The spatial and temporal repeatability of PHA-responses

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Abstract
The capacity to mount a cell-mediated immune response in birds is often quantified as the swelling following an injection of phytohaemagglutinin (PHA) into the wing-web. Most studies have found the repeatability of consecutive measurements of the same swelling to be high (e.g., Alonso-Alvarez and Tella, 2001; Fargallo et al., 2002; Saino et al., 1997; Smits et al., 1999; Tella et al., 2000). However, as pointed out by Siva-Jothy and Ryder (2001), repeatability calculated this way only estimates the precision with which an experimenter measures the size of a particular swelling; it does not encompass other components of measurement error. Furthermore, such estimates of repeatability do not capture temporal variation in individuals' PHA-responses. To our knowledge, no study has reported the latter types of repeatability estimates. Thus, the aim of the present study was to investigate the repeatability of the response to PHA both at a given time and between different occasions separated in time.