Reliability of steatocrit as an indicator of intestinal health in young birds - Relationships with morphology and growth rate of canary Serinus canaria nestlings.

Intestinal health and capacity to efficiently absorb nutrients from diet, including fat, has been recently suggested as a promising physiological health indicator in wild birds, due to its association with gut parasitism and feather coloration. However, little information is available about the sources of variation of non-absorbed fat in birds' faeces, measured by the acid steatocrit technique, and particularly in nestling birds. We assessed steatocrit in captive nestling canaries Serinus canaria and evaluated if it was affected by breed and its relationship with growth (linear growth rate, LGR) and body mass. We also assessed the presence of coccidia infections. Steatocrit differed significantly between day 9 and 14 of the nestling period, being lower when nestlings were 14 days old. Age must be taken into account when assessing steatocrit in young birds; this variation may be due to a development of the digestive system with age or the amount of fat provided later in the nestling period being more adequate to the nestlings' needs. Steatocrit was higher in nestling canaries from yellow lipochromic than red lipochromic breeds. No coccidia infections were detected. Steatocrit did not affect LGR, but steatocrit measured at day 9 was positively related with body mass at day 14. The positive relationship between steatocrit and body mass in canary nestlings suggests that nestlings in breeding facilities may be fed a larger amount of fat than that they can absorb. This physiological assessment of intestinal health may be a better general health indicator in wild adult birds exposed to a higher level and diversity of parasites.