For some waterfowl species, they use diving as the main foraging technique. Diving behavior of waterfowl may be affected by a number of different factors such as prey availability, its distribution, density and type (Benvenuti et al. 2002; Kato et al. 2003; Thompson and Price 2006), water temperature, quality and depth (Guillemette et al. 2004; Green et al. 2005; Thompson and Price 2006), as well as light levels (Wilson et al. 1993; Wanless et al. 1999), body size, physiological functions (Butler and Jones 1982; Sato et al. 2003; Halsey et al. 2006) and habitat type (Ropert-Coudert and Kato 2009). A number of studies have reported that across species, body mass is an important predictor of diving behavior (Trillmich et al. 1991; Costa 1993; Boyd and Croxall 1996; Schreer and Kovacs 1997; Watanuki and Burger 1999). Halsey et al. (2006) analyzed the diving ability of 195 species of birds and mammals and found that both dive and pause durations increased with body mass. Diving theory predicts that large-size birds with a potentially enormous capacity for oxygen storage and low metabolic rate, should be able to stay submerged and surface longer than smaller birds (Cooper 1986). Within a species, however, dive and pause duration of males and adults are usually longer than that of females and juveniles because of different body size (Duncan and Marquiss 1993; Kato et al. 1999; Polak and Ciach 2007; Gomes et al. 2009; Laich et al. 2012).

The Scaly-sided Merganser (Mergus squamatus) is listed in the first category of nationally protected wildlife species in China and is regarded as an endangered species by the IUCN in 2002 (Wang and Xie 2009). This species mainly breeds in southeastern Russia, North Korea and Heilongjiang, Jilin and Inner Mongolia in the northeast of China (Birdlife International 2014). Wintering areas include a wide stretch along the south bank of the Yangtze River (He et al. 2006), while the north of Jiangxi Province provides one of the major wintering habitats for this species in China. Its population is found mainly along four rivers of Jiangxi, i.e., the Xinjiang, Fuhe, Raohe and Xiuhe rivers and their tributaries (Liu et al. 2008; Zeng et al. 2013; Shao et al. 2014). At a global level, their number is about 1200 pairs of which about 250 individual birds are wintering in Jiangxi. At present, the population of the Scaly-sided Merganser continues to decline due to a very high level of habitat loss, illegal hunting and human interference (Birdlife International 2014 ). Although a number of previous studies about wintering ecology of the Scaly-sided Merganser have been carried out (Liu et al. 2008; Shao et al. 2010; Yi et al. 2010), relatively little is known about their diving behavior and the factors that impact this species during winter (Shao et al. 2014).

From December 2015 to March 2016, we studied the diving behavior of the wintering Scaly-sided Merganser in Wuyuan, Jiangxi. The objectives of this study were to (1) provide basic data on diving behavior of wintering Scaly-sided Merganser, and (2) reveal the effect of sex, temperature, time and flock size on its diving behavior.

In our study, we found that mean dive duration of the Scaly-sided Merganser was shorter than that studied in Yihuang County (23.6 s) (Yihuang County is located in the Fuhe River valley), while mean pause duration was similar to that study (11.6 s) (Shao et al. 2014). The relationship between this difference and water depth, access to food sources, type and availability in the two water areas we investigated requires further study (Benvenuti et al. 2002; Kato et al. 2003; Green et al. 2005; Thompson and Price 2006). Dive durations of the Scaly-sided Merganser were similar to the short time bursts, i.e., 17.5−24.3 s, reported for Barrow’s Goldeneyes (Bucephala islandica) (Bourget et al. 2007), Common Goldeneyes (Bucephala clangula) (19.7‒23.9 s) (Bourget et al. 2007) and the Great Crested Grebe (Podiceps cristatus) with 19.5 s (Gomes et al. 2009), as well as that for small diving birds, such as the Little Grebe (Tachybaptus ruficollis) with 13.84 s (Jiang and Shao 2015). These short time bursts were considerably shorter than those reported for large diving birds such as the Subantarctic Cormorant (Phalacrocorax albiventer) with 67‒108 s (Kato et al. 1999), Rock shags (Phalacrocorax magellanicus) with 47.2 s (Quintana 1999) and Red-legged Cormorants (Phalacrocorax gaimardi) with 26.8 s (Frere et al. 2002). Pause durations of the Scaly-sided Merganser were similar to those reported for Barrow’s Goldeneyes (12.4‒14.0 s), Common Goldeneyes (10.4‒12.8 s) and the Tufted Duck (Aythya fuligula) (12.96 s) (Halsey et al. 2005). These differences may be related to their body mass, an opinion supported by Cooper’s prediction (Cooper 1986).

Diving efficiency (2.297) of our Scaly-sided Merganser was similar to Yihuang section in Fuhe River (1.88‒2.21) while higher than that of the Great Crested Grebe (1.2) (Gomes et al. 2009) and the Little Grebe (1.13) (Jiang and Shao 2015). The American coot (Fulica Americana) enhances its diving efficiency when access to food sources decreases due to prolonged dive duration and shortened pause duration. These differences in diving efficiency may be caused by water depth, light levels and the ecological habitat of the species (Jiang and Shao 2015). Pause duration was positively correlated to earlier dive durations as well as to the duration of the subsequent dive; this agreed with the results obtained by Shao et al. (2014) and those of Jiang and Shao (2015), whose results suggest that the time spent on the surface is a period which the bird uses to recover from the previous dive and to prepare for the next one.

Mean dive duration and mean time on the surface of male Scaly-sided mergansers were significantly longer than those for females and similar to those of Common Goldeneyes (Duncan and Marquiss 1993), the Subantarctic Cormorant, the Japanese Cormorant (Kato et al. 1999) and the Blue-eyed Shag (Laich et al. 2012). These results may be related to body mass differences (Scaly-sided Merganser: W _♂: 1.0‒1.2 kg, W _♀: 0.8‒1.0 kg; Common Goldeneyes: W _♂: 0.8‒1.0 kg, W _♀: 0.5‒0.9 kg; Subantarctic Cormorant: W _♂: 2.9 kg, W _♀: 2.5 kg; Japanese Cormorant: W _♂: 3.1 kg, W _♀: 2.5 kg; Blue-eyed Shag: W _♂ significantly larger than W _♀) (Kato et al. 1999; Zhao 2001; Laich et al. 2012) and accord with Cooper’s prediction (Cooper 1986).

Dive duration was negatively correlated to the average daily air temperature, which was similar to the results obtained by Green et al. (2005) and may be related to the minimum rate of oxygen consumption. This minimum rate was far smaller in relatively low temperatures than in high temperatures; therefore, with a decrease in the energy cost of diving, the dive duration increased (Green et al. 2005).

Dive duration was negatively related to time of year and daytime. The temperature in December and January were lower than in February and March in our study area. Dive duration was significantly higher in December and January, which may be related to the low minimum rate of oxygen consumption for diving waterfowl under low temperature conditions (Green et al. 2005). However, the temperature in February was lower than in March while dive duration was significantly lower than in March. This unique result, appearing in February, may be caused by increased human activity during China’s traditional spring festival. Studies have shown that the diving behavior of waterfowl is limited by light conditions, with dive durations longer at lower light intensity (Wilson et al. 1993; Wanless et al. 1999). Our study supports this general assumption. The longest mean dive duration of the Scaly-sided Merganser was from 7:30‒8:59 when light intensity was low. The dive duration of Scaly-sided Mergansers at 16:00‒17:30 was significantly lower than at any other time, which may be related to their ecological habits or prey behavior.

During this study, we found that the foraging time of our mergansers clearly increased and their vigilance decreased when the flock size was larger, which agrees with the point of view by Roberts (1996) on this issue. That is, with a larger flock, the probability of detecting a predator for individual bird increases while the risk of becoming prey diminishes. That leads to an increase in the foraging time for individual birds and a decrease in the necessity for vigilance (Clark and Mangel 1986). Dive duration of the Scaly-sided Merganser was negatively correlated to flock size, i.e., with a larger flock the dive duration shortens significantly, thereby increasing diving time considerably, quite in agreement with the hypothesis of competitive disturbance. In an environment of limited food sources and larger flocks, individual birds will increase their speed in foraging in order to acquire more food, leading to an increase in forage activity for individual birds and a decrease in vigilance (Clark and Mangel 1986). Furthermore, intraspecific competition was more intense when the group of Scaly-sided Mergansers was larger. In order to acquire more food effectively and decrease intraspecific competition, the Scaly-sided Merganser has adopted a flexible foraging strategy that shortens dive duration and adds to diving times. With an enlargement of flock size, the predation ability and efficiency increases due to collective predation, allowing the Scaly-sided Merganser to acquire more prey in a limited time.

Sex, temperature, time and flock size are factors that affect the diving behavior of our wintering Scaly-sided Merganser. The mean dive duration and mean time on the surface of males were significantly higher than those of females, which may be related to the weight differences between sexes. With an increase in temperatures, dive duration increased considerably, along with the passage of time of year and daytime. The difference of diving behavior at various temperatures and daytime may be related to a low minimum rate of oxygen consumption. With an increase in flock size, dive duration decreased significantly and may be caused by rising interspecific competition.