Understanding the Role of Ecology in Variation of Breeding Phenology and Success in Two Bird Populations

Abstract

Climate change has a well-established impact on the trait dynamics of wild populations. Trophic mismatches driven by climate shifts have been associated with observations of a negative relationship between phenology and breeding success at the population level, driving directional selection for early breeding. However, phenology and its relationship with breeding success is variable even under population-level selection. Can these two observations be reconciled? This thesis presents two studies investigating the possibility of environmentally-driven variation in selection at fine spatiotemporal scales (e.g., between habitats/territories) in small passerines. I first used additive modelling frameworks to quantify spatial variation and autocorrelation in the breeding ecology of blue and great tits, and then attempt to explain this with environmental covariates. I found limited evidence of spatial variation in phenology and success, despite spatially non-random environmental effects on nestbox occupancy. However, the relationship between phenology and success interacted with environmental covariates. Following this, I used random slopes modelling to test how the strength and shape of selection for early breeding varies at multiple spatiotemporal scales which I then attempt to explain using scale-dependent environmental variation. I found that the phenology-success relationship (and thus selection) varied among territories and breeding seasons, with different environmental effects operating at each scale. My research shows that ecologically-driven variation in selection within populations can persist alongside directional selection for early breeding at the population level. This may explain persistent variation in phenological strategy under mismatch-driven selection pressure. My findings therefore constitute a significant advancement towards formulating predictions of how climate effects could continue to shape breeding ecology in the wild.