| Hybrid zone movement can provide powerful opportunities for investigating fitness relationships, including Haldane's rule. I used longitudinal data (1998 -- 2016) from four Pennsylvania populations to study spatiotemporal variation in hatching success and nestling sex ratio associated with northward movement of the Black-capped x Carolina chickadee contact zone. Hatching success changed inversely in two populations where hybridization decreased or increased respectively, but did not vary with time in sites outside of the contact zone. In the leading-edge population, adults with hybrid genotypes had reduced hatching success. Nestling sex ratios did not vary spatiotemporally or with pair genotypes; however, increasingly hybrid genotypes of male parents were associated with elevated production of sons. Associations between variation in chickadee genotypes in time and space with hatching success reveal fitness costs from hybridization. Little support for Haldane's rule emerged, but parents may contribute to reduced fitness of the heterogametic sex to different degrees. |
