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.