Action: Provide supplementary food for raptors to increase adult survival
Two randomised, replicated and controlled studies in the USA found that nesting northern goshawks Accipiter gentilis were significantly heavier in territories supplied with supplementary food, compared with those from unfed territories.
Well-fed animals are likely to be in better physical condition than those with too little food: having greater muscle mass and larger fat supplies to help them survive lean periods. However, it is worth noting that species that forage in groups can have dominance hierarchies, which alter the relationship between weight and fitness. For example Gentle and Gosler (2001) found that, amongst great tits Parus major in Oxfordshire, England, more dominant birds had a lower mass than subdominants, particularly when perceived predation risk was high. Birds with lower masses are better able to take off and therefore escape predators than heavier birds (Krams 2002). However, because of their dominance, they were able to usurp other birds from food resources when hungry. Care should therefore be taken when interpreting results which do not directly examine survival.
Krams, I. (2002) Mass-dependent take-off ability in wintering great tits (Parus major): comparison of top-ranked adult males and subordinate juvenile females. Behavioral Ecology and Sociobiology, 51, 345–349.
Gentle, L.K. & Gosler, A.G. (2001) Fat reserves and perceived predation risk in the great tit, Parus major. Proceedings of the Royal Society of London. Series B: Biological Sciences, 268, 487.
Supporting evidence from individual studies
A randomised, replicated and controlled trial in mixed conifer forests and scrub in New Mexico, USA, in 1992-3 (Ward & Kennedy 1996), found that northern goshawk Accipiter gentilis adults from territories provided with supplementary food during nesting were heavier than adults from control (unfed) territories, but sample sizes were too small for statistical tests (females: average of 1,007 g for six birds from fed territories vs. 975 g for five controls; males: average of 689 g for five fed territories vs. 660 g for two controls). Supplementary food consisted of dead Japanese quail Cortunix japonica provided every other day starting the day after hatching and continuing until most control birds left the area. This study also examined differences in chick growth and survival, discussed in ‘Provide supplementary food to increase reproductive success’.
A randomised, replicated and controlled trial in mixed conifer forests in Utah, USA, in 1996-7 (Dewey & Kennedy 2001), found that northern goshawk Accipiter gentilis females from territories provided with supplementary food (Japanese quail Coturnix japonica provided from close to hatching to chick independence) were significantly heavier than those from control (unfed) territories (1,104 g for eight fed females vs. 993 g for nine controls). This study also examined differences in chick growth and survival, discussed in ‘Provide supplementary food to increase reproductive success’.