CONSEQUENCES OF WITHIN-FAMILY COMMUNICATION AND REARING ENVIRONMENT ON OFFSPRING PERFORMANCES AND PARENTAL INVESTMENT IN TWO BIRD SPECIES

Conflicts of interest among family members are expected to occur over parental investment. In altricial species, individual offspring are selected to receive a larger share of parental resources than their siblings and to obtain more care than it would be optimal for parents to provide. On the other hand, parents are expected to allocate their parental investment depending on the reproductive value of individual offspring and according to the environmental conditions so to maximize their own fitness. Perinatal environment is mainly determined by parental decisions over the time and place of breeding as well as over the number of offspring produced and the effort devoted in attending the progeny, which can determine the intensity of within-brood competition and ultimately the amount of resources received by each member of the progeny. The amount of resources received by individual offspring is crucial in determining its growth, general state and viability, with consequences that can eventually carry-over into adult life. It is widely believed that the expression of the so-called begging behaviour, consisting of a diverse set of morphological and behavioural displays that offspring use to solicit care provisioning, is crucial in the resolution of the intra-familiar conflicts. The first part of the present thesis (Chapters 1-3) is concerned with intra-family communication in an altricial bird species, the barn swallow (Hirundo rustica), with special focus on the factors that may influence the expression of begging behaviour (e.g. sex, hunger level and immune challenge), and thus mediate sibling competition and parental food allocation, and ultimately cause variation in offspring general state and growth trajectories. In addition, I have examined the function of sib-sib interactions, mediated by a peculiar form of within-brood communication that occurs in the absence of parents, and their consequences for access to food in order to investigate state-dependent expression of selfish/altruistic behaviour among siblings. I have shown that solicitation of parental care considerably increases when nestlings are experimentally deprived of food and thus reliably reflects short-term need (Chapters 1-3). In addition, I have provided evidence for a fine-tuned modulation of begging behaviour by nestlings in relation to nestmate’s need: less hungry nestlings in fact reduce their competitive effort when facing a sibling in poor condition (Chapter 1) and when a nestmate performs begging displays in the absence of the attending parents (Chapter 2). Such a reduced competitiveness favours the access to food by needy kin and allows them to recover from a food deficiency. The modulation of selfishness thus assures indirect fitness benefits by enhancing survival prospects of siblings. This observation emphasizes the importance of kin selection in moulding the complex interactions among siblings for the access to limiting parental resources and shows that offspring play a crucial role in the control of allocation of parental investment (Chapters 1 and 2). Furthermore, begging intensity increases when nestlings are subjected to an immune challenge that causes a measurable deterioration in their general condition (e.g. decrease in body mass and plumage quality; Chapter 3), implying that begging behaviour can convey reliable information on individual reproductive value. Such information can thus be exploited by parents to optimally modulate allocation of care towards each member of the progeny. I have also demonstrated a sex-dependent variation in the effects of an immune challenge (e.g. feathers growth), leading to differences in the expression of begging behaviour between the sexes. However, variation in general condition does not to affect parental food allocation, likely because barn swallows adopt a ‘brood survival’ strategy whereby parents tend to promote successful fledging of all offspring rather than discriminating among their progeny (Chapter 3). The second part of my thesis (Chapters 4-5) focuses on variation of parental investment towards offspring of either sex according to spatial heterogeneity and seasonality of ecological conditions. In particular, I have examined variation in the sex ratio of first and second broods of the same barn swallow pairs in relation to nesting habitat quality (Chapter 4). In a further study I have investigated the seasonal variation in growth patterns and individual quality of male and female nestlings of the common starling (Sturnus vulgaris; Chapter 5). The proportion of male offspring significantly increases in barn swallow colonies surrounded by large extent of hayfields, the main foraging habitat for this insectivorous bird, and, only in second broods, in colonies showing recent negative demographic trends. These findings suggest an adaptive sex allocation strategy by parents, because males have a higher natal philopatry than females and are thus more likely to benefit from favourable local environment (Chapter 4). I have also documented a seasonal decline in offspring quality (e.g. reduction of antioxidant defence and body growth), particularly marked for females, in the common starling. Such a sexual-related decrease in condition is accompanied by a female-biased mortality in the late-hatched broods and it is possibly caused by a skew of parental food allocation towards higher-quality males or by the monopolization of resources by stronger male competitors (Chapter 5). Finally, in the last part of my work (Chapters 6-7) I have considered the carry-over effects of rearing conditions on longevity and reproductive success in the barn swallow. More specifically, I have shown that conditions experienced at the nestlings stage are important determinants of lifetime fitness: indeed, hatching date and individual condition (e.g. stronger immune response and larger body size compared to nest mates) predict lifespan and the number of offspring successfully reared during the entire life (Chapter 6). Moreover, I found that a bacterial immune challenge during the early post-natal stage, may cause the failure of the first breeding attempt, thus considerably reducing reproductive success in this short-lived species (Chapter 7). In conclusion, the present work provides novel information on the function of begging displays performed both in the presence or in the absence of parents, and the ultimate role of begging behaviour in the resolution of intra-familiar conflicts. It further provides new evidence for the relevance of the rearing environment, mediated by parental decisions over breeding, in affecting nestling growth and reproductive value, potentially causing long-term effects on survival and reproduction, and in determining the parental sex allocation strategies.