Effects of manipulated levels of predation threat on parental provisioning and nestling begging.

Parental provisioning behavior is a major determinant of offspring growth and survival, but high provisioning rates might come at the cost of increased predation threat. Parents should thus adjust provisioning activity according to current predation threat levels. Moreover, life-history theory predicts that response to predation threat should be correlated with investment in current reproduction. We experimentally manipulated perceived predation threat in free-living great tits (Parus major) by presenting parents with a nest predator model while monitoring different aspects of provisioning behavior and nestling begging. Experiments were conducted in 2 years differing greatly in ecological conditions, including food availability. We further quantified male territorial aggressiveness and male and female exploratory tendency. Parents adjusted provisioning according to current levels of threat in an apparently adaptive way. They delayed nest visits during periods of elevated perceived predation threat and subsequently compensated for lost feeding opportunities by increasing provisioning once the immediate threat had diminished. Nestling begging increased after elevated levels of predation threat, but returned to baseline levels by the end of the experiment, suggesting that parents had fully compensated for lost feeding opportunities. There was no evidence for a link between male exploration behavior or aggressiveness and provisioning behavior. In contrast, fast-exploring females provisioned at higher rates, but only in the year with poor environmental conditions, which might indicate a greater willingness to invest in current reproduction in general. Future work should assess whether these personality-related differences in delivery rates under harsher conditions came at a cost of reduced residual reproductive value.

Sources of (co)variation in alternative siring routes available to male great tits (Parus major).

Males of socially monogamous species can increase their siring success via within-pair and extra-pair fertilizations. In this study, we focused on the different sources of (co)variation between these siring routes, and asked how each contributes to total siring success. We quantified the fertilization routes to siring success, as well as behaviors that have been hypothesized to affect siring success, over a five-year period for a wild population of great tits Parus major. We considered siring success and its fertilization routes as "interactive phenotypes" arising from phenotypic contributions of both members of the social pair. We show that siring success is strongly affected by the fecundity of the social (female) partner. We also demonstrate that a strong positive correlation between extra-pair fertilization success and paternity loss likely constrains the evolution of these two routes. Moreover, we show that more explorative and aggressive males had less extra-pair fertilizations, whereas more explorative females laid larger clutches. This study thus demonstrates that (co)variation in siring routes is caused by multiple factors not necessarily related to characteristics of males. We thereby highlight the importance of acknowledging the multilevel structure of male fertilization routes when studying the evolution of male mating strategies.

Does coping style predict optimization? An experimental test in a wild passerine bird.

A number of studies have suggested that avian brood size is individually optimized. Yet, optimal reproductive decisions likely vary owing to among-individual differences in environmental sensitivity. Specifically, 'proactive' individuals who do not track environmental changes may be less able to produce optimal brood sizes than 'reactive' individuals who have more precise local environmental knowledge. To test this, we quantified exploratory behaviour (a proxy for proactivity) in a great tit (Parus major) population, manipulated brood sizes (reduced, control, enlarged) and evaluated whether individuals of dissimilar coping style differed in their level of optimization. If reactive females behaved optimally, any deviation from their original brood size should lower fitness, whereas this should not be the case for proactive females. Reactive females indeed performed best at their natural brood size, whereas proactive females performed best when raising an enlarged brood. These findings imply that proactive females produced sub-optimal brood sizes. We speculate that proactive females might (i) take decisions based on biased perception of their environment, (ii) face energetic constraints in offspring production and/or (iii) be more willing to invest into current reproduction when given the option. Our findings provide experimental evidence for coping style-related differences in optimal reproductive decisions and life-history strategies.

Individual variation in plasma testosterone levels and its relation to badge size in House Sparrows Passer domesticus: it's a night-and-day difference.

The steroid hormone testosterone (T) plays a central role in the regulation of reproduction in animals. Although seasonal variation in T levels is well-studied, differences between day and night have only been described in relatively few species, and daily within-individual variation has been largely neglected when evaluating the relationship between T and the expression of sexual ornaments or behavior. We measured plasma T levels during day and night in a captive population of House Sparrows, and analyzed their relationship with an important male ornament--badge size. T levels were on average twice as high at night than during daytime. This was true in all seasons, and in both males and females. Disturbance of the birds at night, but not during the day, led to significantly lower T levels, suggesting a rapid drop after an individual wakes up. The relationship between T levels and badge size depended on the time when T was measured. During the breeding season, badge size was strongly positively correlated with night-time, but not with daytime T levels. This suggests that badge size signals information related to an individual's maximum potential T level such as social dominance. Our study highlights that integrative research on the endocrine control of ornament expression needs to take diel variation in hormone levels into account.