period if predators are involved in provisioning their own offspring.

• Variation in the movement of individuals between populations must be considered when attributing seasonal changes in local population sizes to mortality. Birds may be increasingly mobile during the winter months when resources are limited. Movement of individuals is likely to be less important when considering variation in national population indices.

Variation in survival probability may differ between specific groups of individuals:

• Age-dependent mortality - the survival rate of first-year birds may be more susceptible to decrease in times of stress due to their inexperience or to their reduced competitive ability. Adult birds may also represent a subset of relatively higher quality individuals as they have already survived their first breeding season.

• Sex-dependent mortality - birds of one sex may be more susceptible to increased mortality rates if the sexes differ in size, behaviour or dispersal patterns.

• Condition-dependent mortality - individuals in poor condition are more likely to experience a reduction in survival rates as selection pressures increase.

Decreasing survival rates have been implicated in the decline in population sizes of a number of bird species of conservation concern.

• Siriwardena et al. (1998) observed that survival rates during periods when populations were increasing in size were significantly greater than those during periods in which populations were declining for 13 of the 28 species of farmland bird analysed, including 3 species of conservation concern: song thrush, blackbird (Turdus merula) and goldfinch. (Declines in goldfinch abundance, however, are now considered to be less severe than previously thought).

  A more detailed analysis (Siriwardena et al., 1999) indicated that variation in the survival rates of both goldfinches and house sparrows (Passer domesticus) were of sufficient size to cause the decreases observed in the population sizes of these species since the 1960s, although current research suggests that declines in productivity may also be involved in house sparrow population declines.

Goldfinch population trends are likely to be driven by changes in survival rates.

• The annual survival rate of first-year song thrushes decreased from 0.484 to 0.405 between 1975 and 1993, a fall of sufficient magnitude to account for the observed 65% decline in the population size over the same period (Thompson et al., 1997). More recent analyses confirm this finding.

• Decreasing survival rates of first-year reed buntings, and to a lesser extent of adult birds, are the most probable cause of a 54% decrease in abundance of this species observed between 1975 and 1995 (Peach et al., 1999).

The survival rates of first-year reed buntings have declined significantly since 1975.

• A recent study by Freeman and Crick (2002) identified a decrease in the survival rates of either fledgling or first-year spotted flycatchers (Muscicapa striata) as the principal factor in their decline.

Declines in the UK spotted flycatcher population may be caused by an increase in the mortality rate of fledgling or first year birds.

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