Supplemental Food Alters Egg Size Hierarchies within Harrier Clutches

Adaptive brood reduction strategies may be facilitated either by asynchronous hatching or by a decrease in the size of the last laid eggs. Large species typically show brood reduction (small final eggs) strategies and small species the reverse (large final eggs). The adaptive significance of these strategies are poorly known since egg size hierarchies within clutches are rarely subjected to experimental manipulation of food resources. Unlike many other large species, African marsh harriers Circus ranivorus living in predictable subtropical environments showed a natural increase in the size of last laid eggs, suggesting a brood survival strategy for the last hatched young. However, food supplemented birds reversed the hierarchy and laid increasingly small eggs, together with an optimistic clutch size. The latter pattern mimics the usual egg-size hierarchy found among their temperate congeners. Since birds were supplemented throughout laying, the significantly smaller final eggs could not be explained by a food constraint model, and they cannot be merely species-specific trends since the hierarchies varied with food resources. I suggest therefore, that African harriers are able to adjust their egg size hierarchies depending on food resources, which are usually initially poor, but predictable. When this is reversed, (food resources good but unpredictable), harriers lay optimistic clutches with a brood reduction strategy. Food predictability may also play a key role in explaining the general pattern of large final eggs in small species where the time interval between clutch planning and brood rearing is short, hence more predictable; in larger species in which the interval is longer, predictability may be reduced. Clearly the strategy employed may depend on temporal fluctuations in the prey base, the "worth" of the final egg and whether species respond to extra food by laying optimistically large and "risky" clutches.