Mass-dependent predation risk as a mechanism for house sparrow declines? - PubMed (original) (raw)

Mass-dependent predation risk as a mechanism for house sparrow declines?

Ross MacLeod et al. Biol Lett. 2006.

Abstract

House sparrow (Passer domesticus) numbers have declined rapidly in both rural and urban habitats across Western Europe over the last 30 years, leading to their inclusion on the UK conservation red list. The decline in farmland has been linked to a reduction in winter survival caused by reduced food supply. This reduction in food supply is associated with agricultural intensification that has led to the loss of seed-rich winter stubble and access to spilt grain. However, urban house sparrows have also declined, suggesting that reduced food supply in farmland is not the sole reason for the decline. Here, we show that changes in house sparrow mass and thus fat reserves are not regulated to minimize starvation risk, as would be expected if limited winter food were the only cause of population decline. Instead, the species appears to be responding to mass-dependent predation risk, with starvation risk and predation risk traded-off such that house sparrows may be particularly vulnerable to environmental change that reduces the predictability of the food supply.

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Figures

Figure 1

Figure 1

Seasonal and diurnal mass change in house sparrows and comparable species. (HS, house sparrow; BB, blackbird Turdus merula; CH, chaffinch Fringilla coelebs; DU, dunnock Prunella modularis; GF, greenfinch Carduelis chloris; RO, robin Erithacus rubecula; ns, no significant difference; ***, very highly significant difference.) Residual mass controls for size (measured by wing length), location of capture (longitude and latitude within Britain) and for part (a) time of capture (since dawn). (a) Inter-specific comparison of residual mean mass (±95% CI) in non-breeding and winter seasons. House sparrows do not show any significant mass gain in winter consistent with compensation for high predation risk, while all five other species do. (b) Inter-specific comparison of percentage diurnal mass gain (±95% CI) at start of day in winter. House sparrows show a low mass gain pattern early in the day consistent with compensation for high predation risk. Mass gain between the first and second parts of the day is expressed as the proportion of mass gained over the whole day by that species.

Figure 2

Figure 2

House sparrow mass in the presence and absence of sparrowhawks. The significantly higher mass of individuals living in areas without sparrowhawks demonstrates that house sparrows are not phenotypically constrained from maintaining greater body mass. Error bars represent ±95% CI.

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