Zoology - Theses

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    The effect of disturbance on the breeding biology of hooded plovers
    Weston, Michael A. ( 2000)
    Disturbance of birds by humans is a growing problem. This thesis examines the mechanisms through which disturbance may reduce reproductive success in the Hooded Plover Thinornis rubricollis, a solitary-nesting shorebird for which disturbance is thought to constitute a conservation threat. As a precursor to examining disturbance in this species, breeding biology, life history and parental care were investigated in a colour-banded population of Hooded Plovers in coastal Victoria, Australia. Pairs bred in widely dispersed territories, and intact pairs bred in the same territories year after year. The breeding season was asynchronous and protracted, lasting up to eight months. Breeding success was low, but pairs re-nested frequently and rapidly after nest and brood failure, and sometimes after successfully fledging young. Survival of juveniles was similar to that reported for other plovers, and adult survival was high compared with other plovers. However, it was estimated that only 34% of adults were expected to live 11 years, the average period needed for a pair to replace itself (produce two young). One unusual feature of the life cycle of Hooded Plovers was the complete temporal overlap of adult wing moult with breeding. Complete biparental care occurred throughout the breeding cycle. There is some evidence that males undertook more of the nocturnal incubation while females undertook more of the diurnal incubation but overall, pair members shared incubation equally. Levels of incubation were high and constant over the incubation period. Once chicks hatched, the average number of parents nearby and the frequency of distraction displays increased. Additionally, adults fed less after their chicks hatched, and adult body mass decreased. Brooding decreased as chicks grew. A number of mechanisms through which disturbance could reduce reproductive success were examined. Disturbance resulted in egg temperatures moving above and below benchmark values, potentially leading to thermal stress in eggs. At least some recorded egg temperatures were potentially lethal. The proportion of abandoned eggs was highest in foredune nests, followed by beach then dune nests. This pattern mirrored the pattern of disruption of incubation by humans between habitats, suggesting thermal stress due to disturbance caused egg mortality. Thermal stress could also kill chicks, and disturbance disrupted brooding. Failure of adults to defend eggs or chicks due to disturbance did not lead to nest or brood failure. Adults defended nests and broods, but predation events were only observed during undisturbed conditions. For broods, foraging time decreased and the level of the habitat used for foraging changed with increasing levels of disturbance. Energetic stress is therefore another potential mechanism that could decrease chick survival. This study uncovered potential mechanisms through which disturbance could decrease hatching and fledging success in Hooded Plovers. Identification of the mechanisms through which disturbance acts, suggests that a management approach that emphasises managing the effect of disturbance rather than the response to it, will be a necessary component of the recovery of this species.
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    The feeding and breeding biology of the sacred ibis Threskiornis aethiopicus in southern Victoria
    Lowe, Kim Waldock ( 1984)
    This study aimed to elucidate the major selective factors affecting the biology, and particularly the breeding ecology of the Sacred Ibis (Threskiornis aethiopicus) in Australia. A comparative study of the breeding ecology of the Family Plataleidae in Australia was undertaken to define the ecological niche of the Sacred Ibis. The Australian ibis and spoonbills have very similar breeding ecology to their conspecifics and congeners elsewhere in the world. However, the breeding success of Australian birds appears to be limited by food supply whereas, in African members of the Family, for example, breeding success is limited by predation. In Australia, Sacred Ibis exhibit two broad responses to their environment: some populations showed adaptations in their breeding to an unpredictable environment; and, other populations have developed complex breeding strategies that are dependent on a highly predictable set of environmental conditions. In unpredictable environments, Sacred Ibis do not breed regularly but rather breeding is closely tied to the variable environmental conditions; the ibis move away from the breeding site when the area dries out. In unpredictable environments, clutch size is adjusted to the conditions and the ibis may employ multiple brooding. Where the environment is much more predictable, Sacred Ibis breed on a very regular cycle. In these environments, ibis are highly sedentary and clutch and brood size and multiple brooding are adjusted to maximise seasonal reproductive success, that is, Sacred Ibis raise fewer offspring per brood than the apparent maximum number possible as a trade-off against raising more broods in each breeding season. The timing and duration of the breeding attempts has had a major effect on the mating system. Sacred Ibis are essentially monogamous but one case of apparent polygyny was found. The mating system is also characterised by a high rate of promiscuous copulation and changes of mate between breeding attempts. The roles of male and female in post-fledging care of young and costs/benefits to life-time fitness are discussed. The selective factors maintaining colonial nesting are reviewed and the single most important evolutionary pressure affecting the breeding ecology of the Sacred Ibis is finding food; predation was shown to be unimportant.
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    Individual variation in reproductive success in the sacred ibis
    Beilharz, Margaret Irene ( 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.