Discussion
Contrary to past literature that described the two cordon-bleu species
as having nearly the same nesting behavior (Goodwin 1982), my
observations revealed that two cordon-bleus adopted different nesting
strategies. It seemed one of the factors to enable them to breed
sympatrically. The species difference against nesting near a wasp nest
likely reflects the difference in their choosiness or order of priority
for nest-site selection. Because wasps are generally assumed to build
their nests earlier than cordon-bleus (i.e., wasp nests require a longer
time to build; Beier and Tungbani 2006, van Someren 1956), and both
cordon-bleus built nests at similar positions in the same season.
Although my preliminary observations are not conclusive to explain why
and how they maintain the niche partitioning, it raises new research
questions regarding nesting behavior and highlights the importance of
natural history recordings in Estrildid finches.
A past study of red-cheeked cordon-bleus has shown that nesting near a
wasp nest decreases predation risk and contributes to increasing
reproductive success (Beier and Tungbani 2006). If nesting near a wasp
nest is advantageous for their breeding, it is puzzling why blue-capped
cordon-bleus can inhabit sympatrically without a wasp nest. One likely
reason is that blue-capped cordon-bleus may compensate for reproductive
success by adopting other strategies. Using old weaver nests might
contribute to saving the energy of parents for nest building. Throughout
my fieldwork, I observed that blue-capped cordon-bleus but not
red-cheeked used flowers and their stems of Mollugo nudicaulisand grasses on the outside of the nest (Fig. 2a; Fig. 5a, c). These may
increase the robustness and safety of the nest and contribute to a
better natal environment for chicks. Alternatively, using wasp nests may
not contribute as much to reproductive success for red-cheeked
cordon-bleus in areas sympatrically habit with blue-capped cordon-bleus,
while may be effective in areas dominated by red-cheeked cordon-bleus
(Beier and Tungbani 2006). In order to test these, it is required to
investigate the functions of the use of weaver nests and various nest
materials. Testing if there are species differences in reproductive
success is necessary too. It should also be noted that my finding might
be area-specific rather than species-specific. Environmental differences
cause the inter-and intra-species variation in the nest material choice,
choosiness, and behavioral plasticity (e.g., Okano et al. 2011). It
would be interesting to examine if the nesting strategies vary in
different regions with different environmental conditions and
populations.
My finding of species difference in nesting strategy implies that there
are still overlooked systems and features in the natural history of
Estrildidae that we consider well understood. Accumulating natural
history knowledge is critical for ecological and behavioral fields
because it can tell us not only how we should interpret the results of
laboratory experiments but also which research question to ask (Travis
2020). Since Estrildid finches are easy to keep and breed in captivity,
studies of their nest-building and its neural mechanisms have been
conducted in the laboratory (Guillette and Healy 2015). Well-controlled
experiments in captivity have revealed many interesting facts such as
the roles of experience, social conditions, and cognitive abilities on
nest-building (Bailey et al. 2014, Guillette and Healy 2015, Hall et al.
2015). Those are hard to test in the field. However, collecting the
information of the wild behavior is still beneficial and required to
expand our idea of the evolutionary background of nest-building
behavior. Only a few studies empirically tested how and why wild
Estrildid finches build nests under wild conditions (Schuetz 2005, Beier
and Tungbani 2006). Furthermore, understanding the nesting strategies of
wild Estrildids may also give us a hint to elucidate the evolution of
their courtship behavior, in which laboratory experiments have
accumulated copious amounts of information. Some Estrildid finches
including cordon-bleus are known to hold a piece of nest materials in
the beak as a courtship component during song and dance (Ota et al.
2015, Ota 2020), which is presumably derived from nest-building behavior
(Soma 2018). It remains unclear how and why they choose the nest
materials during courtship, especially from the viewpoint of ecological
plausibility. Examining ecological contexts that shape nesting behavior
of wild Estrildid finches would give us a new picture of the wide range
of behavioral mechanisms and functions.