Changes over time and space in a rapidly moving chickadee hybrid zone: variation in genotypes, hatching success, and nestling sex ratio

Climate warming is causing rapid pole-ward movement of some avian hybrid zones. These systems provide powerful opportunities for investigating fitness consequences of hybridization. We studied variation in hatching success and nestling sex ratio associated with northward movement of the contact zone between Black-capped and Carolina chickadees (Poecile atricapillus, P. carolinensis) using long-term data (2000 – 2017) from Hawk Mountain, Pennsylvania, along with comparative information from three other regional sites. Based on eight species-diagnostic single nucleotide polymorphism (SNP) markers, the Hawk Mountain breeding population changed over the 18 years from pure Black-capped Chickadee to strongly mixed, as Carolina Chickadees moved northward. Mean hatching success declined from > 90% to < 60%. Pair-level analysis confirmed that very low hatching success in clutches produced by mixed pairs drove the overall decline in hatching success. Contrary to expectations from Haldane’s Rule, within-brood nestling sex ratio did not vary with genetic compatibility of parents or with hatching success; however, sire genotype was associated with brood sex ratio, with hybrid fathers overproducing sons. Our additional data were consistent with Hawk Mountain results: at two sites that were outside the hybrid zone throughout the study, hatching success did not change; at a fourth site, population composition shifted from mixed to nearly pure Carolina Chickadee, and annual mean hatching success improved over time. Our study shows that interbreeding by chickadees confers substantial fitness costs in a pattern matching hybrid-zone movement, but our data do not yield clear evidence for Haldane’s Rule.