Climatically driven changes in population composition and offspring sex-morph ratio in a polymorphic species

Abstract

Females from polymorphic species could theoretically benefit by adjusting offspring morph composition, since morphs often differ in ecological and behavioral traits, and may be differentially susceptible to environmental conditions. Although offspring sex ratio adjustments have been increasingly supported, whether, and in which contexts, genetically determined morph ratios are adjusted has not been established. We used a long-term dataset to examine whether broad-scale climatic patterns characterized by teleconnection indices, population composition, and other environmental factors affect offspring sexmorph ratios in the white-throated sparrow (Zonotrichia albicollis), a species with chromosomal morph determination. We predicted that females might overproduce the underrepresented sex or morph, since pairing is disassortative by morph, and might also produce more white morph male offspring given favorable climatic conditions, because white males (WMs) are large, promiscuous, and expensive to produce. The proportion of WM-by-tan female breeding pairs and recruitment of WMs declined following winters with high Pacific North American teleconnection indices, and females of both morphs produced more WM offspring after these relatively cold winters. These winters and the following springs were also associated with low Southern Oscillation indices, indicating El Ni~no. Females of both morphs also produced fewer males late in asynchronously hatching broods. Adjustment of brood composition in response to climatically mediated changes in population composition could stabilize the white-throated sparrow's disassortative breeding system and genetic polymorphism.