Six key results emerged from this.
First, group has a major impact on its movement characteristics, with intermediately sized groups expressing lower daily travel distances and larger home ranges. These advantageous characteristics suggest that there is an optimal group size for movement—something that has only previously been found in baboons. Groups of intermediate size are also much more flexible when facing more extreme conditions, for example they are able to better balance the risk of hyperthermia and the need to forage, under harsh conditions.
Second, groups have striking adaptations to drought. As conditions dry, groups start expressing substantially larger home ranges, and often move into completely new areas. As soon as rain starts, they move back into their regular home range, highlighting that these extra-range movements likely correspond to undesired movements outside of a preferred range. The project has addressed many questions in relation to this, including how groups navigate between distant areas and their preferred home ranges, their fidelity to their preferred home ranges (and what might explain this), how information is transferred among groups and facilitates movements towards areas of greater resources, and some of the costs that groups incur when making these movements (see key result five).
Third, fine-scale monitoring using state-of-the-art biologging tools has allowed the project to gain an unprecedented understanding of the trade-offs faced by arid-adapted birds facing extreme climatic conditions (notably drought). Drought conditions have a two-fold impact, they reduce the resources that are available (causing an increase in foraging effort) and they expose animals to more extreme thermal conditions (increasing the physiological risks associated with foraging). The trade-off between avoiding starvation (foraging) and maintaining safe body temperature (thermal homeostasis) becomes increasingly challenging to navigate during droughts. As a result, birds lose substantial body weight and experience an increase in mortality. However, this increase is not felt equally among all groups, with larger groups better able to minimise the impacts.
Fourth, many processes take place within groups when they make decisions. While all individuals can contribute to making a group decision, not all do for each decision. Several hypotheses outline how groups should made decisions. These capture how they should move when there is a conflict among group members about where to go (the geometry of decision-making), where they should choose to go when faced with this conflict (using a majority rule), and who is most likely to have try to lead and when (leadership according to need). The project provided empirical support for the predictions derived from each of these three hypotheses, by demonstrating that groups express the same geometric properties in terms of the direction of movement when faced with conflicting directional preferences, that when they have to choose one direction they choose the direction of the majority, and finally that individuals are most likely to contribute to leadership when they have been excluded from foraging on resources. Finally, the project expanded theoretical links between these different aspects of collective decision-making by demonstrating that variation in who initiates movements (tries to lead) can be predicted by optimal foraging theory.
Fifth, the project has revealed highly novel insights into the energetic consequences of of individual and collective movements. It has long been assumed that making larger distance movements incurs higher energetic costs. The project revealed that this is not necessarily true, and that animals can express a suite of movement strategies that can help them to mitigate the energetic cost of transport (how much energy they use to displace a given distance). This involves moving faster and straighter, thereby increasing the efficiency of movement. The project showed that by using such strategies, dispersing individuals can cover almost their entire increase in daily movement distance with almost exactly the same amount of energy spent on moving. However, groups also make very large movements in search of resources during droughts. The project found that although groups expressed similar movement strategies, the constant directional negotiations made as the group moves limits their ability to maintain higher movement speeds, and thus limits their ability to improve their movement efficiency. This finding was one of the first to reveal a major cost to collective movements.
Sixth, and finally, the project revealed previously undescribed costs associated with attempting to lead the group. To date, most theory proposed that being a leader is beneficial to individuals because the group is more likely to do what the leader prefers. However, this has raised a paradox—why do we see that many individuals often opt out from leadership and collective decisions across species? The project made an important revelation that answers this paradox by finding that individuals pay a physiological cost when attempting to lead, with the cost being particularly high (higher than the costs from movement alone) when their attempt goes against the majority of group members.
