In order to provide high quality ecological knowledge on the determinant of individual fitness and their trade-offs, and on factors explaining offspring recruitment,
I estimated the contribution of trade-offs to inter-individual variation in life-history traits, opposed to most research on trade-offs focused on their detectability and evolution. I estimated the covariation among the survival and reproduction traits most frequently reported in the literature using top quality, long-term monitoring data of five raptor species. I furthermore accounted for the effects of adverse environmental conditions of three different lengths (1 to 3 years) upon the start of the breeding career and for natal conditions, and used meta-analysis techniques to combine and generalize species-specific results.
I found limited evidence for covariation among fitness-related traits under normal environmental conditions. However under prolonged adverse environmental conditions trade-offs between early reproduction and subsequent reproduction or survival became apparent.
Before the project belief was that trade-offs shape individual life-histories within populations, although how commonly there are expressed and the strength of the relationships remained unknown. After our analyses I realised that trade-offs contribute little to inter-individual variation in trait values over ecological time scales, unless individuals experience prolonged adverse conditions when trade-offs may account for ~4 % of the observed variation in trait-values. These findings also have an applied value as researchers can use them to determine whether in their research scenario it might be unwise to disregard possible contributions of trade-offs to measured traits. For instance when assessing impact of extrinsic factors (e.g. habitat type or management actions) on breeding output in a population that has experienced adverse conditions for a prolonged period, trade-offs are likely and bias results if not accounted for.
Regarding offspring survival and recruitment, I analysed the effect on recruitment of brood size, offspring body condition, their trade-offs, and hatching date. I also accounted for the variation in food availability, weather and other environmental conditions that may create cohort effects. Trade-offs resulted in significant negative correlations between brood size and body condition in two of the four species, but their effect in recruitment were negligible. Body condition of nestlings had no or marginal effect on recruitment probability, even after controlling for interactions with hatching date, post-fledging weather conditions or food conditions. In 3 of the 4 species individuals who produced larger broods size had a higher probability that one or more of their offspring were later recruited some offspring whereas large brood size it did not diminish, or only marginally, individual probability of recruitment. The number of recruits produced per brood was related to brood size and hatching date in three species, and together explained 0.040 (±0.029) of the observed variation in recruitment and accounted for 24 % (±18) of the variation in recruitment explained by fixed factors. On average hatching date was slightly better predictor of recruitment than brood size, as it accounted for more partial variation.
The best proxy for local recruits produced was LRS. I have modelled estimated LRS to show which is the minimum of change a given monitoring design can detect.
