Nest sites
Streaked Wren-Babblers only build nests in rocky cavities or holes, ignoring other potential suitable places in the study area. Collar and Robson (2007) noted that Streaked Wren-Babblers often place their nests on the ground or in the banks of roads or trails. Wells (2007), working in the Thai-Malay Peninsula, found that Streaked Wren-Babblers there occasionally bred in crevices of dead trees. Although these habitats were also common in limestone forest in Nonggang, Streaked Wren-Babblers did not use them. Interestingly, there are also some other species that are reported to elsewhere build nests in holes or the ground, but prefer rock cavities in limestone areas (Jiang et al. 2013b, 2014). For example, the Oriental Pied Hornbills (Anthracoceros albirostris) mainly build nests in rocky holes in limestone forests in Nonggang, but prefer tree holes in non-limestone forests (Chen et al. 2007).
One possible hypothesis to explain this strong attachment to rocky cavities is that such cavities provide both heat insulation and escape from rain (Jiang et al. 2013a). Generally, both boulders and limestone cliffs overhang the nest entrance, making them well protected from rain and direct sunshine. In our study, all the holes or cavities used by Streaked Wren-Babblers had only one entrance, making the nest drier and warmer than if it had two or more entrances, which would increase exposure to rain and wind. Although these rocky holes help to supply a comfortable (warm/dry) nest for birds, they may be accessible for some predators, such as small mammals, snakes that use the ground, and large invertebrates. Indeed, the nest predation of some babblers breeding in rocky holes was very high in this limestone area, e.g. 75% nests of the Nonggang Babbler were destroyed by nest predators (Jiang et al. 2013a); note that the Nonggang Babbler also nests in a later season, after April, and this may be a large factor in its high predation rate, as argued by Jiang et al. (2017). We believe the comfortable nest environment and potential high nest predation risk after April in this area are two key factors influencing the life history strategy of Streaked Wren-Babblers.
Breeding season
Most Streaked Wren-Babblers laid their first eggs before April in Nonggang limestone forest. This is unusual for birds in Nonggang: most species breed after April (Jiang et al. 2013a, b, 2015). It is also unusual for babblers, as most babbler species’ breeding behavior is associated with an increasing day length and temperature in the northern hemisphere (Collar and Robson 2007).
Despite relatively constant temperatures, tropical birds are known to show strong seasonality of breeding, influenced by different kinds of environmental factors, especially rainfall (Stutchbury and Morton 2001). Some of this seasonality may be related to food availability (Lack 1954). As yet, however, there has been no experimental study conducted to determine the effects of food-supply on forest bird ecology in Southeast Asia forests (Sodhi 2002). As for the case of the Streaked Wren-Babbler, it would seem that food availability would be higher when they complete their breeding process. Both parents and nestlings of the Streaked Wren-Babbler forage on invertebrates (Collar and Robson 2007; Lu et al. 2013). In our research, adult Lepidoptera were the most common insects that appeared in the diet of Streaked Wren-Babbler nestlings. Generally, the population and abundance of adult Lepidoptera has a strong positive correlation with daily temperature and monthly rainfall in tropical forests of Southeast Asia (Intachat et al. 2001), and the weather of Nonggang becomes warmer after April with more rainfall starting from May. All this suggests that the key factor controlling this species’ breeding is not food availability, but more likely potential nest predation after April in this area (Jiang et al. 2017), as has been also suggested for other tropical birds, e.g. the Clay-Colored Robin (Turdus grayi) (Morton 1971).
Nest predation
High nest predation is an important life history trait of tropical birds (Stutchbury and Morton 2001). Some birds breeding in limestone areas have been reported to suffer a higher nest predation pressure compared to temperate areas (Jiang et al. 2013a, b, 2015). Streaked Wren-Babbler appears to be an exception, however, as it showed a very low nest predation rate in our research (12% of 33 nests).
Nest predation has been shown to be affected by several different factors, such as nest sites, breeding season, and checking frequency of the researchers (Morton 1971; Ibáñez-Álamo et al. 2012; Vetter et al. 2013). Regarding the last factor, in a previous study (Jiang et al. 2017) we found that checking the nest once a day (predation in 37% of 59 artificial nests), or checking once every 3 days (31% of 49 nests), was associated with lower predation rates than checking just once in a 12-day period (57% of 74 nests). However, this effect was not as large as seasonal differences (predation in 17% of 83 artificial nests in March, vs. 66% of 99 nests in May). Hence, we believe that this study’s once-a-day checking may be one factor that contributed to the low predation rate, but that natural rates of predation are likely to be low, at least before April.
What about Streaked Wren-Babbler’s nesting biology might explain their low nest predation rate? Some nests are located in inaccessible places, such as high on a vertical cliff or on the roof of big cave; such locations would make predation from common predators, such as raptors and big or medium carnivores, difficult. Even if predators arrive at the hole, the small and deep entrance may block predator access. Some of the larger invertebrates and snakes are the main threats for the eggs and nestlings of Nonggang Babbler, which builds nests at sites similar to those selected by Streaked Wren-Babblers (Jiang et al. 2013a). These animals also appear to be the main nest predators for other babblers in southwestern China (Fu et al. 2017). These ectotherms become more active with increased temperature (Cox et al. 2013). We suggest that Streaked Wren-Babbler finishes the breeding process before April to avoid the attacks from these ectothermal animals, and thus escapes nest predation.
Clutch size
The Streaked Wren-Babbler mainly laid three eggs in our study area, similar to birds in India and Myanmar (Collar and Robson 2007). However, this is different from birds in Peninsular Malaysia, where two eggs are usually laid (Wells 2007). Nonggang has the similar latitude with India and Myanmar, but is substantially further north than Peninsular Malaysia. Generally, clutch sizes of birds increase with an increasing latitude (Lack 1947), similar to the pattern shown here. However, the clutch size of Streaked Wren-Babbler is less than most birds breeding in the Nonggang area, which mostly laid more than three eggs (Jiang et al. 2013a, b, 2014, 2015). Small clutch sizes may be explained by high nest predation or limited food supply for nestlings (Styrsky et al. 2005). In our research, the Streaked Wren-Babbler has low nest predation but breeds in early spring, suggesting that limited food for nestlings may drive the relatively small clutch sizes (relative to other species) of the Streaked Wren-Babbler in Nonggang limestone forest.
Incubation
The incubation rhythm of the Streaked Wren-Babbler is similar to that of the Nonggang Babbler in having only a few relatively long recess periods (Jiang et al. 2013a). Indeed, adults were away from the nest about 20% time of whole day. The incubation rhythm of birds is affected by environmental factors of the nests and the body size of the incubator (Afton 1980), and the incubation strategy of birds is a trade-off between keeping adequate egg temperature and foraging (Jia et al. 2010). Generally, small-sized birds leave nests to search for food more frequently than bigger or medium sized birds (Afton 1980). Perhaps rocky holes regulate heat well and thus allow incubating birds to forage more. However, to confirm this hypothesis it is necessary to do more observations with individually identified birds.