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In animals, reproductive performance typically improves over time early in life. Several ultimate and proximate mechanisms may contribute to such an age-related improvement and these mechanisms can act in a relative or in an absolute sense. Low performance of young individuals may be the consequence of a comparison or competition with older individuals (relative), or it may be due to specific traits of young individuals and be unrelated to the presence of older competitors (absolute). Here, we perform a test to disentangle whether the effect of age class (yearling or older) on male extra-pair siring success is relative or absolute. Male age is the most consistent predictor of male extra-pair siring success across bird species, yet the mechanisms underlying this pattern are not well understood. Low extra-pair siring success of yearling males may be a consequence of the presence of older (“adult”) males (hypothesis 1), because adult males are more successful in intra- and intersexual interactions or because females prefer to copulate with adult males when available (relative preference). Alternatively, low extra-pair siring success of yearlings may be independent of the presence of adult males (hypothesis 2), for example, if yearling males on average invest less in extra-pair behavior or if females avoid them as extra-pair mates, independent of the availability of older males (absolute preference). To distinguish between these 2 hypotheses, we experimentally manipulated the age structure of a nest-box-breeding population of blue tits ( Cyanistes caeruleus ) by removing almost all adult males, and compared patterns of extra-pair paternity in the experimental year with those from the preceding 15 “control” years. Removal of adult males resulted in a substantial increase in the extra-pair siring success of yearling males compared to the “control” years, but did not affect the population-level frequency of extra-pair paternity or its spatial patterns. Our results provide clear evidence that extra-pair siring success of yearlings can increase and that it depends on the presence of older males in the population, indicating a relative effect of age on reproductive performance. These results suggest that older males outcompete yearling males in direct or indirect interactions, in sperm competition or as a result of differences in attractiveness to females.

When host nests are scarce, avian brood parasites would benefit from behaviours that increase the availability of suitable nests. Several studies reported ejection of host nestlings from nests by brood parasites; however, whether brood parasites do so to induce the host to re‐nest and thus increase opportunities for future parasitism (i.e. ‘farming’ behaviour) remains unclear. Here, we report observational evidence of farming behaviour by a common cuckoo Cuculus canorus female in a Daurian redstart Phoenicurus auroreus population: (1) the cuckoo destroyed a host nest by ejecting all nestlings, (2) the host then produced a new nest and (3) the cuckoo successfully parasitized the replacement nest. We suggest that farming behaviour may be more common, but often goes undetected because it requires intense nest monitoring. The farming hypothesis suggests that avain brood parasites may destroy nests that are unsuitable for parasitism, thereby inducing the host to re‐nest. In this study, we provided the first explicit evidence that common cuckoos farm Daurian redstarts by ejecting host nestlings.

In mammals, glucocorticoid (i.e. GC) levels have been associated with specific life-history stages and transitions, reproductive strategies, and a plethora of behaviors. Assessment of adrenocortical activity via measurement of glucocorticoid metabolites in feces (FGCM) has greatly facilitated data collection from wild animals, due to its non-invasive nature, and thus has become an established tool in behavioral ecology and conservation biology. The aim of our study was to validate a fecal glucocorticoid assay for assessing adrenocortical activity in meerkats (Suricata suricatta), by comparing the suitability of three GC enzyme immunoassays (corticosterone, 11β-hydroxyetiocholanolone and 11oxo-etiocholanolone) in detecting FGCM increases in adult males and females following a pharmacological challenge with adrenocorticotropic hormone (ACTH) and biological stimuli. In addition, we investigated the time course characterizing FGCM excretion, the effect of age, sex and time of day on FGCM levels and assessed the potential effects of soil contamination (sand) on FGCM patterns. Our results show that the group specific 11β-hydroxyetiocholanolone assay was most sensitive to FGCM alterations, detecting significant and most distinctive elevations in FGCM levels around 25 h after ACTH administration. We found no age and sex differences in basal FGCM or on peak response levels to ACTH, but a marked diurnal pattern, with FGCM levels being substantially higher in the morning than later during the day. Soil contamination did not significantly affect FGCM patterns. Our results emphasize the importance of conducting assay validations to characterize species-specific endocrine excretion patterns, a crucial step to all animal endocrinology studies using a non-invasive approach.

In species that are subject to brood parasitism, individuals often vary in their responses to parasitic eggs, with some rejecting the eggs while others do not. While some factors, such as host age (breeding experience), the degree of egg matching and the level of perceived risk of brood parasitism have been shown to influence host decisions, much of the variation remains unexplained. The host personality hypothesis suggests that personality traits of the host influence its response to parasitic eggs, but few studies have tested this. We investigated the relationship between two personality traits (exploration and neophobia) and a physiological trait (breathing rate) of the host, and egg-rejection behaviour in a population of Daurian redstarts Phoenicurus auroreus in northeast China. We first show that exploratory behaviour and the response to a novel object are repeatable for individual females and strongly covary, indicating distinct personality types. We then show that fast-exploring and less neophobic hosts were more likely to reject parasitic eggs than slow-exploring and more neophobic hosts. Variation in breathing rate—a measure of the stress-response—did not affect rejection behaviour. Our results demonstrate that host personality, along the bold-shy continuum, predicts the responses to parasitic eggs in Daurian redstarts, with bold hosts being more likely to reject parasitic eggs.

The perceived risk of predation can affect breeding behaviour and reduce reproductive success in prey species. Individuals exposed to predators may also adopt different mating tactics with potential consequences for the distribution of paternity in socially monogamous species that engage in extra‐pair copulations. We experimentally increased perceived predation risk during the fertile period in blue tits Cyanistes caeruleus. Every morning between nest completion and the onset of egg laying, we presented a model of either a predator or a non‐predator (control) near active nestboxes. Broods from pairs exposed to predators had higher levels of extra‐pair paternity than control broods. This mainly resulted from a higher proportion of extra‐pair offspring in broods with at least one extra‐pair young. Females exposed to predators first emerged from the nestbox later in the morning, stayed away from the nestbox for longer and were less likely to be visited at the nest by their social mate, but we detected no behavioural differences once the model was removed. Our results suggest that the higher rates of extra‐pair paternity resulted from the disruption of morning routines, which may have inhibited within‐pair copulations or increased opportunities for females to engage in extra‐pair copulations. We conclude that the perceived risk of predation can have substantial effects on levels of extra‐pair paternity. Perceived predation risk can have many behavioural consequences, but how it affects mating behaviour has rarely been tested. The authors exposed blue tits to predator models during the peak fertile period and found that levels of extra‐pair paternity were substantially increased, possibly because early morning behaviour was disrupted.

1. A central goal in evolutionary ecology is to identify factors that explain variation in reproductive success, i.e. in the number of offspring produced. In altricial birds, a substantial part of this variation is determined by the number of nestlings that die before fledging, but surprisingly little is known about the proximate causes of offspring mortality during the nestling period. 2. We used a uniquely comprehensive dataset of parental nestbox visits from seven breeding seasons to investigate the association between parental behaviour and nestling mortality in a population of blue tits (Cyanistes caeruieus). 3. In almost all nests that suffered complete brood mortality one of the parents had suddenly disappeared during the nestling stage. In contrast, parental disappear- ance in nests with partial brood mortality was rare and equally common as in nests with no brood loss. 4. With few exceptions, parents that disappeared during the nestling stage were never observed again and never returned to breed. In contrast, parents that remained after their partner disappeared were equally likely to be observed again or return to breed as parents of nests where both parents stayed. Visit rates at nests where a parent would disappear did not differ from those at nests where both parents stayed. 5. Taken together, our results show that - in contrast to partial brood failure - complete brood failure is almost always associated with the sudden and permanent disappearance of one of the parents, probably due to prédation. Partial and complete brood mortality should be treated as distinct processes that have different underlying causes.

Summary Oxidative damage, caused by reactive oxygen species during aerobic respiration, is thought to be an important mediator of life‐history trade‐offs. To mitigate oxidative damage, antioxidant defence mechanisms are deployed, often at the cost of resource allocation to other body functions. Both reduced resource allocation to body functions and direct oxidative damage may decrease individual fitness, through reducing survival and/or reproductive output. The oxidative costs of reproduction have gained much attention recently, but few studies have investigated the long‐term consequences of oxidative damage on survival and (future) reproductive output under natural conditions. Using a wild population of the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis), we tested the prediction that high levels of reactive oxygen species, or high antioxidant investments to avoid oxidative damage, have fitness consequences because they reduce survival and/or reproductive output. We found that individuals with higher circulating non‐enzymatic antioxidant capacity had a lower probability of surviving until the next year. However, neither current reproductive output, nor future reproductive output in the surviving individuals, was associated with circulating non‐enzymatic antioxidant capacity or oxidative damage. The negative relationship between antioxidant capacity and survival that we observed concurs with the findings of an extensive comparative study on birds, however the mechanisms underlying this association remain to be resolved. A lay summary is available for this article. Lay Summary

The common hippopotamus Hippopotamus amphibius can significantly influence the dynamics of ecosystems and engender serious conflicts with people but, in Kenya, one of the species strongholds, it has been little studied or monitored. We surveyed the hippopotamus population in the Masai Mara National Reserve and the adjoining pastoral ranches in 2006 using foot counts along 155.3 km of the main rivers. We counted 4,170 hippopotamuses in 171 schools. Comparisons with earlier surveys suggest that this population increased by 169.6% between 1971 and 1980 within the reserve and, although it did not increase within the reserve during 1980–2006, it increased by 359.4% outside the reserve during this period against a background of deteriorating habitat conditions. The overall density in 2006 was 26.9 hippopotamuses km-1 of river, equivalent to a biomass of 26,677 kg km-1 of river. The ratio of calves to 100 adults was 9:100 inside the reserve, 10:100 outside the reserve and 6:100 along tributaries of the Mara River, implying that the population is either increasing or that its spatial distribution is being compressed because of range contraction. The apparent increase in the hippopotamus population contrasts with marked contemporaneous declines in the populations of most other large mammalian herbivore species in the Reserve. We discuss possible reasons underlying the increase in the hippopotamus population.

Reproductive success within populations often varies with the timing of breeding, typically declining over the season. This variation is usually attributed to seasonal changes in resource availability and/or differences in the quality or experience of breeders. In colonial species, the timing of breeding may be of particular importance because the costs and benefits of colonial breeding are likely to vary over the season and also with colony size. In this study, we examine the relationship between timing of breeding and reproductive performance (clutch size and nest success) both within and between variable sized colonies (n = 18) of fairy martins, Petrochelidon ariel. In four of these colonies, we also experimentally delayed laying in selected nests to disentangle the effects of laying date and individual quality/experience on reproductive success. Within colonies, later laying birds produced smaller clutches, but only in larger colonies. The general seasonal decline in nest success was also more pronounced in larger colonies. Late laying birds were generally smaller than earlier laying birds, but morphological differences were also related to colony size, suggesting optimal colony size also varies with phenotype. Experimentally delayed clutches were larger than concurrently produced non-delayed clutches, but only in larger colonies. Similarly, delayed clutches were more likely to produce fledglings, particularly later in the season and in larger colonies. We suggest that the reduced performance of late breeding pairs in larger colonies resulted primarily from inexperienced/low quality birds preferring to settle in larger colonies, possibly exacerbated by an increase in the costs of coloniality (e.g., resource depletion and ectoparasite infestations) with date and colony size. These findings highlight the importance of phenotype-related differences in settlement decisions and reproductive performance to an improved understanding of colonial breeding and variation in colony size.