A 27-year study tracking 35 bird species across North America finds that female birds generally have lower survival rates than males. Researchers report that the pattern is consistent across a large dataset, suggesting the discrepancy is broader than previously recognized. The study focuses on how the demands of reproduction and migration may affect females more strongly, given the energy and risk involved in nesting, raising offspring, and related biological processes. The findings also highlight an apparent contrast with mammals, where females often live longer and have higher survival than males, a difference that researchers say remains difficult to explain. The study points to potential biological mechanisms, including genetic factors tied to sex chromosomes, as one possible contributor to the survival gap. Overall, the researchers describe the results as an ongoing puzzle in why, in birds, sex-based differences in survival run in the opposite direction from the common mammal pattern.