Repeatability of hematocrits and body mass of Gray Catbirds

Hematocrits may provide information about the physiological condition of birds, but, to be a useful measure, information is needed concerning how hematocrits vary among individuals and over time. We examined the repeatability of hematocrits in a population of Gray Catbirds (Dumetella carolinensis) in Pennsylvania at several time scales and also examined the repeatability of body mass, another measure commonly used as an indicator of condition. Both hematocrit (r= 0.64) and mass (r= 0.65) were repeatable (P 0.05) between captures in different months within a year (r= 0.11 for both). Repeatability of both measures differed by sex and age. Females exhibited repeatability of hematocrit and body mass only between years, while male hematocrits were repeatable between years and between first and second captures within a season. Male mass was repeatable for all time periods. Hematocrits of younger birds were repeatable between captures within a season and their body mass was repeatable between months and weeks while hematocrits of older birds were not repeatable and their body mass was repeatable only between captures in a season. Our results indicate that hematocrits and body mass had similar repeatability coefficients overall, but that hematocrits of Gray Catbirds were a consistent trait of individuals only across years. Because repeatability between captures and months depended on sex and age, we conclude that the hematocrit is a useful measure of individual performance only in limited circumstances. RESUMEN El hematocrito podria proveer informacion sobre la condicion fisica de las aves pero para ser una medida util se necesita informacion sobre como el hematocrito varia entre individuos y a traves del tiempo. Examinamos la repetibilidad del hematocrito en una poblacion de Dumetella carolinensis en Pennsylvania durante diferentes escalas temporales. Tambien examinamos la repetibilidad del peso corporal, otra medida comunmente usada como indicador de la condicion del individuo. El hematocrito (r= 0.64) y el peso corporal (r= 0.65) eran repetibles (P 0.05) entre capturas realizadas en diferentes meses dentro del ano (r= 0.11 para los dos). La repetibilidad de las dos medidas difiere por sexo y edad. Las hembras se caracterizaron por tener repetibilidad del hematocrito y del peso corporal solo entre anos, mientras que el hematocrito de los machos era repetible entre anos y entre la primera y segunda captura dentro de la temporada. El peso corporal de los machos era repetible durante todos los periodos a traves del tiempo. El hematocrito de las aves mas jovenes era repetible entre capturas dentro de la temporada y su peso corporal era repetible entre meses y semanas, mientras que el hematocrito de las aves mayores no era repetible y su peso corporal era repetible solo entre capturas dentro de la temporada. Nuestros resultados indican que los hematocritos y el peso corporal en general tuvieron coeficientes de repetibilidad similares, pero que los hematocritos de D. carolinensis eran un rasgo consistente de los individuos solo entre anos. Por la razon que la repetibilidad entre capturas y meses dependia del sexo y edad, concluimos que el hematocrito es una medida util del performance individual solo en circunstancias limitadas.