Jessie Barton
A Review of Unequal Parenting and Breeding Behavior as an Indication of Individual Quality in Little
Penguins; Eudyptula minor
Introduction
Eudyptula minor has a unique status among long-lived shore birds as double breeders. It is this
ability to raise more than one clutch consistently and successfully without hatch failure being a cause in a
single breeding season. This is of particular interest in studying the reproductive success of these
individuals compared to the more commonly seen one clutch laying birds (Johannesen, Houston, Russell,
2003). In recent years much data has become available about the mating period of the smallest penguins
in the world (Heber, Wilson, and Molles, 2008), from colonies concentrated in New Zealand and
Australia. Unequal parenting, in addition to double breeding, give support in defining the quality of the
individuals within the population because they are more than a simple measure of reproductive fitness. It
is these two criterion, unequal parenting and double breeding, that may hold the key to the conservation
efforts in of some of the declining little penguin populations in New Zealand and Australia.
Current Research
Presently, research focusing on the reproductive aspects of Eudytula minor has investigated the
breeding biology and success of the little penguins as well as the relationship between unequal parenting
effort in little penguins. The effects of habitat and reproductive success of double breeders compared to
single breeders has also been a topic of study. All of the reviewed material thus far has taken an
observational approach as the most effective method in gathering information as a starting point for the
colonies off of Australia and New Zealand.
The research by Heber provides the framework for understanding the breeding cycle as well as
the potential threats for the little penguin population of South Island, New Zealand. This group's body of
work most importantly provides background information on a declining population. It is important to
note that while the populations in a few areas have been declining for several decades, the security of this
small species of bird is assured by the large number of birds over many colonies totaling over 100,000.
This species is known for using a wide range of resources in varying habitats to create nesting burrows
and among the most prominent habitats are vegetation, sand, driftwood and caves (Heber 2008 as cited in
Marchant 1990). Findings from Heber's research on the West Coast, South Island little penguins were
found to have the highest mortality rate from lack of adult survival which is critical to their conservation
and not from the choice of nesting sites as previously hypothesized. The adult survival is crucial during
the extended breeding season because both parents are needed to rear the young. Thus, with the loss of
one parent there is an additional loss of the remaining parent with two eggs and not just a single
individual (Saraux, Chiaradia, Maho, and Ropert -Coudert, 2011). With their breeding productivity
comparable to other colonies, at roughly 66.2% overall and of the eggs that hatched 83.9%, Heber cites
human encroachment upon the habitat by paved roads and higher population densities for the decline of
the West Coast population (Heber, 2008). Predation was not a contributing factor to the declining of this
population during the study due to the measures placed after being identified in other colonies as a major
factor. Canis familiaris as well as vulpus and mustela species have been problematic for some colonies in
conjunction with increased human activity in the near vicinity (Braidwood, Kunz, Wilson, 2011).
Additionally, research by Saraux and associates covers a population of Eudytula minor on Phillip
Island, Australia. They define parental care using the definition given by Trivers in 1972 as, any
involvement in the young that can contribute to the offspring's survival but at the cost of the parent
(Saraux et al., 2011). Using this as the model for parental care, they observe the unequal parental
contributions measured by the meals brought back by each parent as well as the frequency of the visits
during the post-guard phase. During this phase the chicks will be left alone throughout the day with the
parents coming back only to feed them. Saraux suggests that the parent's contribution of food to the
offspring is related to the amount brought back to the nest. The individuals contributing more are
categorized as the higher quality mate among pairs that did not share parental care equally. This was
defined as one in the pair that made three or more trips to shore than their partner and it is important to
note that the number of trips between the equal effort pairs and the unequal pairs were the same ( Saraux,
2011). This helps lay the ground work for individual differences and not a difference due to unequal
parental effort. It would be expected that if age and other factors are controlled for that the difference
would stem from individual quarks and not environmental or biological factors.
A higher investment in feeding of offspring ultimately mirrors the amount of energy the parent
can give without putting future reproductive success at stake (Saraux, 2011). This allows one of the mates
to invest time into a second clutch by leaving the primary responsibility to the other partner; doing so
increases the overall success of the individual without causing death to the offspring. Most of the
breeding Eudyptula minor retained being either the high or low contributing partner regardless of the
mate they were with and also in favorable and unfavorable years (of those who were in unequal
relationships). This means that there is not a disparity as a response to a changing partner or as a result of
the food supply available. In any given year the penguins are known to remain faithful to the site they
choose for nesting meaning that one partner is not going in-between sites and can be accounted for. This
provides evidence of a reproductive choice. There was not a significant difference in the success rate of
chicks where one of the parents in the pair brought meals more frequently than the other compared to
pairs that were equal. However, the weight post-fledge was lower than the weight of chicks raised by
equally contributing parents (Johannesen, 2003). In birds, especially long-lived shore birds, a higher postfledge weight is an important factor in a chick's survival, although this is not the case with this species.
Eudyptula minor is limited to breeding in a habitat that is in close proximity to their supply of
food from the ocean (Braidwood, 2011). It is important to rule out type of habitat and proximity to the
ocean and natural versus artificial burrows respectively as a source for the variation of breeding little
penguins behavior. This is where the work of Braidwood and Johannesen come in. The breeding success
rate for all habitat types was high and not significantly variable at the sites of operation for Braidwood
even though a preference was shown by the occupancy percentage in each type of habitat. This would
mean that the habitat type does not directly affect the number of eggs lain or provide for the difference in
parental care. At the sites Johannesen surveyed, all of the nesting boxes were artificial burrows with the
assumption that there were natural burrows because none were observed. This too would indicate that
quality of nesting burrows does not convey a difference unless the burrow is flooded or otherwise
compromised. As further evidence of the success artificial burrows provided, between 1993 and 1997 the
number of nesting boxes nearly doubled from 87 to 165 meaning more fledglings survived to reproduce
(Braidwood, 2011; Johannesen, 2003). The definition of fledgling is variable in ornithology and here is
taken to mean the point in the chick's life when it is independent of its parents and first heads to sea.
Fledgling can also be the period when the musculature and feathers are developed enough for flight, but
penguins are a secondarily flightless species and so this was not an appropriate application of the word.
Research by Braidwood proposed that there should be a difference between the survival of the
offspring of the single and double breeders. In this location the entire population breeds in the artificial
nests and precautions have been taken to prevent predation of the birds since 1993 when a ferret began
hunting chicks (2003). This measure allows for any chance effects by a difference in habitat to be
controlled for. Single clutch breeders will only raise two eggs which are laid approximately two to seven
days apart. Double breeders will generally start earlier in the year, although not always, and raise up to
four chicks: two sets of two. In this study there was a significantly higher number of fledged chicks by the
double breeders than the single breeders at 1.7 times that of the single breeders. In addition to this, the
survival of the single breeders was 94% (Braidwood, 2003). Virtually all of the double breeding
individuals were guaranteed to come back and reproduce the next year provided that they were still living
(Braidwood 2003). Established breeders are more likely to return to the site of a successful hatch the
following year than where a failure has occurred. This result was not quite significant at p=.054 where a
value less than .05 was considered significant, but this gives a strong correlation between the quality of an
individual and the overall survival of a double breeder. This means there are qualitative differences that
are unaccounted for. Another interesting finding of the study, echoed by Nisbet's study, was that of the
lay date and first year survival of the fledglings. Nisbet found, that for the single breeders, as the date of
laying eggs came later in the year, the offspring were less likely to survive than the offspring of the
double breeders (Braidwood 2003; Nisbet and Dann, 2009). The double breeders maintain a longer
breeding season and have a lay date both early and late in the season, thus, making it unlikely that
survival is solely based on how far into the breeding season the laying date is. The survival rate of the
first and second clutch of the double breeders had the same success rate (Braidwood 2003).
Complementary to this, Nisbet and Dann found that late nesting birds were less likely to lay a clutch of
only a single egg, but they experienced more hatch failure than that of early laying single breeders and
double breeders (2009). So, with higher risk there was a larger reward.
Implications
From a conservation standpoint, little penguins that are double breeders could be selected and
integrated into a declining population. This would boost the number of young in the colony and add
individuals of quality as well as adding allelic diversity to the population. In addition to this as Heber
suggests, there could be additional sites equipped with predator management materials to ensure safe
hatching and fledging of the chicks. Traps for predators and fences to keep penguin nesting sites
undisturbed are paramount to ensuring declining habitats are maintained. In places prone to human
interaction, especially by roads, more fences can be put in place to limit the encroachment upon prime
breeding habitat. The last way to boost the declining populations could be the introduction of more double
breeders to the breeding sites from other successful colonies as their success is almost assured as is their
return if they are able to successfully hatch young in the new environment. Not all populations of
Eudyptula minor are declining, but there is not a single population that would not benefit from predation
management and protection from people. Finally, the provision of nesting boxes is a great way to ensure
that there are enough quality spots for the breeding penguins to return to as well as to entice new breeders.
Considerations for further Research
Is the behavior that is observed in the Eudyptula minor found in the conservation of the genes in
the population relating to reproductive fitness and individual survival? In an effort to solidify the
information available on Eudyptula minor, it is highly recommended to pursue genetic testing on the
populations to quantify the results that have been presented in support of distinguishing the quality of
individuals based on double breeding status and unequal parenting. The alleles that are conserved in the
double breeding individuals in the populations could be measured against others to see if there is a
significant difference on the molecular level. If double breeding and unequal parenting are good measures
of individual quality, then it would be expected that allelic frequencies would be different than the
population as a whole. It is further proposed by Saraux et al. that in order to more fully comprehend the
costs of unequal parental care over many seasons that phenotypic differences and not fitness itself should
be a measure of individual quality. In the future studies not dependent upon correlating quality of
individuals with life history traits would be more definitive proof of quality and not just a correlational
effect. Additionally, it may be of some benefit to experimentally study some individuals from large
populations to garner additional information about the outside influences impacting the reproductive
success of the little penguins as much of the current research is focused on observational material
gathered to provide a baseline in data.
Conclusion
Both unequal parenting and double breeding help define a difference in the overall quality of an
individual and thus define the maximum amount of effort that can be exerted toward a clutch in order to
maximize individual fitness. With this difference not being accounted for by sex, burrow site choice,
number of clutches, or age it is suggestive that this contributing difference is due to the characteristics of
an individual. As long as there are measures in place such as fencing, controlling the predator population
and the general conservation or artificial selection of higher quality and double breeding birds, there is the
potential for population recovery in declining New Zealand and Australian populations of little penguins.
Conservation efforts cannot wait until a species has severely declined. It is while populations are healthy
that conservation efforts can provide the best outcome.
Peer Review Response
Julia gave several helpful comments in the restructuring of the paper. Her suggestion to carefully evaluate
sentence structure and vocabulary choice were the most helpful and I took most of the grammatical advice
she had to offer. As she put it, "just because you understand what you mean does not mean that your
audience does." Sometimes I overlook this, although I did not feel the need to define quite as many terms
as she would have liked. My reasoning for this is that my paper is focused for an audience familiar with
penguins/birds, Especially a scientific audience and not a general public kind of audience. As such, I do
not feel the need to identify terms that would be common place in the literature unless they have several
meanings (like fledgling). Instead of adding a table, I had summarized the data in text as a table would
only lead to further confusion. Looking back at that specific area (page 4), this information was taken
from a table and because most of the information was not relevant to my scope I did not include it (there
was quite a bit of extra information that I was not willing to interject into my paper, especially as I had a
hard time deciphering it at points. Her last suggestion was to clarify what the species was in my use of a
common name. Where possible, I switched out "little penguin" (as in a colloquial name, not a reference to
its size-although true) and put its genus-species binomial. This species had several common names and I
stuck with one throughout as not to be any more confusing. It is also important to note that even though I
talked about populations in New Zealand and Australia, I was generalizing and using data over many
colonies in the two areas not just two populations. Many of these studies built upon one another so it was
difficult to separate the material at times, especially when the data from one would be incorporated into
another.
Bibliography
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the blue penguin (Eudyptula minor) on the West Coast of New Zealand. New Zealand Journal Of
Zoology, 38(2), 131-141. doi:10.1080/03014223.2010.535498
Heber, S., Wilson, K., & Molles, L. (2008). Breeding biology and breeding success of the blue penguin
(Eudyptula minor) on the West Coast of New Zealand's South Island. New Zealand Journal Of
Zoology, 35(1), 63-71.
Johannesen, E., Houston, D., & Russell, J. (2003). Increased survival and breeding performance of double
breeders in little penguins Eudyptula minor , New Zealand: evidence for individual bird quality?.
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