The earlier the better? Nesting timing and reproductive success in subalpine cavity-nesting bees

Reproductive timing can affect an organism's production of offspring and its offspring's success, both of which contribute to its overall fitness. In seasonal environments, the timing of reproductive activity may be restricted to short periods of the year owing to numerous potential selective pressures such as variation in daylength, weather, food availability, predation or competition. We documented the relationships between reproductive timing and individual reproductive success (total reproductive output and offspring success) in subalpine populations of five cavity-nesting solitary bee species. We also examined the relationships between bee reproductive success and environmental variables that are likely ultimate drivers of bee phenology in subalpine environments (i.e. seasonality of floral resource abundance and temperature). Over 6 years, we recorded solitary bee nesting timing, egg production and offspring success using artificial nesting structures ('trap-nests') established at multiple study sites. We also quantified floral resources and recorded temperature throughout growing seasons. Bees nesting earlier in the season exhibited greater reproductive success. Reproductive output generally increased with floral abundance, although this relationship was weak and only significant for some bee species. Elevated temperatures were associated with increased nest construction rate, but not with greater reproductive output. These contrasting effects of temperature may have been driven by the negative relationship between temperature and bee longevity. Bees who nested for shorter durations of time (a proxy for longevity) produced fewer offspring, and individuals exhibiting the shortest nesting durations were also those that began nesting late in the season. Overall, bees who initiated nesting early and sustained activity for a long duration had the highest reproductive output. This work documents the relationship between reproductive phenology and fitness in wild insect populations and highlights the ways in which organisms can cope with the challenges of living in seasonal and highly variable environments.