Change in beak brightness over the first (a) 6 and (b) 10 days of the adult immune assessment as a function of treatment.

<p>The first letter of the treatment label denotes whether individuals received LOW (L) or HIGH (H) levels of carotenoids in the diet, while the second letter denotes the diet received at the adult stage. An increase in brightness is associated with a loss of carotenoid pigment <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038043#pone.0038043-Butler1" target="_blank">[38]</a>. Therefore, over the first six days of the adult immune assessment, individuals receiving LOW diets (A) had beaks that became less carotenoid-rich (i.e., increased in brightness) –relative to those receiving HIGH diets (B; <i>P</i><0.05). Over the full 10 days of immune assessment, however, LH bird beaks became more carotenoid-depleted than HH bird beaks (*; <i>P</i><0.05), while similar trends existed between LH and LL birds, and HL and HH birds (§; 0.05<<i>P</i><0.08).