Parental provisioning was investigated in two congeneric shearwater species, the
winter breeding Little Shearwater Puffinus assimilis tunneyi and summer breeding
Wedge-tailed Shearwater Puffinus pacificus. The Western Australian coast is
characterised by climatic and oceanographic instability largely linked to the variable
flow rate of the Leeuwin Current generated by differences in global climatic
conditions. Ongoing long-term studies at the Western Australian coast have indicated
that this climate driven phenomenon affects seabird reproductive participation and
success. Such an ongoing, parallel study of Wedge-tailed Shearwater reproduction
effectively provided this thesis with a background index of oceanographic conditions,
or a context in which to view and interpret the ability of adults to provide parental
care to the dependent nestling. Such an index of oceanographic conditions is
generally lacking in studies of seabird provisioning. Fortuitously, this study was
conducted in what appeared to be a favourable year (2000), a transition year (2001)
and a less favourable year (2002) for seabird reproduction off the Western Australian
coast. Consequently, it was possible to comment on the effectiveness of parental
provisioning during different oceanographic conditions.
In this thesis, parental provisioning encompassed the delivery of a range of resources
to the dependent nestling, including a burrow, heat during incubation, protection from
predators, brooding after hatching, and the delivery of food to sustain growth and
development. The main focus of the study was the nestling feeding stage, although
other more sensitive and less flexible phases were investigated in the Little
Shearwater. Most research focused on the winter breeding Little Shearwater, as the species least studied. Adult Little Shearwaters were present at the nest throughout the
night during the nestling feeding period, which presented the opportunity to examine
provisioning from an adult perspective. Adult responses are discussed in the context
of this index of oceanographic condition, in terms of regional food resources levels,
and also more localised weather patterns which facilitate shearwater locomotion. A
potential two-tier adult state-dependent mechanism of parental resource allocation is
presented, which would allow a mobile K-strategist, foraging within an acceptable
distance from the breeding colony, to persist in this relatively unstable and unpredictable environment.
This research suggests that parents adjusted parental provisioning in order to
minimise risks to the functional adult, which had attained its reproductive potential at
the expense of the nestling that may or may not attain reproductive status. The
weights of adult encountered at the colony did not differ significantly between
different phases of the breeding season (pre-laying, incubation and nestling feeding)
or between years. It appeared as if the weights of adults engaged in parental provision
fluctuated within an acceptable working range whilst present at the colony, which
makes it difficult to define average adult weights. The theory of threshold adult body
condition suggests that only adults above a sufficient body condition are allocating
resources towards reproduction, and therefore present at the breeding colony. Whilst,
adults below a threshold are expected to be absent from the colony and therefore not
willing to allocate resources to support the dependent nestling.
In the two-tier adult state dependent allocation of parental care, firstly the body
condition of the adult will determine if it is present at the colony, hence providing care for the offspring. Secondly if the adult is able to maintain adult body condition
provisioning can be reactive to the needs (or body condition) of the nestling.
Evidently, such a system of parental provisioning predominantly based on the ability
to maintain adult body condition will be affected by differences in oceanographic and
climatic conditions, which affect the ability of adults to maintain or replenish body
condition within an acceptable time and distance away from the colony. The level
and continuity of parental care provided by the pair will be discussed, especially how
deviations from an optimal provisioning schedule may affect nestling growth, survival and viability.
Ongoing, long term research of summer breeding Wedge-tailed Shearwaters along the
Western Australian coast suggests that reproduction is adversely affected by
prevailing negative southern oscillation indices and associated declines in the flow
rate of the warm, low density Leeuwin Current. Despite breeding in the austral winter
when the maximal flow rates of the Leeuwin Current are expected, Little Shearwater
reproduction was also affected in a manner similar to that documented for tropical
seabirds breeding in summer. Furthermore, decreases in productivity registered by
local fisheries suggested that observed declines in seabird reproduction may be linked
to an overall crash in productivity experienced along the Western Australian coast
during years of weak Leeuwin Current flow. Such years are associated with more
uniform and cold offshore water temperatures. Consequently, a regional lack of
oceanographic resources, within an acceptable distance of the breeding site, are likely
to adversely affect the ability of adults to replenish body condition, however other
factors such as prevailing weather are also likely to affect energy expenditure during travel and foraging. Both factors would affect the ability of adults to maintain body conditions.
In a favourable year, Little Shearwaters were able to meet nestling needs and the
amount of food delivered matched the pattern of weight gain typical for
Procellariiform young. Conversely, in a less favourable year, Little Shearwaters
delivered food to young less often and the amount delivered was erratic, as the
percentage of nestlings fed was consistently lower and temporal delivery was more
irregular in the less favourable year. Nonetheless, food delivery also changed with
nestling age, following a pattern similar to a favourable year, but at a lower rate.
Future research should focus on the level of obesity attained by cohorts under
different conditions and possibly the quality of feathers produced by nestlings.
Feather quality and the state of obesity attained may affect the post-fledging buffer
and ultimately affect fledging survival.
Supplementary feeding suggested that adult Wedge-tailed Shearwaters adjusted
feeding in response to proximal needs of nestlings, as supplementary fed nestlings
were fed less than non-supplementary fed nestling. Furthermore, in both species
adults changed provisioning to meet the changing needs of nestlings throughout
ontogeny however the adults ability to respond to changing needs may be dependent
upon the body condition of the adult and therefore ultimately dictated by resource
availability. These results are in accord with other studies in which adults have
altered provisioning in response to changing nestling needs. Whilst in other studies
adults displayed an inability to react to nestling needs, which may have been linked to the difficulty faced by adult to maintain their own body condition during a less
favourable year.
The tentative mechanism of parental provisioning may help to explain the often
contradictory outcomes recorded in studies of parental provisioning conducted in the
absence of an indicator of resources availability and context in which to interpret
adult response. Since, adults adjusted provisioning to firstly match their ability to
maintain their own body condition in response to oceanographic conditions and
secondly if able to met their own needs, in accordance with nestling needs.
Therefore, parental provisioning is a flexible, state-dependent behaviour ultimately
limited by oceanographic conditions.
Once nestlings have hatched and passed a critical weight, they are more likely to
survive and parental provisioning has the potential to become a highly flexible
behaviour as the offspring can absorb periods of reduced parental provisioning. This
relatively forgiving nestling phase is often targeted by studies of parental
provisioning. Here, an optimal provisioning schedule may be orchestrated under
favourable conditions however deviations may be absorbed without terminating the investment in reproduction.
Although breeding at different times of the year, both species fed their nestling to a
stage of obesity. This occurred, despite feeding nestlings near daily, which should
have allowed adults to obtain regular feedback about the body condition of the
nestling. Obesity may reflect the relatively unpredictable availability of resources
along the Western Australian coast. However, adults may aim to increase the potential fitness of the nestling when adult body condition allows, through the
provision of a post fledging buffer in terms of stored energy and high quality, structurally sound plumage.
At the Western Australian coast, Little Shearwaters used a bimodal strategy to care
for nestlings during incubation and feeding. Adults lost weight during periods of
intensive provisioning, namely incubation and nestling feeding, but if too much
weight was lost, provisioning was withheld and adults departed for a longer absence.
Whilst absent from the colony for a prolonged period of time adults gained weight,
both during the incubation and nestling feeding period. Return of birds from long
absences appeared to be related to the predictable passage of weather systems.
In Little Shearwater, during incubation the efforts of both parents were highly coordinated
in a favourable year, since egg neglect was low, presumably as a
consequence of predation pressure. Weather appeared to assist in the arrival of the
relieving adult during incubation. Similarly during the transition to nestling feeding,
the nocturnal activity at the breeding site indicated that the return of adults from long
absences was associated with predictable changes in wind direction, related to the
inevitable passage of weather systems, rather than wind strength, which is directly
proportional to pressure differences between adjacent weather systems.
In a favourable year, during nestling feeding the co-ordination of change overs
between individuals within a pair also appeared to be high. Consequently, patterns of
provisioning may be entrained throughout the duration of the breeding season,
including incubation and nestling feeding, by an underlying pattern generated by the predictable passage of weather systems. Adults able to meet their own needs in a time
frame dictated by the movement of weather system may minimise travel costs, whilst
returning in time to allow the partner to depart to meet its needs. Both partners are
required for the co-ordination of parental care to produce an optimal provisioning
schedule. Consequently, activities are expected to be structured to protect not only
the body condition of the self, but also of the partner in a species where biparental
care is generally required to successfully raise an offspring.
During nestling feeding, in a favourable year the co-ordinated bimodal strategy
achieved near daily delivery of food, whilst in a less favourable year deviations from
this pattern occurred, as adults spent more time at sea. The average duration of long
absences, used to replenish body condition, appeared to increase during less
favourable years. Further research should focus on variations in temporal absences
and adult weights under different conditions that affect the ability of adults to
replenish their body conditions. Furthermore, not all pairs were able to achieve daily
food delivery during each year, and during a less favourable year a larger proportion
of the breeding population appeared to be unable to deliver continuous parental care.
Consequently, a larger proportion of nestlings remained unfed. The disruption in the
input system, or deviations from a potentially optimal provisioning schedule, resulted
in differences in growth rates between years. Costs incurred by adults during less
favourable conditions appeared to be passed on to the nestling, whilst the risks to the
functional adult that had reached reproductive potential were minimised.
Wedge-tailed Shearwaters, breeding during the austral summer in a favourable year,
were able to meet nestling needs. Here, food was delivered at a pattern that matched weight gain, where total overnight food delivered changed with nestling age, as in the
Little Shearwaters. Wedge-tailed Shearwaters did not deliver food as regularly as
Little Shearwaters. The colony of Wedge-tailed Shearwaters appeared to experience
periods of reduced provisioning, during low wind conditions associated with
dissipating tropical lows. During such conditions, Wedge-tailed Shearwater adults
were often seen rafting at sea away from the breeding site suggesting it was energy
expensive to fly. Such periods of poor provisioning associated with low wind speeds
were recorded at different latitudes along the Western Australian coast and in
different years. Dissipating tropical lows which generate low wind conditions are a
seasonal occurrence however the frequency, persistence and intensity of such tropical
lows appeared to fluctuate with offshore sea surface temperatures, frequency of
tropical cyclones to the north and ultimately prevailing large scale climatic conditions
that affect seas surface temperatures. Consequently, prevailing short term weather
conditions which are affected by large scale climatic events, including climate
change, have the potential to influence Wedge-tailed Shearwater reproduction.
Wedge-tailed Shearwater nestlings supplementarily fed during a dissipating tropical
low were visited and fed less than unfed nestlings of lower body condition. This
implies that adults adjusted provisioning in accordance with nestling body condition.
However, nestlings that had not been supplementarily fed were visited less often once
conditions improved, suggesting that parents were unable to sustain food delivery.
Parents of supplementarily fed nestlings, which had reduced provisioning during the
adverse conditions, were able to feed their nestlings at a higher rate once conditions
recovered then parents with non-supplementary fed nestlings. Thus, parents appeared
to adjust provisioning in relation to nestling body condition however the ability of adults to maintain high levels of parental care appears to be limited, possibly by
parental body condition. Similarly, adults reacted to the changing needs of a nestling
throughout ontogeny in a favourable year, whilst during an adverse year deviation
from a more optimal pattern of provisioning was evident.
Meal sizes delivered by different species are often compared. However, food delivery
appears to be adjusted with nestling age and differences in oceanographic conditions.
Consequently, an optimal amount of food matching the developmental needs of the
nestling may be met during optimal conditions using an optimal pattern of
provisioning, whilst deviations from this may give an index of parental inability to
cope with long or short-term deterioration in oceanic conditions, which ultimately
affect their ability to meet adult needs. Furthermore, meal sizes are often determined
during frequent weighing research of short duration, which is easily affected by
proximal conditions that could affect the ability of adults to travel and would reflect
the developmental stage of the nestling. Therefore, for meal sizes quoted it is not
know if adults were able to engage in an optimal provisioning schedule or if
deviations occur to compensate for below optimal conditions.
Comparison of meal size by weight alone may not be feasible, even throughout a
season within a species, as a seasonal shift in diet was recorded for Wedge-tailed
Shearwaters during the breeding season using stable isotope. The different diets of
different species often living in different geographic locations and the ability of some
species to produce stomach oil, further confounds the ability to compare meal sizes by
weight alone. It appears difficult to define a meal size characteristic for a species and
make direct comparisons based on weight alone a practice widespread in seabird research. At best an optimal meal size for a given developmental stage may be used
in conjunction with calorific values to calculate an optimal energy input for a specific age to compare between species.
The seasonal difference in diet recorded during the Wedge-tailed Shearwater breeding
season revealed a shift from a higher to a lower trophic level. This is consistent with
females engaging in a pre-laying exodus where preferred resources may be targeted,
whilst during nestling feeding foraging constraints are introduced as foraging occurs
around a central location. A broader range of prey may be targeted within an
acceptable distance from the breeding colony, including scavenging behind Western
Rock Lobster fishing vessels which provide a predictable source of food or discarded often imported bait.
The application of stable isotope analysis provided promising insights into shearwater
foraging behaviour otherwise difficult to elucidate. Different resources were targeted
by the two species during egg membrane formation. Wedge-tailed Shearwaters
generally appeared to forage at a higher trophic level than Little Shearwaters,
although outliers suggest that Little Shearwaters also have the potential to target
higher trophic levels. Differences in diet during egg formation were evident between
different colonies of Wedge-tailed Shearwaters breeding at different locations.
Consequently, stable isotope analysis may be used to track changes in diet between
favourable and less favourable years, where increased foraging constraints are
expected to emerge. Stable isotope analysis is a promising technique and
interpretation of data will increase with further understanding of ecosystem dynamics
and the development of a reference library.
Direct observation at sea suggested that Little Shearwaters forage in deep, offshore in
areas most likely influenced by the Leeuwin Current. They were not observed to
forage in the cooler, inshore waters fed by a northwards flowing counter current, on
the continental shelf between the Houtman Abrolhos Islands and the mainland.
Maximum depth gauges revealed a maximal dive depth of approximately 34 m. The
maximal dive depths recorded appeared to increases with time spent at sea, as
indicated by birds engaged in short absences (near daily returns) from the colony and
long absences (approximately weekly returns) when they were replenishing body
condition. Long absences were recorded after an incubation shift or a period of daily nestling feeding.
To summarise, a tentative mechanism of adult state-dependent allocation of parental
provisioning in which pelagic seabirds persist in environments of fluctuating resource
levels was proposed. This may be based upon the ability of adults to operate above a
threshold body condition, which is ultimately dictated by resource availability and
proximal conditions which affect the ability of adults to maintain their own body
condition. Once adults struggle to maintain their body condition, a disruption in the
co-ordination of the parental care orchestrated between the members of a pair can
arise, as adults spend more time away from the colony replenishing their body
condition. This results in a deviation from an optimal provisioning schedule, which
is achievable in favourable years when resource levels and proximal conditions
facilitate easy maintenance of the pairs body condition. In this system, any costs
incurred whilst struggling to replenish or maintain adult body condition are passed on
to the nestling. In a system where biparental care is necessary, incentive exists for one partner to also act in a manner that facilitates the protection of the body condition of the other partner.
The nestling in turn may absorb such deviations from an optimal provisioning
schedule in a manner that affects the development of temporary attributes, rather than
fixed morphological attributes. This may be a testimony of the strong selective forces
exerted by the ocean, which also discourages sexual dimorphism in pelagic seabirds.
Reduced levels of provisioning recorded in less favourable years translated into lower
average peak weights, smaller growth rates and however asymptotes reached
generally did not differ significantly between years. However, the deviation from
the optimal provisioning schedule during less favourable years may reduce the fitness
of the fledgling, through a reduction in the post fledging buffer, including investment
in a smaller post fledging fat deposits and decreased plumage quality. It is possible,
that nestling shunt costs incurred away from skeletal or fixed attributes. The nestling
once independent may overcome lower quality plumage and a smaller post fledging
fat buffer, if conditions are favourable. Thereby, the offspring engages in its own risk
minimisation strategy and makes the most of its only chance for survival.
Within a species, temporal patterns of parent provisioning and consequently foraging
strategies are flexible. Firstly, a species may engage in a different foraging strategy
ranging from unimodal to bimodal in response to prevailing conditions at a given
location that affect the ability of adults to maintain their own body condition.
Consequently, even at the same location under different conditions a shift in foraging
strategy may result. Secondly, an optimal provisioning schedule able to meet the
needs of the nestling may only be realised during favourable conditions when a pair is able to co-ordinate their efforts. The proportion of the colony able to achieve optimal
provision is expected to vary between years and resource availability, as does the
proportion of the breeding population able to return to the colony and lay an egg.
Consequently, nestling quality of the cohort may be affected by the adult state
dependent input system, based upon adult risk minimisation. This has implications
for climate change, which affects sea surface temperatures and consequently
oceanographic productivity and prevailing weather patterns.
Effectively, costs are passed on to the offspring whilst the reproductive potential of
the adult or pair is maintained. Such flexibility allows iteroparous k-strategists, such
as shearwaters, to persist successfully in areas of climatic and oceanographic
variability, but may have implications on the post-fledging survival chances. In an
adverse year, despite reaching lower average weights, a proportion of adults were still
able to feed their nestlings to obesity, possibly highlighting differences in quality
between pairs. Since the state of obesity reached and the quality of feather produced
may translate into increased post-fledging survival chances for the nestling, the
survival chances of nestlings produced by a breeding population may not be equal, as
the level of parental care delivered may vary between pairs and years. The subtle
adjustments in two tier adult state-dependent system of parental provisioning may
alter the post-fledging survival chances of the cohort and this has implications on
population dynamics in terms of climate change, which reach beyond easily observed
reductions in breeding participation and success.
| Identifer | oai:union.ndltd.org:ADTP/221860 |
| Date | January 2006 |
| Creators | 19006626@student.murdoch.edu.au, Indre Kirsten Asmussen |
| Publisher | Murdoch University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.murdoch.edu.au/goto/CopyrightNotice, Copyright Indre Kirsten Asmussen |
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