Details of Award
NERC Reference : NE/C000978/1
Mass change as an indicator of population processes in birds
Grant Award
- Principal Investigator:
- Dr W Cresswell, University of St Andrews, Biology
- Grant held at:
- University of St Andrews, Biology
- Science Area:
- Terrestrial
- Overall Classification:
- Terrestrial
- ENRIs:
- Natural Resource Management
- Biodiversity
- Science Topics:
- Survey & Monitoring
- Population Ecology
- Conservation Ecology
- Behavioural Ecology
- Abstract:
- Animals have to both eat and avoid being eaten. Any animal that stays hidden or that watches constantly is unlikely to get caught by a predator, but then such an animal will never feed and so will starve. In the same way, any animal that feeds constantly is not likely to starve, but may be caught by surprise by a predator. Animals therefore spend time both avoiding predators and feeding, and how much time they spend doing one or the other depends on what the animal considers is its risk of starvation or being eaten. In birds, how an individual views both starvation and predation risk is neatly measured by its weight. Fat birds are unlikely to starve, but getting fat takes time away from avoiding predators and once fat, a bird's ability to fly quickly away from an attacking cat or hawk is reduced. Therefore we tend to find fatter birds when starvation risk is relatively important such as during the cold winter days when feeding is difficult and thinner birds when predation risk is relatively important such as when hawks are common. In theory, we should then be able to work out the relative importance of starvation or predation risk simply from a measure of birds' weight and how this varies. If this is true, it would be useful because measuring either starvation risk or predation risk is complicated involving a lot of detailed field measurements, whereas measuring weight is simple and routinely carried out for many wild birds during bird ringing. What this research aims to do is to establish how we can use the weights of birds to assess starvation and predation risk and so draw conclusions about how they may be determining the habits and population size of birds. An example of how we can do this is provided by the house sparrow. House sparrows have declined by 12 million in the last 30 years and have become extinct in many areas. The reason for this is not well known but was thought most likely to be due to decrease in availability of food. We analysed the weights of over 10,000 house sparrows that were caught as part of the British Trust for Ornithology ringing scheme, where birds are marked by enthusiastic amateurs to determine their movements and how long they live. We compared the sparrow weights to those of five other similar common species that have not declined. Theory predicts that if sparrows are suffering from increased starvation risk, leading to their population decline, then they should show higher levels of fat reserves (increased weight) compared to those species that have not declined. We found the opposite: sparrows did not put fat on over the winter compared to the other five species. This suggests that house sparrows are under greater predation risk and this was confirmed because we found that weights of sparrows in areas where there are no hawks were higher than in hawk areas. House sparrows are also the most common bird prey of domestic cats and a preferred prey of hawks. We concluded that house sparrows are probably suffering from the effects of reduced food supplies because they cannot afford to put on extra fat as insurance because of the danger of predators. Thus we have gained an insight into the importance of predation and starvation risk in determining house sparrow population decline. What we propose to do now is to expand our research to analyse weights from 30 species of UK birds. If we can establish rules that tell us what the chance of starvation and predation is for a population of birds on the basis of easily collected weights, we can then provide a general tool that will allow us to tell why populations might be declining and so take conservation steps. With the house sparrow, for example, the simple recommendations of increasing dense cover to act as refuges for sparrows on farmland, or of putting bells on cats to warn urban sparrows, that are suggested by our research may be sufficient to address the decline of the house sparrow.
- NERC Reference:
- NE/C000978/1
- Grant Stage:
- Completed
- Scheme:
- Small Grants Pre FEC
- Grant Status:
- Closed
- Programme:
- Small Grants
This grant award has a total value of £30,695
FDAB - Financial Details (Award breakdown by headings)
| Total - Staff | Total - T&S | Total - Other Costs | Total - Indirect Costs |
|---|---|---|---|
| £18,040 | £1,282 | £3,076 | £8,298 |
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