Summary1. Circadian (i.e. daily) regulation of behaviours is thought to provide fitness benefits to organisms by enabling them to anticipate diel changes in the environment, such as sunrise. 2. A common behaviour among socially monogamous songbirds that usually takes place in the early mornings is extra-pair mating, that is copulating with partners outside of the social pair bond. 3. Thus, variation in when individuals begin their daily activity may influence their reproductive success; early risers may be better able to gain copulations and guard their partners, thus minimizing their risk of being cuckolded compared with late risers. Sexual selection may thus play an important role in shaping circadian behaviours, but this assumption has yet to be tested in free-living animals. 4. Here, we experimentally weakened endogenous circadian rhythmicity, and thus, anticipation of dawn in male great tits (Parus major) in the wild through the subcutaneous administration of implants filled with melatonin shortly before egg-laying began in this population. Melatonin is a hormone released during the dark phase at night and is one important cue animals use to entrain their circadian clock. 5. Experimental individuals delayed the onset of daily activity compared with controls and were more likely to be cuckolded compared with control males. Manipulation did not alter other behavioural traits observed; no difference between treatments was observed in activity levels during the day or in the end time of daily activity. 6. These results strongly support the assumption that selection, particularly sexual selection, shapes the circadian phenotypes of wild vertebrates which enable anticipation of important and predictive diel changes in an individual's biotic and abiotic environment.

Incomplete reproductive isolation between related species of birds at contact zones is increasingly being documented. Such hybridization typically occurs between sister taxa that diverged in relatively recent times, and hybrids are most often identified based on their intermediate phenotypic characteristics and, increasingly, through genetic admixture analysis. When species have been diverging over relatively longer time scales, prezygotic isolation barriers are expected to evolve, precluding maladaptive interbreeding. Here, we examine the extent of introgressive hybridization in a pair of African barbets, the yellow-fronted tinkerbird (Pogoniulus chrysoconus extoni) and the red-fronted tinkerbird (Pogoniulus pusillus pusillus), which were not previously known to interbreed, across a contact zone in Southern Africa. Although there were significant differences in the coloration of plumage between the species, we found a pattern of extensive admixture in and around the contact zone. Nonetheless, the two species appear to have diverged > 4 Mya and might not even be sister taxa, suggesting that time of divergence alone might not be sufficient for the evolution of prezygotic reproductive barriers. Significantly more phenotypically red-fronted individuals had a P. c. extoni (yellow-fronted) genetic background than vice versa, suggesting possible asymmetry in mate preferences. Sexual selection may thus play a role in breaking down species barriers despite the extent of genetic divergence.

In theoretical frameworks, it is often assumed that citizen science projects contribute to the Global Sustainable Development Goals (SDGs) because of their participatory character and the potential for social transformation. To bring a practical perspective into the dialogue, we designed a survey to obtain direct assessments of the contribution of citizen science to the SDGs by European project participants. We launched the survey across European science networks in 2020 and evaluated 125 questionnaires. Participants reported contributing most to three of the SDGs: ‘Good health and well-being’, ‘Quality education’, and Life on Land’. Additionally, our results provide evidence that, with ongoing advocacy, citizen science projects in Europe could support all SDGs in the future. Seventy-two percent of participants indicated that their projects are involved in data acquisition and 30% stated to report data, but 19% do not pass on data at all. Our findings indicate further that European citizen science projects lack infrastructures and institutional support to facilitate data sharing. We recommend a focus on the promotion or creation of interfaces, for example, between projects and UN databases. Finally, we advise that citizen science projects, some of which operate with little funding, should not be overburdened with inflated expectations as a means of implementing the SDGs.

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