Brain lateralization is considered adaptive because it leads to behavioral biases and specializations that bring fitness benefits. Across species, strongly lateralized individuals perform better in specific behaviors likely to improve survival. What constrains continued exaggerated lateralization? We measured survival of pheasants, finding that individuals with stronger bias in their footedness had shorter life expectancies compared to individuals with weak biases. Consequently, weak, or no footedness provided the highest fitness benefits. If, as suggested, footedness is indicative of more general brain lateralization, this could explain why continued brain lateralization is constrained even though it may improve performance in specific behaviors.

Cognitive abilities probably evolve through natural selection if they provide individuals with fitness benefits. A growing number of studies demonstrate a positive relationship between performance in psychometric tasks and (proxy) measures of fitness. We assayed the performance of 154 common pheasant (Phasianus colchicus) chicks on tests of acquisition and reversal learning, using a different set of chicks and different set of cue types (spatial location and colour) in each of two years and then followed their fates after release into the wild. Across all birds, individuals that were slow to reverse previously learned associations were more likely to survive to four months old. For heavy birds, individuals that rapidly acquired an association had improved survival to four months, whereas for light birds, slow acquirers were more likely to be alive. Slow reversers also exhibited less exploratory behaviour in assays when five weeks old. Fast acquirers visited more artificial feeders after release. In contrast to most other studies, we showed that apparently 'poor' cognitive performance (slow reversal speed suggesting low behavioural flexibility) correlates with fitness benefits in at least some circumstances. This correlation suggests a novel mechanism by which continued exaggeration of cognitive abilities may be constrained.This article is part of the theme issue 'Causes and consequences of individual differences in cognitive abilities'.

© 2016 the Association for the Study of Animal Behaviour a strong motivation for one individual to aggregate with others is to reduce their vigilance because other group members provide coverage and warning of approaching predators. This collective vigilance means that a focal individual is usually less susceptible to predation than when alone. However, individuals differ in their vigilance levels depending on status and context. They may also differ in how they adjust their vigilance levels as group size changes. This flexibility in response means that the collective vigilance of a group, and hence its optimal size, is not intuitive. We demonstrate, in both natural and experimental systems, that male and female pheasants, Phasianus colchicus, in harems differentially adjusted their vigilance levels as harem size changed. Females became less vigilant as harems became larger, and benefited by increasing their foraging time. Conversely, males became more vigilant as harems became larger. We calculated the collective probability that a harem would detect a predator. Within natural harem sizes, a male and two females exhibited the highest probability of collective detection, with decreases as more females joined. This optimal harem size matched the average harem size observed at our study site. Females may join harems for benefits of collective vigilance. Despite both sexes having a shared interest in larger harems for mating benefits, optimal harem size is influenced by trade-offs in a nonsexual behaviour, vigilance. This results in males with relatively small harems, females associating with less preferred males and each male being surrounded by fewer females than he could mate with.

© 2016 the Authors. Subtle variations in early rearing environment influence morphological, cognitive and behavioural processes that together impact on adult fitness. We manipulated habitat complexity experienced by young pheasants (Phasianus colchicus) in their first seven weeks, adding a third accessible dimension by placing elevated perches in their rearing pens mimicking natural variation in habitat complexity. This simple manipulation provoked an interrelated suite of morphological, cognitive and behavioural changes, culminating in decreased wild mortality of birds from complex habitats compared with controls. Three mechanisms contribute to this: Pheasants reared with perches had a morphology which could enable them to fly to the higher branches and cope with prolonged roosting. They had a higher propensity to roost off the ground at night in the wild. More generally, these birds had more accurate spatial memory. Consequently, birds were at a reduced risk of terrestrial predation. The fitness consequences of variation in early rearing on behavioural development are rarely studied in the wild but we show that this is necessary because the effects can be broad ranging and not simple, depending on a complex interplay of behavioural, cognitive and morphological elements, even when effects that the treatments provoke are relatively short term and plastic.

Behavioural and physiological deficiencies are major reasons why reintroduction programmes suffer from high mortality when captive animals are used. Mitigation of these deficiencies is essential for successful reintroduction programmes. Our study manipulated early developmental diet to better replicate foraging behaviour in the wild. Over 2 years, we hand-reared 1800 pheasants (Phasianus colchicus), from 1 day old, for 7 weeks under different dietary conditions. In year one, 900 pheasants were divided into three groups and reared with (i) commercial chick crumb, (ii) crumb plus 1% live mealworm or (iii) crumb plus 5% mixed seed and fruit. In year two, a further 900 pheasants were divided into two groups and reared with (i) commercial chick crumb or (ii) crumb plus a combination of 1% mealworm and 5% mixed seed and fruit. In both years, the commercial chick crumb acted as a control treatment, whilst those with live prey and mixed seeds and fruits mimicking a more naturalistic diet. After 7 weeks reared on these diets, pheasants were released into the wild. Postrelease survival was improved with exposure to more naturalistic diets prior to release. We identified four mechanisms to explain this. Pheasants reared with more naturalistic diets (i) foraged for less time and had a higher likelihood of performing vigilance behaviours, (ii) were quicker at handling live prey items, (iii) were less reliant on supplementary feed which could be withdrawn and (iv) developed different gut morphologies. These mechanisms allowed the pheasants to (i) reduce the risk of predation by reducing exposure time whilst foraging and allowing more time to be vigilant; (ii) be better at handling and discriminating natural food items and not be solely reliant on supplementary feed; and (iii) have a better gut system to cope with the natural forage after the cessation of supplementary feeding in the spring. Learning food discrimination, preference and handling skills by the provision of a more naturalistic diet is essential prior to the release of pheasants in a reintroduction programme. Subsequent diet, foraging behaviour, gut morphology and digestive capabilities all work together as one nutritional complex. Simple manipulations during early development can influence these characteristics to better prepare an individual for survival upon release.

© 2013 the Association for the Study of Animal Behaviour. Recreational hunting can disrupt the population structure or alter the morphology of target populations. More subtly, such hunting may alter the behaviour of individuals in the target population, especially if individuals are culled nonrandomly. We assayed the behavioural temperaments of a sample of hand-reared and released pheasants, Phasianus colchicus. We could place birds on a behavioural continuum between bold or fast and shy or slow. Individual differences could not be explained by sex or mass. Birds were released into the wild and we followed their fate over a single hunting season. Birds that survived the hunting season were shyer or slower as juveniles than the original population mean. Males that died of disease or predation were relatively bold or fast as juveniles, while females dying of disease or predation were relatively shy or slow. Males that were bold or fast as juveniles were shot early in the season compared to females. Unselective hunting can skew the expression of behaviours in released gamebirds. This skew may explain why released birds subsequently fail to reproduce or are especially likely to die of natural causes once the hunting season has finished, and hence why it is difficult to establish wild populations of these species through reintroduction to an area where shooting takes place.

Females vary in their mate choice and consequent fitness outcomes. Individual differences may be explained by conditions experienced early in life. We tested whether the sex ratio at which young pheasants, Phasianus colchicus, were reared affected their adult sexual behaviour. Females reared in equal sex ratios discriminated strongly between males of differing attractiveness in choice tests and had the lowest variance in mating success. Conversely, females reared in female-biased sex ratios showed little discrimination between males based on their attractiveness, and exhibited highly skewed mating success with the majority gaining no copulations, but a quarter each gaining more copulations than any other female in the study. Early life environmental determination of variation in female choice could explain the lack of uniformity in mate choice and hence maintain variation in male traits in the face of directional sexual selection. © 2013 the Association for the Study of Animal Behaviour.