1 | Introduction
As the most challenging season of the year, winter usually brings harsh and unpredictable conditions in subtropical regions, requiring animals to evolve a range of morphological, behavioral, and physiological adaptations to reduce mortality. In winter, birds must face a trade-off between starvation, which favors fat storage, and avoiding predation, which favors a lean body (Brodin, 2007). The random fluctuations of temperatures during the night and unpredictable foraging conditions during the day requires birds to carry a sufficient fat reserve in the event of a worst case scenario. However, there is also a constant risk of predator attack, such as from owls or hawks, in which case a lighter bird carrying less fat reserve will have a better chance of escape (Brodin, 2019).
Based on this starvation-predation trade-off framework, researchers have previously attempted to uncover the dynamic patterns and driving factors of body condition change at the individual level. Theoretical models have included the optimal foraging theory (OFT) and the dynamic state variable model (DSVM). Such models incorporate stochastic variables for weather conditions, predation risk and foraging success, and also include temporal dynamics and behaviors that depend on the state of the animal (Brodin, 2007; Clark and Mangel, 2000; Lima, 1986). Meanwhile, some studies have focused on how birds flexibly fine-tune the size of their fat reserve in direct response to local daily changes in ambient conditions. For instance, temperature, humidity, snow cover, and even wind speed had all been shown to influence the adjustment of energy reserve in birds (Laplante et al., 2019; Rogers et al., 1994). In addition, inner factors such as sex and social hierarchy can also influence the fattening strategy of birds (Clark and Ekman, 1995). However, most studies on wintering birds were conducted in relatively harsh conditions in North America and Europe, locations which usually had long-term snowfall and subzero temperatures. The short-term cold adaptation of birds wintering in a subtropical forest, which typically has milder winter conditions, is still largely unknown.
Besides how individual birds respond to ambient variables, wintering adaptation also can be observed at the population level. Some groups of ducks and small songbirds studied had a skewed sex ratio in their wintering habitat, indicating the existence of sexual segregation (Pattenden and Boag, 1989; Senar and Domènech, 2011). Sexual segregation is a common phenomenon in large-scale animals, leading to unequal sex ratios in certain habitats or areas (Main et al., 1996). The existence of sexual segregation in birds can usually be explained by the influence of social hierarchy or differential tolerance to ecological factors such as extreme temperatures (Catry et al., 2006). Therefore, knowing the sex ratio of one species helps to better understand its life history strategy and population-level adaptability.
As a typical long-distance migratory songbird, the orange-flanked bush-robin (hereafter referred to as “robin”) spends the breeding season in Finland, northern Russia, Mongolia, and northeast China, and spends the non-breeding season in the forests of southwest China and southeast Asia (Collar et al., 2020). The robin is a typical sexually dichromatic bird exhibiting delayed plumage maturation (DPM); female robins and yearling males have olive-brown plumage, while 2yr+ adult male robins exhibit bright blue plumage (Hellström and Norevik, 2013). Delaying the acquisition of distinctive plumage coloration until after the first potential breeding period may play an important role in the adaptation and sex ratio among DPM birds (Hawkins et al., 2012). A study of robins wintering in central China demonstrated that the sex-ratio was significantly biased towards males among olive-brown individuals (Li et al., 2016). However, there are still many unresolved questions, including whether this sex-ratio is a universal rule for DPM birds or an occasionally observed phenomenon, and how male and female DPM birds adapt to winter environments.
We conducted the first wintering strategy study of the orange-flanked bush-robin in southwest China at both the individual and population levels. Using mist net surveys and bird banding, we aimed to (1) determine the basic population composition of wintering robins, (2) reveal potential morphological differences between females and female-like males, and (3) illustrate dynamic body condition patterns and factors which affect daily fattening.