Experimental reduction of haematocrit affects reproductive performance in European starlings

Reproductive success usually declines in the course of the season, which may be a direct effect of breeding time, an effect of quality (individuals with high phenotypic or environmental quality breeding early), or a combination of the two. Being able to distinguish between these possibilities is crucial when trying to understand individual variation in annual routines, for instance when to breed, moult and migrate. We review experiments with free-living birds performed to distinguish between the 'timing' and 'quality' hypothesis. 'Clean' manipulation of breeding time seems impossible, and we therefore discuss strong and weak points of different manipulation techniques. We find that the qualitative results were independent of manipulation technique (inducing replacement clutches versus cross-fostering early and late clutches). Given that the two techniques differ strongly in demands made on the birds, this suggests that potential experimental biases are limited. Overall, the evidence indicated that date and quality are both important, depending on fitness component and species, although evidence for the date hypothesis was found more frequently. We expected both effects to be prevalent, since only if date per se is important, does an incentive exist for high-quality birds to breed early. We discuss mechanisms mediating the seasonal decline in reproductive success, and distinguish between effects of absolute date and relative date, for instance timing relative to seasonal environmental fluctuations or conspecifics. The latter is important at least in some cases, suggesting that the optimal breeding time may be frequency dependent, but this has been little studied. A recurring pattern among cross-fostering studies was that delay experiments provided evidence for the quality hypothesis, while advance experiments provided evidence for the date hypothesis. This indicates that late pairs are constrained from producing a clutch earlier in the season, presumably by the fitness costs this would entail. This provides us with a paradox: evidence for the date hypothesis leads us to conclude that quality is important for the ability to breed early.

Reproductive effort can have profound effects on subsequent performance. Field experiments on the collared flycatcher (Ficedula albicollis) have demonstrated a number of trade-offs between life-history traits at different ages. The mechanism by which reproductive effort is mediated into future reproductive performance remains obscure. Anti-parasite adaptation such as cell-mediated immunity may probably also be costly. Hence the possibility exists of a trade-off between reproductive effort and the ability to resist parasitic infection. Serological tests on unmanipulated collared flycatchers show that pre-breeding nutritional status correlates positively with reproductive success and negatively with susceptibility to parasitism (viruses, bacteria and protozoan parasites). Both immune response and several indicators of infectious disease correlate negatively with reproductive success. Similar relations are found between secondary sexual characters and infection parameters. For brood-size-manipulated birds there was a significant interaction between experimentally increased reproductive effort and parasitic infection rate with regard to both current and future fecundity. It seems possible that the interaction between parasitic infection, nutrition and reproductive effort can be an important mechanism in the ultimate shaping of life-history variation in avian populations.