Zoology - Theses
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ItemIndividual variation in reproductive success in the sacred ibis( 1988)Factors affecting the reproductive success of individually colour-banded sacred ibis, Threskiornis aethiopicus stricitpennis (Gould), at Healesville Sanctuary, Victoria, were studied from November, 1983, to March, 1987. Although the majority of clutches were of three eggs, the modal fledging success was two young, due to starvation of the third chick. The likely adaptive value of brood reduction is discussed. Characteristics of individuals (e.g. weight, body condition, culmen length, estimated age and dominance status) had no discernible effect on the number of chicks raised to fledging from one attempt (fledging success). The structure of the dominance hierarchy among male sacred ibis at Healesville is described. Dominance relationships among females were not consistent and, therefore, analysis of dominance status was restricted to males. Weight was the major trait contributing to a male's dominance status. High-status males achieved a higher seasonal reproductive success than did low-status males in two seasons when resources were apparently at low levels. Reproductive success over five consecutive years was largely dependent on survival, but, no significant relationships between individual traits and survival were found. Male sacred ibis achieving high seasonal reproductive success tended to survive better than did less successful males. Thus, variation in success measured over a short term (one season) underestimated the variation between males in success measured over a long term (five years). Variation in success of females also increased with the longer sampling period, but not at as fast a rate as variation in male success. Variation in success is therefore significantly greater in males than females, even though the sacred ibis is a monogamous species. Variability in reproductive success and number of attempts among individuals is facilitated by the changes of mate frequently observed in individuals breeding more than once in a season. Female sacred ibis were found to invest more in any one attempt than were male ibis. Consequently, individual females averaged fewer nesting attempts per season than did males and, with the sex-ratio of unity, females were a limiting resource for males. The resulting opportunity for selection on males is apparently fulfilled to some degree, with high-status, heavy and old males being particularly successful in competition for mates.