House finch leg color changes with infection

In numerous animals, dramatic coloration (e.g. bright red or yellow) often indicates potential fitness, but it is less clear whether subtle coloration in the same individuals (e.g. grey or brown) could also encode such signals. To determine if subtle coloration could indicate health in a bird species with colorful feathers, house finches (Haemorhous mexicanus), we used spectrometry to objectively quantify leg brightness and saturation before, six days after, and 13 days after captive individuals were inoculated with a naturally-occurring bacterial pathogen, Mycoplasma gallisepticum. We found that legs became less bright six days after infection (near the typical peak of infection), then more bright and less saturated at 13 days (entering the typical recovery period). These results suggest that subtle colors could reliably indicate current infection status, and therefore survival potential.