Causes and Consequences of Differential Growth in Birds

Flexible growth and body mass predict physiological condition at fledging in the synchronously breeding European starling, Sturnus vulgaris

Recent studies have reported beneficial carryover effects of juvenile development that predict interspecific survival differences at independence. Yet, traits relating to body size (i.e. morphological traits) have proven to be unreliable predictors of juvenile survival within species. Exploring individual variation of growth trajectories and how they covary with physiology could reveal species-specific developmental modes which have implications for our assessments of juvenile quality. Here, we investigated morphological development of European starlings (Sturnus vulgaris) approaching fledging in relation to three components of physiological condition at independence: aerobic capacity, energy state and oxidative status. We found evidence of flexible mass and wing growth which independently covaried with fledgling energy state and aerobic capacity, respectively. By comparison, tarsus and wing length at fledging were unrelated to any physiological trait, while mass was positively associated with principal component scores that comprised aerobic capacity and energy state. Thus, flexible growth trajectories were consistent with 'developmental plasticity': adaptive pre-fledging mass recession and compensatory wing growth, which seemingly came at a physiological cost, while fledgling body mass positively reflected overall physiological condition. This highlights how patterns of growth and absolute size may differently reflect fledgling physiology, potentially leading to variable relationships between morphological traits and juvenile fitness.

Strategic growth in social vertebrates

Individual differences in growth and size of vertebrates often represent adaptive, plastic responses to contrasts in ecological conditions. Recent studies show that vertebrates can also modify their growth and size in an adaptive fashion in response to fine-grain changes in social conditions (which we refer to as strategic growth). Here, we review experimental evidence for strategic growth in social vertebrates. We describe a set of conditions under which strategic growth commonly occurs, and highlight potential examples of convergent evolution of strategic growth across the tree of life. This synthesis has implications for the way we think about organismal growth and size, because it underscores that the size of individuals can often be fine-tuned to their social environment.

Adaptation and constraint shape the evolution of growth patterns in passerine birds across the globe

Background

Growth trajectories should be adapted to selective factors of each species’ environment. However, major shaping forces of growth and development are unclear, especially when studying several traits at once. Birds provide an ideal opportunity to analyze growth patterns across species due to there being enough available data. We tested the relative importance of nest predation risk, the number of care-givers, nest height, foraging substrate, clutch size, and latitude on growth patterns of passerine birds (Passeriformes) using phylogenetic comparative methods. Specifically, we studied the evolution of fledging time, average and peak growth rates, and relative development at fledging of body mass and tarsus, wing, and tail length.

Results

Using a comprehensive literature search and data quality control, we obtained data on growth in 231 species based on 295 populations. Species with long development in the nest grew slowly and had well-developed traits at fledging. Species breeding under high nest predation risk, building their nests close to the ground, and those living in northern temperate regions fledged early and grew fast, sometimes fledging with less developed body mass and traits critical for locomotion (tarsus, wing, and tail). On the other hand, the number of caring adults, clutch size, and species’ foraging substrate had very limited predictive value for growth patterns across passerine species.

Conclusions

Shortening of the nestling period was a primary means of accelerating development (in relation to nest predation, nest height, and latitude), sometimes supplemented by higher peak growth rates of body mass, tarsus, and wing (especially in relation to latitude). Overall growth patterns of passerines were adaptively tuned to nest predation risk and nest height, with northern temperate species having especially short nestling periods and fast growth rates of body mass, tarsus, and wing.

Sex-specific patterns of minimal compensation of care during and after short term mate removal in biparental blue tits

Early theoretical models predicted that over evolutionary timescales, changes in effort by one biparental parent should result in incomplete compensation by the other. Empirical studies, however, report responses ranging from no compensation through to complete compensation which may mean that parents respond to each other's efforts over short time scales, as predicted by some recent theoretical models. Few studies have examined behavioural changes over short time periods which mimic the onset of reduced effort so we removed one blue tit (Cyanistes caeruleus) parent for 20 min during nestling provisioning. We then quantified the provisioning rates of both parents for 60 min 'pre-removal', the non-removed partner during the 20 min 'removal' period and both parents for 60 min 'post-removal'. When compared to pre-removal, both sexes reduced their provisioning rates during the removal stage and also during the post-removal stage. There were, however, sex-specific provisioning patterns in the hour after the parent was returned because after females were released, males began provisioning at a relatively high rate and then maintained that rate across the hour after removal whereas after males were released, females began provisioning at a low rate but significantly increased thereafter. There was no long term effect on offspring fitness, which probably reflects the short time parents were removed and so we conclude that parents with biparental care adjust their provisioning rates to successfully overcome very short term decreases in care.

Local weather conditions have complex effects on the growth of blue tit nestlings

Adverse weather conditions are expected to result in impaired nestling development in birds, but empirical studies have provided equivocal support for such a relationship. This may be because the negative effects of adverse weather conditions are masked by parental effects. Globally, ambient temperatures, rainfall levels and wind speeds are all expected to increase in a changing climate and so there is a need for a better understanding of the relationship between weather conditions and nestling growth. Here, we describe a correlative study that examined the relationships between local temperatures, rainfall levels and wind speeds and the growth of individual blue tit (Cyanistes caeruleus) nestlings in relation to their hatching order and sex. We found that changes in a range of morphological characters were negatively related to both temperature and wind speed, but positively related to rainfall. These patterns were further influenced by the hatching order of the nestlings but not by nestling sex. This suggests that the predicted changes in local weather conditions may have complex effects on nestling growth, but that parents may be able to mitigate the adverse effects via adaptive parental effects. We therefore conclude that local weather conditions have complex effects on avian growth and the implications for patterns of avian growth in a changing climate are discussed.

The design and function of birds' nests

All birds construct nests in which to lay eggs and/or raise offspring. Traditionally, it was thought that natural selection and the requirement to minimize the risk of predation determined the design of completed nests. However, it is becoming increasingly apparent that sexual selection also influences nest design. This is an important development as while species such as bowerbirds build structures that are extended phenotypic signals whose sole purpose is to attract a mate, nests contain eggs and/or offspring, thereby suggesting a direct trade-off between the conflicting requirements of natural and sexual selection. Nest design also varies adaptively in order to both minimize the detrimental effects of parasites and to create a suitable microclimate for parents and developing offspring in relation to predictable variation in environmental conditions. Our understanding of the design and function of birds' nests has increased considerably in recent years, and the evidence suggests that nests have four nonmutually exclusive functions. Consequently, we conclude that the design of birds' nests is far more sophisticated than previously realized and that nests are multifunctional structures that have important fitness consequences for the builder/s.