Raptor predation was studied by direct observation of sparrowhawks, peregrines, and merlins, hunting a known prey population, and subsequent recovery of kills. Raptor predation was shown to be the most significant cause of mortality in most wader species. Kleptoparasitism of raptors carrying prey, by carrion crows, significantly increased the over winter mortality of some waders. Redshank populations were most affected by raptor predation; over 50% of the total population and over 90% of the juvenile population were taken in two winters. No selection for body size in redshank was found, but juveniles were more likely to be killed by raptors. This was a consequence of adult redshank risk-averse foraging, and excluding juveniles from low-risk and low-feeding profitability areas. Juveniles, even though feeding in a relatively profitable area compared to adults, still showed risk-prone foraging within that area. Flocking reduced an individual redshank's probability of being killed by a raptor. Larger flocks were preferentially attacked, but an attack was significantly more likely to succeed on a smaller flock. Within a larger flock a redshank was less at risk through the 'dilution' effect, vigilance effects (which were shown to be a direct consequence of flock size) and probably also the 'confusion' effect. Redshank did not gain any foraging benefits within larger flocks. Reduced individual risk of predation appeared to be the main reason for flocking. Redshank responded differently when attacked in a similar way by the three species of raptor. During an attack the probability of capture depended on the escape response. The response that was most likely to lead to escape from a sparrowhawk was most likely to lead to capture on peregrine attack, showing that raptor discrimination was an important determinant of over-winter survival.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:645136 |
| Date | January 1993 |
| Creators | Cresswell, Will |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/14744 |
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