Bold or cautious : behavioural characteristics and dominance in great tits

Social dominance affects territory acquisition, reproduction and survival in many species. It plays a major role in the life of an individual, and has important consequences for its fitness. Several factors that can influence dominance relationships between individuals have been well studied, such as differences between rivals in size, weight, fighting experience, prior residence, and resource value. Individual behavioural characteristics can also influence dominance. Studies of domesticated and laboratory animals show that individual animals can be characterised by their aggressiveness, as measured in standardised tests, and that aggressive individuals are generally dominant over non-aggressive ones. These behavioural characteristics have a genetic basis, and are not only reflected in aggressiveness but also in a whole range of other behaviours, such as exploration or nest building. Based on these behavioural characteristics, different types of animals can be distinguished. So far, behavioural characteristics have mainly been studied in mammals, not in birds. Moreover, the effect of such behavioural characteristics on social dominance has received little attention in studies of natural populations. This thesis aims to take a first step by studying the existence of consistent individual behavioural characteristics and their effect on social dominance in the great tit (Parus major). If behavioural characteristics are really individual traits, or in other words 'consistent', they should already exist early in life, before any dominance relationship develops. In that phase of life, exploration and foraging are the main activities of great tits. Indeed, during the first 18 weeks of their life, hand-reared young male great tits, collected from a natural population, could be characterised by their exploratory behaviour (chapter 2). In repeated tests, they showed consistent reactions to a strange object in their home cage, even with different objects and after an interval of some weeks. This extended to other exploratory behaviour. Birds that approached a strange object quickly, were also fast to explore an aviary with which they were unfamiliar. Accordingly, birds that approached a strange object more slowly, took more time to explore thoroughly the unfamiliar aviary. These behavioural differences were also reflected in the strength of foraging habits, built up during a training in which food was always offered in the same place. After a change in the location of food, the fast and superficial explorers (FE) would stick to their habit, and keep going to the place where the food used to be. The slow and thorough explorers (SE) soon changed their behaviour and stopped going to the usual feeding place. They seemed to remain alert and to pay more attention to stimuli in the known environment than the fast, superficial explorers. The juveniles could also be characterised by their aggressive behaviour in experimental pair-wise confrontations (chapter 3). FE started more fights and won more often than SE, also when possible effects on dominance of other factors such as weight and size were taken into account. For FE clearly the first blow was half the battle. For natural populations, the relevance of these findings could still be small, since dominance relationships normally develop in a flock of juveniles, resulting in a more complex social hierarchy. Therefore groups of birds were observed in aviaries, which probably better resemble natural conditions (chapter 4). In all observed groups, a stable hierarchy only established after a dynamic phase of several days, in which many reversals in dominance relations occurred. During the first day in the aviary, the situation was similar to that observed in the tests with pair-wise confrontations. FE initiated more fights than SE and won more often, again after correction for factors such as weight and size. Surprisingly though, once the hierarchy had stabilised, SE were on average dominant over FE; SE had higher ranks than FE in the hierarchy. Apparently, the same behavioural characteristics result in different dominance relationships under different circumstances. Further observations supported and specified this conclusion. Other studies have shown that familiarity with the environment increases chances of becoming dominant. The birds in the groups had been unfamiliar with the aviary before being put together. The SE initiated fewer fights than FE and initiated their first fight more often in the place where they had spent most time. This suggested that SE made more use of their knowledge of the environment than FE, who were more focused on fights. Their more thorough manner of exploring may gradually have led to a better, or more detailed, knowledge of the environment in SE, which in turn may increase their chances of winning. In this way, the initial advantage that FE had by giving the first blow, could be reversed by the alertness and increasing spatial knowledge of SE. Such a gradual process could not happen in groups of birds that were first separately familiarised with the aviary before being put together (chapter 5). In those groups, all birds had a good knowledge of the environment and FE and SE did not show the differences in their behaviour that they showed in the unfamiliar aviaries. In these familiar aviaries, FE won on average over SE, both on the first day and after stabilisation of the hierarchy. These results suggest that SE may be best adapted to new or unstable and changing environments, while FE may do better in familiar and stable ones. The presence and behaviour of flock mates in the groups also modified dominance relations of FE and SE (chapters 4, 5). Several studies have shown that previous experience in a fight influences the outcome of a subsequent fight. FE and SE differed in their fighting behaviour and reaction to previous fights. FE seemed to take more risks in their fighting behaviour. In all groups, they attacked quickly and won from SE on the first day. But if FE lost severely, they needed more time to recover before starting a new fight, which strongly suggests that they had problems in coping with the defeat. The more cautious SE needed less time to recover and seemed not only to make more use in their fights of information about their physical, but also about their social, environment. They would take advantage of the vulnerability of a FE that had just lost from a third bird, by starting a fight with that loser. In this way, a FE with a high rank that lost severely from a FE with an even higher rank, could subsequently also lose from SE and fall in hierarchy to the lowest positions. This resulted in a stable hierarchy in which fast explorers had either high or low ranks, while slow explorers had middle ranks. This characteristic dominance pattern was found in all observed groups. In the groups in unfamiliar aviaries, only a small proportion of the FE had extreme high ranks, while a large proportion had extreme low ranks. In the groups in familiar aviaries it was the other way round. Hence, familiarity with the environment only influenced the proportion of FE in highest or lowest places, resulting in different average dominance of FE and SE in the two experimental situations. Such average dominance is therefore not the most sensitive way to describe dominance relations in a stable hierarchy. The last chapter (6) discusses whether the behavioural characteristics reflect two different, but equally successful, behavioural strategies to cope with environmental challenges, and what their consequences could be for foraging success, territoriality and survival in different natural situations. This could be the starting point for future studies. In conclusion, the results of this study show that juvenile male great tits show consistent individual differences in exploratory behaviour. These differences extend to fighting behaviour, which is consistent over different social situations. These behavioural characteristics predict dominance, the outcome depending on familiarity with the environment and behaviour of possible flock mates.