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.