Our study provides new insight into stress levels in relation to group living versus territorial strategies. We found significant differences in faecal corticosterone metabolites between fractions of the same population with different life styles. At baseline concentrations, corticosterone primarily relates to metabolism and, by regulating energy intake, storage and mobilization, supports energetically demanding processes [1
]. Higher stress levels in non-territorial ravens may be associated with higher energetic demands, perhaps through greater competition, more unpredictable environments, higher predation risk, food shortage or poor foraging skills. They may be also influenced by their younger age; however, most studies on long-lived birds have not found a significant effect of age on baseline corticosterone levels (e.g. [18
]). Raven flocks are unstable, non-cohesive groups where a dominance hierarchy—which determines the access to food—develops, and aggressive interactions are common [13
]. This supports the idea that stress levels are elevated in unstable groups [6
]. Additionally, elevated corticosterone concentrations may also denote adaptive responses to promote dispersal or movements and to improve foraging behaviour [7
]. According to Goymann & Wingfield [1
], our findings suggest that it may be more energetically demanding to live in groups than to maintain a territory.
We found an interaction between sociality and sex only in territorial birds, suggesting differences in resource competition and selection pressures between females and males in raven pairs, but not in non-breeders. The higher baseline corticosterone levels exhibited by territorial males in relation to females may be a response to higher energetic demands; males are most immediately involved in aggression associated with the establishment and maintenance of a territory, defence of food resources against raven flocks and mate-guarding behaviour [1
]. Although the effects of parasites are energetically demanding, and can be better coped with by increasing baseline corticosterone concentrations (e.g. [7
]), parasite burdens were not a prime factor mediating stress levels.
Because hormones mediate life-history trade-offs, including transitions between life stages and relationships with the environment [2
], species with variable life histories, including breeding strategies and social structure, may show different hormonal patterns. For example, some studies demonstrated the absence of high levels of corticosterone in non-breeders of cooperatively breeding species [9
], or no sex differences in baseline corticosterone concentrations in adult birds of colonial species [7
]. Our study highlights the prevalent role of sociality in stress levels, and sheds light on the hormonal mechanisms underlying group living and territorial strategies within a population. Elevated corticosterone levels may simply help non-territorial birds maintain homeostasis in an unstable environment and social context; on the other hand, by affecting individual fitness, they may act as a selective filter to enter adulthood. High stress levels may also act as an important force pressing non-breeders to leave the group and acquire a territory. Our findings should inspire further research on the link between hormone levels and selective pressures modulating gregarious and territorial strategies in long-lived birds.