| Seasonal variation in food availability plays an influential role in the population dynamics and the adjustment of the behavior patterns of many avian species, especially the waterbirds. Water level fluctuation changes waterbirds foraging habitat and food availability, affecting their temporal-spatial patterns of foraging activities. It is of great significance to investigate the influence of water level fluctuation on the food availability of wintering waterbirds, and their foraging responses to the wetland degradation. Understanding the behavior patterns is favorable to protecting the population of wintering waterbirds and restoring their wintering habitats.Hooded Crane (Grus monacha) is a large migrating wader, IUCN vulnerable species and the national grade I protected birds in China, wintering in the lakes of Yangtze River floodplain. In recent years, the wetlands are exploited rapidly. Seine fishery and the construction of water conservancy projects have changed the natural hydrological fluctuation. Aquatic vegetation resources decreased sharply, especially the submerged vegetation resources. Water level fluctuation changes Hooded Cranes foraging habitats and food availability, thus affecting their habitat use and behavior patterns.The field survey of the foraging activities was carried out in the lakes of Yangtze River floodplain from November in 2013 to April in 2014. According to the trend of water level rising in the upstream of Shengjin Lake and the corresponding water level fluctuation time, we defined five stages in the wintering period. At the same time, based on the differences in vegetation and micro-geomorphological features, the foraging habitats were distinguished into three types, i.e., mudflat, meadow, and paddy field. Direct counting method was used to record the number of crane flocks, as well as the number of crane individuals in each flocks weekly. Within each stage,30 quadrats were arranged in the grounds occupied by the cranes at their foraging range. The habitat varibles recorded included the food type, abundance and soil hardness. The ratio of the dry weight of prey resources in a quadrat (0.25 m2) was defined as the prey abundance (g/m2). Voice Recorder was used to record the behavioral events for 20 min. Survey results in each 20 min was used as an independent sample, and the foraging frequency, duration of foraging and foraging time budget were calculated respectively. The Kruskal-Wallis H test and Mann-Whitney U test were used to test the difference of relative abundance in different habitats and time budget during five stages. Person correlation analysis was used to test the correlation between foraging behavior and the environmental factors. In addition, the relationships of foraging behaviors (foraging time budget, duration of foraging, foraging success and foraging frequency) with water level fluctuation and habitat type were analyzed using generalized linear model. All analyses were performed using PASW Statistics 18 (IBM Inc.,2009). A significance level of 0.05 (p) was used for all statistical tests, with means expressed as mean±SE.Food resources abundance in meadow habitat was the highest in Shengjin Lake (241.2±12.5), and the second was paddy fields (83.7±9.4). The food resources in mudflat habitat were the least in the three habitat types (13.4±0.8). Food resources abundance had obvious spatial heterogeneity (X2=195.826, P=0.000). However in Caizi Lake, food resources abundance was the highest in paddy habitat (62.33±2.35), and the second was meadow habitat (44.0±1.4). The food resources in mudflat habitat were also the least(12.7±0.7). Throughout the winter period, food resources abundance in mudflats and paddy habitats all showed a trend of decline, but had a tendency of increasing during the late recovery stage in meadow habitat.In Shengjin Lake, the abundance of Hooded Cranes reached the maximum in paddy fields, became the main foraging habitat. Meadow habitat was the second. Utilization of mudflat was the least. With the water level fluctuation, utilization of mudflat habitat showed a significant difference during five stages (X2=10.767, P= 0.029). Utilization of Hooded Cranes has obvious spatial heterogeneity (X2=13.640, P=0.017). The main foraging habitat in Caizi Lake was also the paddy fields, and had maintained a relatively high utilization. Utilization of mudflat and meadow habitat stayed lower during the whole wintering stages, but with overwintering period going on, the utilization of the two habitats showed the tendency of relative increase.During the whole wintering period, the foraging time budget showed a significant difference in three habitats in Shengjin Lake (X2=65.103, P=0.000). The foraging time budget in meadow habitat was the highest (0.72±0.24), the second was the paddy field (0.70±0.25), which was slightly lower than the meadow habitat. In mudflat habitat, the foraging time budget was the least (0.49±0.22), but the time budget in maintenance and locomotion were far higher than other two habitats. In Caizi Lake, the foraging time budget of Hooded Cranes in paddy field was the highest (0.78±0.19). Most of the time budget in meadow habitat ware foraging (0.71±0.19), but ware also slightly lower than in the paddy field. The time budget in mudflat habitat was the lowest in the three habitats (0.47± 0.21), the time budget in maintenance and resting were far higher than other two habitats, just like in the Shengjin Lake. Time budget had obvious spatial differences in Caizi Lake (X2= 82.623, P=0.000).Foraging frequency of Hooded Cranes in three habitats was also different. In paddy field where the cranes cost the most time foraging, the behavior frequency was the highest (2.98±0.15). The foraging frequency was lower in meadow habitat than in the paddy fields (2.50±0.10). However, behavior frequency in mudflat was also the lowest (2.53±0.10). There were also significant differences of foraging success in three habitats(X2=64.430, P=0.000). The food resources abundance in paddy field was relatively richer, correspondingly the cranes foraging success were the highest (7.71±0.50). The foraging success had the same change trend with foraging frequency in different habitat types, it was lower in meadow habitat than in the paddy habitat (6.33±0.60), and was the lowest in the mudflat habitat (3.62±0.37).The results of person correlation analysis had proved that the food density was significantly correlated with the foraging time budget and the duration of foraging time (P=0.000; P=0.042), but bad not significant correlation with the foraging frequency and the foraging success (P=0.065; P=0.059). The foraging frequency, foraging time budget and the foraging during time all had no significant correlation with the soil hardness (P=0.731; P=0.079; P=0.407), however the foraging success had significant correlation with it (P=0.001). General linear regression showed that the foraging strategies of Hooded Cranes were greatly affected by water level fluctuations, including the duration of foraging, foraging frequency, foraging time budget and especially the foraging success. The habitat type had extremely remarkable effects on foraging time budget and foraging success, but less impact on the duration of foraging.With the temporal-spatial change of the abundance of food resources, the utilization of three types of habitats and behavior patterns by Hooded Cranes changed correspondingly. This was an adaption to the collapse in food resources. Hooded Cranes would adjust their foraging patterns and make full use of different habitat types to get enough food in respond to the water level fluctuations and the change of food resources abundance. |
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