Performance tradeoffs and resource availability drive variation in reproductive isolation between sympatrically diverging crossbills

Theoretical models indicate that speciation, especially when the scope for gene flow is great (e.g., sympatric speciation), is most likely when strong performance tradeoffs coincide with reproduction. We tested this classic hypothesis using measures of the strength of three prezygotic reproductive isolating barriers (habitat isolation, reduced immigrant fecundity, and behavioral isolation) between two young (~2,000 yrs) and sympatric red crossbill (Loxia curvirostra) ecotypes. All three isolating barriers increased with increases in performance tradeoffs, with total reproductive isolation varying between 0.72 and 1 (0 represents random mating, 1 represents complete reproductive isolation). Strong tradeoffs led to strong habitat isolation, an inability to breed in the “wrong” habitat, and more assortative flocks, with the latter leading to stronger behavioral isolation. Reproductive isolation decreased as resource availability increased relative to the demands of breeding, with higher re...