食物可获得性对迁徙丹顶鹤取食策略影响的研究

The Contingency Model (CM) and Digestive Rate Model (DRM) are the two well-known foraging strategies and most adopted by foragers. The CM model assumes that the foragers select their preys based on energetic profitability (e/h) whereas the DRM model assumes that foragers select their preys according to digestive quality (e/k) when constrained by digestive bottleneck. It is quite necessary for migratory birds to adopt rate-maximizing foraging strategies for acquiring the needed energy resource at the stopover sites, since high fluctuation in prey availability is often present in these stopover sites, which can affect the choice of foraging strategies used. As a result, frequent switch between CM and DRM is often necessary under normal condition. The endangered Red-crowned Crane (Grus japonensis) provides an excellent example for exploring the switch in foraging strategies between CM and DRM, because these birds mainly feed on the abundant but hard-carapace Mudflat Tidal Crab (Helice tientsinensis) when they arrive at the Yellow River Delta during their migration. Due to their habit of swallowing the whole prey, a lot of bulky and ballast materials also enter the digestive tract. In this way, Red-crowned Cranes are more likely to adopt the DRM model. However, the prey availability of H. tientsinensis fluctuates seasonally and daily due to the effect of temperature. This in turn would cause the Red-crowned Cranes to adopt the CM model in order to maximize their consumption rate during the active foraging time. H. tientsinensis is a a sex-dimorph crab species, with the female being smaller than the male, but has a high digestible quality (e/k). This variation makes it possible to predict whether Red-crowned Cranes would choose the CM or DRM model in response to the fluctuating prey availability. We will first collect fecal samples of Red-crowned Cranes from low- and high-prey availabilities and analyze these in the laboratory to obtain the variation of crabs in sex ratio and prey size selection. Next, we will conduct field experiments to obtain the data for the energy contents (e), ballast mass (k), handling time (h) and prey availability (λ), and then feed these data into the two models to obtain the predicted prey size selections and compare these with the observed values to obtain the Euclidian distance (the difference between the two), a measure of which will reflect the accuracy of the models. The Euclidian distance also reflects the foraging strategy chosen by the Red-crowned Cranes. It can be used to predict which foraging strategy is more likely to be adopted by the cranes based on the temporal availability of prey. This project will greatly enrich our understanding of the foraging strategies employed by Red-crowned Cranes and can enable us to manage the conservation of the habitats of Red-crowned Cranes' stopover sites in term of safeguarding their food resource.

权变模型（CM）和消化速率模型（DRM）是目前两种最受关注的取食策略，二者分别从食物的能量收益率（e/h）和可消化质量（e/k）来探讨食物选择机制。候鸟迁徙期倾向于选择能量摄入率最大化的取食策略，而停歇地食物可获得性的剧烈波动将影响鸟类取食策略的选择。丹顶鹤为国家Ⅰ级保护鸟类,在黄河三角洲迁徙期主要取食天津厚蟹。该螃蟹既含有大量外壳，又具有明显的食物可获得性变化，是研究丹顶鹤CM和DRM模型选择的绝佳材料。本项目拟采用行为观察和粪样分析法，分别获取不同性别和大小螃蟹的e/h 和e/k值, 通过比较迁徙期和日间两个时间尺度上粪样中的螃蟹性比和大小分布的差别，探讨其取食策略选择以及其随食物可获得性变化的可塑性, 并对CM和DRM两个取食模型进行检验。本研究不仅将丰富取食策略理论中的选择决策机制，同时，还有利于了解丹顶鹤中停期的食物利用及能量积累策略，对于丹顶鹤停歇地的栖息地保护亦具有重要意义。

本项目采用野外调查、行为观察、粪样分析和视频监测等多种手段对丹顶鹤迁徙期栖息地利用及取食策略进行研究。调查发现近年丹顶鹤的种群数量呈明显下降的趋势，迁徙高峰期种群大小由2006-2009年的120-180只下降到30-50只。丹顶鹤在黄渤海地区迁徙停歇期主要利用翅碱蓬滩涂湿地，较少利用互花米草生境。丹顶鹤迁徙、越冬期的食物组成存在一定的地区和季节性差异。丹顶鹤在黄河口春、秋季迁徙期均主要取食滩涂蟹类，而在辽河口仅在秋季取食蟹类，春季主要取食玉米、芦苇和香蒲根茎等植物性食物，在盐城越冬期食物组成较为丰富，包括少量蟹类。丹顶鹤在黄河口主要取食天津厚蟹（86.5%）和少量宽身大眼蟹（13.5%）；在辽河口则全部取食天津厚蟹（97.2%）。丹顶鹤对螃蟹的性别和个体大小也具有选择性，倾向于取食雌性天津厚蟹（≥85.9%）和雄性宽身大眼蟹（≥90%），显著区别于两种螃蟹的在生境中偏雄性比（天津厚蟹：74.4~75.2%；宽身大眼蟹：63.3~68.6%）。此外，丹顶鹤倾向于取食个体较小的天津厚蟹和较大宽身大眼蟹。通过能值测定，雌性天津厚蟹的去灰分干重显著高于雄性个体，因此代表更优质的食物资源。丹顶鹤对雌性天津厚蟹的取食选择符合消化速率限制取食模型。进一步对滩涂蟹类的分布和活动规律研究发现，滩涂蟹类的分布受滩涂潮位和植被生境的影响，天津厚蟹主要分布可分布在不同潮位的翅碱蓬滩涂生境，而宽身大眼蟹主要分布在低潮滩的互花米草生境。天津厚蟹的活动强度与气温密切相关，日平均温度在4 ~12°C之间，螃蟹在洞口活动的时间比例最高，此时，最容易被丹顶鹤取食。丹顶鹤春季在黄河口的停歇期正好与天津厚蟹结束冬眠开始活动的时间吻合，而在辽河口，春季迁徙期螃蟹普遍还处于冬眠状态。因此，通过本研究发现丹顶鹤对滩涂蟹类的取食策略既受食物质量的影响，也受食物空间分布和食物可获得性时间变化的影响。