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Different colors reveal different information: how nutritional stress affects the expression of melanin- and structurally based ornamental plumage

Kevin J. McGraw^1,*, Emiko A. Mackillop^1,2, James Dale^1,3 and Mark E. Hauber^1,4

¹ Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA
² Department of Biological Sciences, Binghamton University, Binghamton, NY 13902, USA
³ Department of Biological Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada
⁴ Department of Integrative Biology, Museum of Vertebrate Zoology, University of California, Berkeley, CA 94720, USA

View larger version (16K): [in a new window]   Fig. 1. Representative reflectance spectra for the types of ornamental color exhibited by male house sparrows P. domesticus and brown-headed cowbirds M. ater. Data were collected following McGraw et al. (1999). Note the absence of discrete peaks in the ultraviolet portion of the spectrum (300-400nm) for all plumage types. Also note that we have described the spectral profile of iridescent cowbird plumage at only one angle (90° to the reading surface); the general height of the curve for this plumage region would be expected to vary, depending on the angle at which the feather is analyzed.

View larger version (15K): [in a new window]   Fig. 2. Effect of nutritional stress on the (A) brightness and (B) size of melanin-based ornamental plumage in male house sparrows. Nutritionally stressed males (dark bars) were deprived of food for randomly assigned 6h periods on 3 out of every 4 days (sensu Hill, 2000), while unstressed males (white bars) were allowed access ad libitum to the same type and amount of food. Badge size and brightness were measured with Adobe® Photoshop® from digital photographs. Values are means + 1 S.E.M. For badge brightness, more negative values indicate blacker plumage.

View larger version (16K): [in a new window]   Fig. 3. Effect of food deprivation on the (A) brightness and (B) size of melanin-based ornamental coloration in male brown-headed cowbirds. Plumage brightness was scored with the ColortronTM reflectance spectrophotometer; hood length was measured with calipers. See Fig. 2 for description of nutritional treatment and bar charts.

View larger version (17K): [in a new window]   Fig. 4. Influence of nutritional stress on the expression of structurally based plumage (A) hue, (B) saturation and (C) brightness in male brown-headed cowbirds. Structural color was scored with the ColortronTM. See Fig. 2 for other details.