Ornaments are useful signals only if they are costly to fake. In the common yellowthroat, the black mask has been examined against the major histocompatibility complex, the gene family that lets the immune system recognise pathogens. Work on a Wisconsin population found that males with larger masks carried greater MHC diversity and had higher reproductive success, including paternity gained outside their social pair.
The interesting complication is geography. In a different population studied in New York, the yellow bib rather than the mask was the trait associated with male quality. Two populations of the same species, therefore, appear to place weight on different ornaments, which cuts against the assumption that a species has one honest signal.
This matters for how bird ornament studies are read. A result demonstrated in one population is often reported as a species trait. In yellowthroats the same experimental design in two locations produced different answers, and the working interpretation is that local conditions, including pathogen pressure, shape which signal is informative.