The social organisation and mating system fo the brown kiwi (Apteryx mantelli) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Albany, New Zealand
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Date
2011
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Massey University
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Abstract
Mating systems are shaped by the strength and direction of sexual selection,
the evolution of differential sex roles, and by the sexual conflict over mating
rates and parental duties. Ecological factors and the behaviour of the entire
population will then determine whether a certain mating strategy will be adopted
by individuals, leading to variable mating systems between populations of the
same species. The five kiwi species (Apteryx spp.), endemic to New Zealand,
have experienced dramatic population declines due to habitat destruction and
predation from introduced mammals, resulting in fragmentary low-density
populations. Despite that the Brown Kiwi (A. mantelli) is still the most numerous
among the kiwi species, little is known about aspects of their social organisation
and mating system. The Brown Kiwi exhibits male-only parental care for
precocial chicks and a sexual size dimorphism with larger females. While such
characteristics are typical for polyandrous species, the only reported mating
system for Brown Kiwi is monogamy. However, due to their nocturnal and
secretive nature, in addition to the scarcity of this species, field observations on
their social and mating behaviour are particularly challenging to obtain. Here, I
chose one of the few remaining high-density and easily accessible populations
of Brown Kiwi on Ponui Island. The exceptional density of this population should
increase the potential for interactions between birds and reveal insights into
their social organisation and mating system that are difficult to obtain in
declining and/or low-density populations. Radio-telemetry was employed to
investigate the formation and stability of pairs and groups, intersexual spacing
behaviour in relation to the reproductive period of Kiwi, and the breeding
behaviour and nesting success of radio-tagged birds. Genetic analysis was
used to confirm paternity of incubating males, and to assess genetic
relationships between group members. Nocturnal and diurnal spacing and
roosting behaviours indicated long-term stable bonds between female-male
pairs. Home range sizes of females were only slightly larger than those of
males, most likely because of their larger body size. Stable monogamous pairs
had largely overlapping ranges in the breeding and the non-breeding season.
While such aspects of spacing behaviour are typical for a monogamous mating
system, high degrees of nightly interactions between radio-tagged birds were
detected, and range overlap was likely to be underestimated given the high
number of tagged birds detected within focal birds’ ranges and the inability to
account for untagged birds. Additionally, some birds formed stable polyandrous
trios with largely overlapping ranges and frequent roost site sharing of all trio
members. Two males of a polyandrous trio were found cooperatively attending
an active nest. Genetic kinship analysis revealed that such groups consisted of
unrelated individuals rather than family groups as found in the Tokoeka (A.
australis). Furthermore, one intra-group offspring of another polyandrous trio
was genetically identified. Such findings lead to the assumption that birds may
engage in cooperative polyandry. In closed habitats such as on Ponui Island,
birds are unable to disperse and leave. Hence, population density may affect
the social behaviour and mating system of individuals because of a potential
shortage of resources that are important for reproduction. Thus, unpaired and/or
floater Kiwi males may have joined existing breeding pairs due the unlike
opportunity of independent breeding in a possibly saturated environment. While
females may benefit from exhibiting cooperative polyandry at the expense of the
caring males, males may have reduced reproductive success due to shared
paternity. Nevertheless, the majority of breeding birds consisted of seemingly
monogamous pairs and nesting success was high compared with other studied
Brown Kiwi populations. In addition, levels of extra-pair paternity were low,
indicating that most socially monogamous pairs were also genetically
monogamous, despite the potential for social interactions and the freedom of
females from parental care. My results confirm previous findings that Brown
Kiwi maintain long-term monogamous pairs, most likely because of energetic
demands of females during production of disproportionally large, energy-rich
eggs, but also because of the high costs of reduced reproductive success for
both sexes when a cuckolded male deserts a clutch. Nonetheless, although
sample size was small, Brown Kiwi may potentially engage in cooperative
polyandry in a high-density population, which demonstrates more flexibility in
their reproductive ecology than previously known. The understanding of a
species’ social and mating system is crucial as reproductive behaviours directly
influence the dynamics of a population. This study provides valuable information
on kiwi life histories and demonstrates the key paradoxes between Brown Kiwi
reproductive characteristics and their mating system and may highlight the
conflicts between the sexes over aspects of reproduction.
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Keywords
Kiwi behaviour, Kiwi ecology, Kiwi breeding, Kiwi reproduction