Experimental support for the makeup hypothesis in nestling tawny owls (Strix aluco)

Abstract

Body condition can affect coloration of traits used in sexual selection and parent-offspring communication by inducing rapid internal changes in pigment concentration or aggregation, thickness of collagen arrays, or blood flux. The recent ''makeup hypothesis'' proposes an alternative honesty-reinforcing mechanism, with behaviorally mediated deposition of substances on body surfaces (''cosmetics'') generating covariation between body condition and coloration. In birds, the uropygial gland wax is actively spread on feathers using the bill and changes in its deposition rate may cause rapid changes in bill and plumage coloration. Using tawny owl nestlings, we tested 3 predictions of the makeup hypothesis, namely that 1) quantity of preen wax deposited accounts for variation in bill coloration, 2) an immune stimulation (induced by injection of a lipopolysaccharide [LPS]) impairs uropygial gland wax production, and 3) different intensities of immune stimulations (strong vs. weak stimulations induced by injections of either LPS or phytohemagglutinin [PHA], respectively) and high versus low food availabilities result in different bill colorations. We found that 1) preen wax reduced bill brightness, 2) a challenge with LPS impaired uropygial gland development, and 3) nestlings challenged with LPS had a brighter bill than PHA-injected nestlings, whereas diet manipulation had no significant effect. Altogether, these results suggest that a strong immune challenge may decrease preen wax deposition rate on the bill of nestling birds, at least by impairing gland wax production, which causes a change in bill coloration. Our study therefore highlights that cosmetic colors might signal short-term variation in immunological status.

Natural soiling has a small effect on structurally-based plumage coloration

“…The darkening effect of preen wax was experimentally confirmed for carotenoid red plumage of house finch Carpodacus mexicanus, by both applying preen wax or by blocking access to uropygial gland (López-Rull, 2010). In agreement with my results, preen wax reduced the overall reflectance of the bill's of nestling tawny owls (Strix aluco, Piault et al, 2008).…”

“…Because dirt removal and preen wax production are time and energy demanding processes (Walther and Clayton, 2005;Piault et al, 2008), it is reasonable to assume that color changes due to these factors may reinforce the reliability of the signal produced solely by pigment concentration or (and) keratin structure. To date, the effect of soiling on plumage color has been tested mainly by artificially soiling birds' plumage in indoor conditions.…”

Use of TLC‐FID and GC‐MS/FID to examine the effects of migratory state, diet and captivity on preen wax composition in White‐throated Sparrows Zonotrichia albicollis

“…Among these are maintenance of feather quality (Jacob & Ziswiler 1982, Moyer et al. 2003), mate attraction (Piault et al. 2008), scent suppression during incubation (Reneerkens et al.…”

Chronic coccidian infestation compromises flight feather quality in house sparrowsPasser domesticus

“…We found that the size of the uropygial gland significantly increased in Med compared to Cocc birds during the second postnuptial moult, which may indicate that the negative effect of coccidians on condition appears after a long-term chronic infestation. As a result of the large amount of oil produced during this period, and because of the costly gland secretion (Piault et al, 2008), coccidians affected the size of the gland only during that period of the annual cycle when the gland size is the largest. These findings support our expectation that, during long-term chronic infestation, the negative effect of parasites on the host's condition is cumulative.…”