Introduction
Niche partitioning has an important role in the coexistence of multiple species with similar ecology (Levine and HilleRisLambers 2009). It is rather natural that ecologically similar species use different resources such as foods (Hasui et al. 2009) and nest sites (Kosinski and Winiecki 2004, Vierling et al. 2009) depending on the availability in the shared habitat to avoid conflicts and competitions. Knowing strategies that animals adopt to coexist with other species is essential not only for understanding the biodiversity and ecosystems but also for elucidating factors that shape their behavior and cognitive functions.
Estrildid finches are one of the most well-studied songbird families regarding their behavior from its neural mechanisms to evolution (e.g., courtship song: Catchpole and Slater 2003, Soma and Garamszegi 2015; nest-building: Bailey et al. 2014, Guillette and Healy 2015, Hall et al. 2015), while we have limited knowledge of their wild behavior and the environmental conditions in many species. Blue-capped cordon-bleus (Uraeginthus cyanocephalus ) and red-cheeked cordon-bleus (Uraeginthus bengalus ) are socially monogamous, biparental Estrildid finches from Africa. Male blue-capped and red-cheeked cordon-bleus, as their names suggest, have blue caps and red cheeks, respectively (Fig. 1). Female blue-capped cordon-bleus have brighter blue plumage and pink beak than red-cheeked (Fig. 1). They are sister species and have similar life history and breeding behavior, and their habitat overlaps in East Africa (Goodwin 1982, Hockey et al. 2005, Billerman et al. 2020). Past literature generally mentioned the ecological and behavioral similarities between the two species and usually neglected the possibility of adopting different behavioral strategies to avoid conflicts in the shared habitat.
The nest building behavior of cordon-bleus is characteristic in several ways. They build their nest in trees mainly using grasses, but sometimes take over weaver (Ploceus  spp. or Bubalornis  spp.) old nests (Fig. 2b, c). They often build a nest near wasp nests (Hymenoptera: Vespidae) to reduce the predation risk (Fig. 2a, c, d; Goodwin 1982, Hockey et al. 2005, Beier and Tungbani 2006, Billerman et al. 2020). During my fieldwork in Tanzania, I observed that cordon-bleus used the same nesting strategies as previously reported (Fig. 2). The primary purpose of the expedition was to observe the multimodal courtship display of wild cordon-bleus (Ota 2020), but I could also observe nest-building behavior and the nests during the study (Fig. 2, Movie S1). When attempting to observe the courtship behavior of cordon-bleus, I usually focus on individuals holding a piece of nest materials on their beak since cordon-bleus perform courtship displays with holding them (Ota 2020). Cordon-bleus holding a piece of nest materials often carried them to their nests (Movie S1) rather than using them for courtship. In addition, I confirmed that two species of cordon-bleus actually inhabited and breed sympatrically at my field site (see results, Fig. 3), so I expected it would be an ideal opportunity to examine their nesting strategies and the species differences.
I hypothesized that the adopting nesting strategies and their frequency vary between two cordon-bleus, which can avoid resource competition. I recorded the positions and surrounding conditions of the nests and tested if there were species differences. I will also describe some observational findings of nest characteristics in two cordon-bleus.