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 resource availabilities eliminating the positive relationship between reproductive isolation and performance tradeoffs. This latter result is consistent with previous work suggesting that increasing resource abundance dampens the effect of strong performance tradeoffs on evolutionary divergence. Because many organisms, with the notable exception of host-specific phytophagous insects, rely on abundant food resources with weak performance tradeoffs while breeding, our results may explain why sympatric speciation is uncommon.