Quantifying the relative importance of direct and indirect effects influencing bird nestling growth

Nestling growth parameters are integral components of avian life-history strategies as they are crucial determinants of individual survival.Although many factors impact on nestling growth,the relative contribution of each one is still debated in the literature.Most studies rely on the assumption that each factor directly affects nestling growth,but indirect effects mediated by other factors are usually the rule in nature.In this study,we present a comprehensive view of both direct and indirect factors affecting nestling growth using the Red-crested Cardinal(Paroaria coronata)as model system.We evaluated the relative importance of different habitat(forest structure),biotic interactions(botfly larvae ectoparasitism,number of siblings,hatching order),and temporal factors(time of breeding)on nestling growth parameters in 278 nestlings of 128 nests by using piecewise structural equation models.We found that botfly ectoparasitism had the strongest direct effect on nestling growth and,in turn,forest structure increased the probability of botfly occurrence.Besides,the interaction between the number of siblings and hatching order influenced nestling growth,indicating that the first and second nestlings had disproportionately higher growth rates in large than in small clutches.Time of breeding also showed a strong positive indirect effect on botfly occurrence,as well as a weak direct positive effect on nestling growth.Our results demonstrate that,under natural conditions,nestling growth is driven by different factors acting not only directly,but also indirectly on this essential life history trait,and that these factors weave a complex web of interrelated variables.

Linking the hierarchical decision- making process of fruit choice and the phenotypic selection strength on fruit traits by birds

Aims Animals in search of fleshy fruits forage mostly according to the number of available fruits and then select individual fruits based on reward quality or advertised subtle traits.This hierarchical pat-tern of fruit choice would be translated into patterns of selection strength mediated by frugivores on fruit display traits.Thus,frugi-vores would exert higher selection pressures on fruit crop size and lower selection pressures on within-plant variation of phenotypic traits(infructescence,fruit and seed size).However,no attempt to link this behavioral mechanism of hierarchical trait selection to natural selection patterns has been made.Therefore,we sought to determine the relationship between the hierarchical decision-mak-ing process of fruit choice and patterns of natural selection on fruit traits.Methods We recorded bird visits and measured fruit-related traits(fruit crop size,fruit diameter and seed weight)in a natural population of Psychotria carthagenensis,a bird-dispersed treelet,in a Yungas for-est from Argentina.To assess phenotypic selection patterns on fruit display traits,we performed multivariate selection analysis,and to explicitly identify a hierarchy of fruit trait choice we used a classifi-cation tree as a predictive model.Important Findings Selection patterns on fruit display traits were in agreement with a hierarchical process of fruit choice made by birds.The strength of directional selection on the total number of fruits in a plant(i.e.fruit crop size)was nearly two times higher than on fruit size,and the classification tree analysis supported this hierarchical pattern.Our results support previous evidence that seed dispersers shape fruit crop size with higher intensity than subindividual fruit traits.Also,high levels of subindividual phenotypic variation of fruit display traits may be explained by relaxed selection pressures exerted by frugivores.Empirical studies also show that this pattern may consti-tute a general phenomenon among other plant-animal interactions.