Consistently with our predictions and previous studies21, we showed that the choice of breeding strategy might be mediated by brain size (Fig. 4). The female head size was connected to the first egg laying date. Big-headed females started laying eggs later in the season than small-headed females. Later first egg laying date may be connected to either later arrival to the breeding site or a later decision on breeding onset in big-brained females. If the latter was true, it might be connected to differences in habitat sampling between small and big-headed individuals. Although the big brain provides information processing capacity, the process is also time-consuming 17,19. The slower decision-making process was, in fact, found in large-brained guppies in comparison with small-brained guppies42. On the other hand, under laboratory conditions, in artificially selected guppies, it has been shown that fast-exploring guppies are the ones with larger brains, and slow-exploring guppies have smaller brains. The authors suggested the opposite – greater cognitive abilities require less time to assess the environment, however, they also pointed out, that large-brained individuals may take longer at the beginning of a novel environment assessment, but they learn and habituate faster 41. The results of a study of female wild Common eiders, conducted under natural conditions contradict an assumption that large brain, facilitates fast assessment of the environment21. The study showed that the onset of breeding was linked to the head size, breeding dispersal distance and body condition. Large-headed females started breeding later than small-headed females. The effect of body condition on the breeding onset was greater in small-headed females and negligible in large-headed females. The relationship between breeding dispersal distance and breeding onset, on the other hand, was less visible in smaller-headed females but was strong in large-headed females, where the greater was the dispersal distance, the later was the breeding onset. These results suggest that it is beneficial for females in better body condition to start breeding earlier but for large-headed females to accurately choose nesting sites, which delays the breeding onset, especially if the breeding dispersal distance was greater, i.e. the habitat was less familiar. These large-headed females may be more efficient at utilising available resources and thus may afford to nest later in the season. Similar may be true also for Red-backed shrikes. Large-headed females may invest more in a mate and nest-site choice at the beginning of the breeding season and better feeding habitat, or a male of higher quality enables them to stay in better body condition throughout the breeding season, as shrike males feed females during mating and incubation51, and small-headed females gain the advantage from the early breeding onset that provides best conditions for breeding32,33. However, our results did not show any link between the timing of laying eggs and body condition. Similarly to the previous study in Common eider21, our findings may suggest that larger brains, through securing resources, can show high reproductive capacity despite the unfavourable environmental conditions during the latter part of the breeding season. In fact, studies showed that early breeding onset is linked to a higher probability of breeding success, also in Red-backed shrikes, while the onset of breeding was not connected to body condition45. However, we did not confirm that the time of breeding onset was related to breeding success in our population. This may have arisen from the unfavourable weather conditions or the habitat features in the studied population. Late incidents of freezing temperatures in the first half of May and severe droughts slowed vegetation development and decreased shrikes' prey abundance. In fact, we observed significant differences in the phenology of breeding between the years of the study. The probability of success, although insignificantly, also differed between the years. The overall tendency, however, although again – insignificant – was that the likelihood of success decreased with the breeding season advance, which is in line with previous findings of Müller et al.45. Moreover, shrubs in the study area have linear composition, being located almost exclusively alongside dirt roads or ditches, which has been previously shown to be linked with higher predation probability50, which may have also stand behind the lack of significant relationship between the timing of breeding and breeding success.
Nest concealment was positively related to the first egg laying date, and the nest was concealed better if the onset of breeding was earlier. As the later breeding onset may also have consequences in nest-site selection due to, e.g. decreasing availability of preferable nesting sites, also nest concealment may be affected by the decision of when to start breeding. Nest concealment is considered one of the crucial anti-predatory behaviours52,53. Our results show that, in fact, the nest concealment decreases with the advance of the breeding season. This finding is in line with a previous study that also showed decreasing concealment with progressing breeding season in Red-backed shrike54. It may suggest that those late, big-headed birds do not invest in nest concealment but rather in risk assessment. However, head size was not directly linked to nest concealment.
We also found that the breeding success depended on nest concealment, but the relationship was visible only in smaller-headed males. A previous study on Red-backed shrike has shown that more fledglings were produced in better-camouflaged nests45. Our results show that the probability of breeding success overall depends on how well the nest was concealed, however, results also suggest that big-headed males can somehow compensate for not having a well-concealed nest. Even though we failed to show how big-brained shrikes can compensate for lower nest concealment (neither the presence of thorns on the nesting shrub species nor the height at which the nest was located were correlated with the head volume), the literature suggests other behaviours important for successful reproduction. Another study of Red-backed shrike has shown that not the nest concealment but the aggressive behaviour of parents affects breeding success55. The important factor influencing breeding success is the distance to human settlements47. However, it does not apply to our population, as all monitored nests were located far from any buildings. On the other hand, nest concealment can also be viewed as a trade-off between the visibility of the nest to the predator and the visibility of the surroundings to the nesting bird56,57 or even a trade-off between the nest concealment and predator detection strategies58. It has been suggested that it may also be beneficial to the Red-backed shrikes to have higher visibility of the hunting grounds or predators to react quicker and more efficiently when potential predator approaches54. Therefore, birds with better cognitive abilities, which means greater capacity for acquiring and processing information, may as well benefit from having less concealed nests, or at least can compensate for that fact. It can also be linked to microclimatic conditions inside the nest, as it is expected that exposed nests may dry off faster, preventing the chicks from cooling during harsh weather conditions44,54. A few studies also indicated the importance of forming interspecific associations to defend the nest. Red-backed shrikes occupy the same habitat as Barred warbler Sylvia nisoria, and up to half of the shrike population may choose nesting sites in the vicinity of Barred warbler nests. In consequence, they have higher chances of breeding success59,60. On the other hand, nest concealment was not linked to Barred warbler presence in a previous study of Red-backed shrikes48, which suggests the local effect of this relationship. In case of this study, neighbouring with breeding Barred warblers was relatively low due to the lack of suitable habitats for this species.
Our results are, for the most part, in line with those reported by Jaatinen & Öst21. It is striking that we see a similar pattern despite of phylogenetic distance between these species. These results suggest that the cognitive capacity determined by brain size may be a neglected force that shapes decision-making processes on an individual's level.
The link between breeding strategies and brain size (or head size) should be further explored as it is clearly linked in avian species. This link between cognitive abilities, capacity for the informed decision-making process and breeding strategies may give insight into the evolution of these strategies and how these are kept by selection. Future studies may want to focus on other species less susceptible to predation, as the pattern may greatly differ.