有机氯农药在大滨鹬和红腹滨鹬体内的富集程度

为探讨东亚-澳大利西亚迁徙路线上的鸻鹬类体内有机氯农药的含量及来源,本研究以该迁徙路线上的大滨鹬(Calidris tenuirostris)和红腹滨鹬(C.canutus)为研究对象,用索氏提取法对这两种鸟的胸肌和皮下脂肪中的有机污染物进行萃取,并用气相色谱法对19种有机氯农药进行检测。结果表明,HCHs、DDTs、硫丹Ⅱ等14种有机氯农药在大滨鹬和红腹滨鹬的组织中均有不同程度的检出,所有样品中的含量最高值达1 573.5 ng/g脂重;在检出的14种有机氯农药中,α-HCH、β-HCH、γ-HCH、p,p′-DDE、硫丹Ⅱ、硫丹硫酸酯和/或p,p′-DDT的检出率达100%;在大滨鹬的肌肉组织、红腹滨鹬的肌肉和脂肪组织中,p,p′-DDE的残留量最高;而在大滨鹬的脂肪组织中硫丹硫酸酯和/或p,p′-DDT的含量最高;目标物中的艾氏剂、异狄氏剂、七氯、反式氯丹等未达检出限或含量较低。我们对比了不同物种及不同组织样本中有机氯农药的富集程度,红腹滨鹬的肌肉组织中HCHs的沉积量显著高于大滨鹬,而大滨鹬的脂肪组织中硫丹硫酸酯和/或p,p′-DDT的含量显著高于红腹滨鹬。此外,分别对比两个物种的肌肉组织和脂肪组织中有机氯农药沉积量,部分有机氯农药在脂肪组织中的沉积量显著高于肌肉组织,说明相比于肌肉组织,有机氯农药可能更易于在脂肪组织中累积。     

黑腹滨鹬亚种分类研究进展

黑腹滨鹬Calidris alpina因繁殖季节其腹部有黑斑而得名,是北半球最常见的鸻形目鸟类之一.黑腹滨鹬历史上共命名过11个亚种,经过2次主要的系统厘定后,目前认为分化有10个亚种.亚种间除在量度性状、体色性状和地理分布有区别外,还在换羽模式、线粒体谱系组成等方面有所不同,从而使其成为研究种群分化的理想对象之一.本文介绍了黑腹滨鹬的亚种分类性状、分类修订以及亚种分类系统,并就非繁殖期的相关研究作了概述.由于混群,非繁殖地的亚种鉴定与繁殖地的亚种鉴定在性状的使用侧重上有很大的不同.非繁殖地亚种的划分,以环志回收结果和线粒体DNA谱系组成为主,其它性状为辅.中国究竟有几个亚种分布还需要进一步研究后才能确定.     

东亚-澳大利西亚候鸟迁飞区中杓鹬 的迁徙追踪

追踪候鸟的迁徙活动是全面认识其生活史年周期的重要途径。中杓鹬(Numeniusphaeopus)在全球广泛分布,但在东亚-澳大利西亚候鸟迁飞区的迁徙活动一直缺乏追踪研究。2018年2月,在澳大利亚西北部的布鲁姆为捕捉到的中杓鹬成鸟佩戴平台发射终端或全球定位系统-全球移动通讯系统追踪器,以确定其迁徙日程、迁徙路线以及迁徙停歇地和繁殖地的地理位置。我们从成功追踪的7只个体获取了6 378条精度高于1 km的位点数据。分析结果表明,在春季,中杓鹬的迁徙时长为(36±4)d,其间在1～3个迁徙停歇地的停留日期为(23±2)d,从越冬地到繁殖地的迁徙距离为(9 795±346)km(n=7)。追踪的中杓鹬在俄罗斯东部和中部区域繁殖,不同个体的繁殖地纬度相近而经度范围较广。在秋季,中杓鹬的迁徙时长为(90±27)d,相比春季迁徙时长更长;其间,在2～4个迁徙停歇地停留(79±29)d,从繁殖地到越冬地的迁徙距离为(10 101±520)km(n=5)。无论在春季还是秋季迁徙,迁徙停歇地广泛分布于东亚、东南亚沿海及内陆区域。大部分个体春季和秋季的迁徙路线相近,成功追踪的个体均在秋季返回了上一年的越冬地,这表明中杓鹬对越冬地具有很高的忠诚度。     

人为干扰对黑腹滨鹬觅食行为的影响

2003年11月至2004年3月,采用目标动物取样法和人为干扰实验法,于浙江省乐清湾滩涂进行人类活动对黑腹滨鹬(Calidrisalpina)觅食行为影响的研究。取样观察结果表明,在每天沿海堤可观察黑腹滨鹬持续觅食的1h里,96.6%的黑腹滨鹬在35m或更小的距离上通过奔跑或惊飞对人类的活动作出反应;黑腹滨鹬花费90%(人类较多的盐盆滩涂为85%,较少的乐成滩涂为94%)的时间觅食,10%的时间用于避免人类的干扰;人数、人类活动类型对觅食时间具有显著影响,人数、人离黑腹滨鹬的距离、人类活动类型对黑腹滨鹬移动的距离、移动的次数以及黑腹滨鹬对人类靠近的反应均具有显著效应。人为干扰最小接近距离实验结果表明,黑腹滨鹬对人类反应的距离在3～30m;干扰人数为1人时的最小接近距离显著大于2人;人类行走和奔跑的两种活动类型对黑腹滨鹬的最小接近距离没有显著影响。本研究验证了滨鸟觅食区内的人为活动严重影响黑腹滨鹬觅食行为的假设。     

白鹤迁徙动态及其停歇地环境条件研究

在吉林省莫莫格保护区于2000年4－5月对途经此地的白鹤（Grus leucogeranus)种群数量和停歇地环境条件进行研究的结果表明：白鹤于4月3日左右迁来,高峰期在4月下旬,最多数量达到577只（4月27日）,5月8日离开莫莫格,停歇期36天。白鹤迁徙活动受风速、气温、降雨等天气条件影响。喜栖湿草甸、浅水湖泡及湖泡中间的“旱堑”。取食鱼类及植物,以植物性食物为主。本文提出了进一步研究和保护白鹤迁徒停歇地的措施。     

迁徙鸟类对中途停歇地的利用及迁徙对策

中途停歇地是迁徙鸟类在繁殖地和非繁殖地之间的联系枢纽,对于迁徙鸟类完成其完整的生活史过程具有重要作用。从鸟类的迁徙对策、中途停歇地的选择、鸟类在中途停歇地的停留时间、体重变化和种群特征以及中途停歇地的环境状况等方面,回顾了中途停歇生态学在近年来的研究进展,并提出了在迁徙对策理论的实验研究,小型鸟类在中途停歇地的停歇时间及体重变化的准确确定等目前有待解决的问题。     

细嘴滨鹬春季迁徙中转停歇期间机体结构的表型弹性

通过描述细嘴滨鹬（Calidris tenuirostris）春季在上海中转停歇期间个体与器官表型变化,分析了中转停歇期间机体结构表型弹性的变化。结果表明：细嘴滨鹬中转停歇期间,体质量显著增加,最大增量约相当于平均体质量的52.2％。体质量增加的最大因素是脂肪质量的增加;其次是胸肌质量的增加,胸肌质量随体质量平行变化;腿肌与心质量随体质量的增加也有增加趋势;消化器官的变化分为2方面,一是消化腺（肝、胰）质量增加,二是消化道质量（砂囊、肠）没有大的变化,推测消化道没有大变化的原因可能是保留有一定的备用容量,细嘴滨鹬通过消化腺的增大来快速提高停歇期的觅食效率,同时避免重建消化器官的能量与时间的消耗,提高整体迁徙速度;脑、肺、肾、脾、眼及盐腺等器官质量保持稳定。     

围垦堤内迁徙鸻鹬群落的生态学特性

以群落生态学的原理为基础 ,研究景观特征及异质性变化导致鸟类群落的变化 ,重点探讨了人为干扰导致的景观特征变化对迁徙鹬群落时空结构的影响。景观异质性的改变是引起鹬群落结构变化的主要原因 ,也是决定鹬群落分布的重要因子。围堤内湿地植被覆盖率大于 6 0 %或水面小于总湿地面积 2 0 %后 ,鹬基本不出现 ,必须含有水域、植被和裸地三种景观要素。植被覆盖率为 1 0 %～ 2 0 %时 ,鹬数量最多。鹬在围堤内滩涂的分布与底栖生物量无关。水深是影响鹬分布的重要因素 ,使鹬的空间生态位得到分离。科鸟类主要活动在裸地和浅水区 ,中小型鹬类主要在中水区 ,大中型鹬类在深水区。科学管理围堤封堵后滩涂 ,控制和改造湿地景观 ,将在一定程度上降低围涂工程对鹬群落的负面效应。     

豆雁在中国的春季迁徙路线及迁徙停歇地

掌握禽流感病毒携带者豆雁(Anser fabalis)的迁徙路线对于全面理解候鸟在禽流感疫情传播与扩散中的作用具有重要意义。2013年3月至4月在江西省鄱阳湖南矶山保护区,利用卫星跟踪法对3只豆雁个体的迁徙路线进行研究,仅有1只个体成功传输数据。数据经Google Earth和Excel软件处理后,结果显示:该只豆雁由江西鄱阳湖出发,途经安徽、江苏、山东、天津等省市到达河北秦皇岛,5月被发现死于山海关,迁徙总距离约1 500 km,有5个停歇地;两相邻停歇地之间最短距离为20 km,最长为700 km;停留时间最短为1 d,最长为14 d;约68%的停歇位点地物类型为农田;飞行高度平均值为15 m,最大值为407 m。     

围垦堤内迁徙(行鸟)鹬群落的生态学特性

以群落生态学的原理为基础,研究景观特征及异质性变化导致鸟类群落的变化,重点探讨了人为干扰导致的景观特征变化对迁徙(行鸟)鹬群落时空结构的影响.景观异质性的改变是引起(行鸟)鹬群落结构变化的主要原因,也是决定(行鸟)鹬群落分布的重要因子.围堤内湿地植被覆盖率大于60%或水面小于总湿地面积20%后,(行鸟)鹬基本不出现,必须含有水域、植被和裸地三种景观要素.植被覆盖率为10%～20%时,(行鸟)鹬数量最多.(行鸟)鹬在围堤内滩涂的分布与底栖生物量无关.水深是影响(行鸟)鹬分布的重要因素,使(行鸟)鹬的空间生态位得到分离.(行鸟)科鸟类主要活动在裸地和浅水区,中小型鹬类主要在中水区,大中型鹬类在深水区.科学管理围堤封堵后滩涂,控制和改造湿地景观,将在一定程度上降低围涂工程对(行鸟)鹬群落的负面效应.     

Stopover Ecology of Red Knots in Southwestern James Bay During Southbound Migration

Many shorebirds rely on small numbers of staging sites during long annual migrations. Numerous shorebird species are declining and understanding the importance of these staging sites is important for successful conservation. We surveyed endangered rufa red knots (Calidris canutus rufa) staging in James Bay, Ontario, Canada, during southbound migration in 2017 and 2018. We used mark-resight data and count data in an integrated Bayesian analysis to quantify migration phenology, estimate passage population size, and model the age structure of the stopover population. Many adult red knots arrived in James Bay in a single wave in early August in 2017, whereas adult red knots arrived in multiple smaller waves in July and mid-August in 2018. These waves may correspond with breeding phenology where more red knots bred successfully and arrived in one large event in 2017 and the higher number of earlier arrivals in July 2018 may have been failed breeders. We included a binomial generalized linear model in the integrated analysis to estimate that 20% and 10% of staging red knots were juveniles in 2017 and 2018, respectively. In future applications, this method could provide a metric to assess breeding performance and develop our understanding of its role in population declines. Overall, we estimated that up to 23% of the estimated rufa red knot population staged in southwestern James Bay for an average of 10–12 days. The region is a key staging site for endangered red knots and could be included in conservation planning. © 2021 The Wildlife Society.     

Among‐year site fidelity of Red Knots during migration in Washington

ABSTRACT Many shorebirds exhibit within‐ and among‐year site fidelity during their annual cycle. Little is known, however, about the migration ecology of Red Knots (Calidris canutus) that migrate along the Pacific Flyway and occur in Washington in numbers that exceed counts elsewhere on the flyway. At two large estuaries in coastal Washington, Grays Harbor and Willapa Bay, we searched for and recorded the locations of Red Knots (N= 547) that had been individually marked with leg flags at their wintering grounds in Baja California Sur, Mexico, during the period from October 2006 to April 2009. In 2010, we resighted 43 Red Knots at Grays Harbor and Willapa Bay that had been observed at these sites in previous years, primarily in 2009. We found a high degree of site fidelity between years, with birds observed in 2010 more likely to return to the same stopover site used in 2009 than to switch stopover sites. For knots that did not switch estuaries between years, the median nearest distance between locations where individuals were observed between years was 1.4 km at Grays Harbor and 0.6 km at Willapa Bay. Our results provide the first evidence of stopover site fidelity by Red Knots of the roselaari subspecies. Fidelity occurred at three spatial scales: coastal Washington, the two estuaries where we conducted our study, and specific mudflat areas within the estuaries. Because our study sites support high populations of bivalves, Red Knots may be returning to the same areas in subsequent years to exploit what we suspect is a predictable food resource. The abundance of Red Knots and high degree of site fidelity suggest that our study sites in Grays Harbor and Willapa Bay are important for the conservation of this species on the Pacific Flyway.     

Residence Probability and Population Size of Red Knots During Spring Stopover in the Mid-Atlantic Region of the United States

ABSTRACT Weekly counts of western Atlantic red knots (Calidris canutus rufa) at their Delaware Bay migration stopover site have suggested a major decline since the 1980s. We estimated red knot spring passage population size in the New Jersey Coast-Delaware Bay region (DENJ; 2004 and 2006) and Virginia (VA; 2006 and 2007), USA, by correcting weekly aerial counts for mean daily residence probability between counts in a Monte-Carlo simulation. We used daily telemetry relocations in mark-resight models to estimate mean daily residence probability. Average daily residence probability was approximately 1.0 in mid-May, 0.96–0.97 in the week of 22 May, and 0.64–0.77 after May 28 in DENJ in 2004 and 2006 and in VA in 2006. Average daily residency was approximately 0.88 in VA in 2007 from 22 May to 5 June. No birds moved from VA to DENJ in 2006 and only 2 birds (5.5%) moved in 2007. Stopover population sizes (±SE) in DENJ were 17,108 ± 1,322 in 2004 and 19,555 ± 831 in 2006, and in VA were 7,224 ± 389 in 2006 and 8,332 ± 718 in 2007, significantly greater than peak aerial counts. Years with similar peak counts had different residence probabilities; hence, adjustments for turnover should be used in the future to assess annual population changes. Our results suggest that VA can support a significant portion of this red knot subspecies during migration in at least some years. Managing red knots for recovery should entail improving our understanding of the use of other Atlantic Coast sites and protecting key coastal habitat from disturbance and development.     

渤海湾大型底栖动物调查及与环境因子的相关性

对渤海湾大型底栖动物和环境因子进行调查,利用多元分析技术对大型底栖动物与环境因子之间的关系进行研究。调查期间共发现大型底栖动物45种,平均密度为406个/m2。大型底栖动物数量空间变化明显,呈现从近岸到外海降低的变化特征。运用典范对应分析(CCA)探讨大型底栖动物与环境因子之间的关系,并采用向前引入法对环境因子进行逐步筛选,Monte Carlo置换检验结果显示,水深、无机氮和溶解氧是影响渤海湾大型底栖动物空间分布的关键环境因子。     

莱州湾水母种类多样性及群集结构的季节变化

基于2011年5月至2012年4月(除冰期12月和翌年1—2月外)在渤海莱州湾逐月采集的数据资料,系统地开展水母种类组成、数量分布的周年季节变动,弥补自20世纪90年代以来该区域相关资料的不足,有利于掌握该水域水母(主要是小型水母)主要种类数量时空分布特点,促进莱州湾生物类群对生境改变响应的相关研究。结果显示:所有调查中,共出现水母30种,无全年出现的种类。各月中,莱州湾水母以8月种数最多,5月数量居多。各月水母总数量的80%多由1—2种优势种贡献所致;水母优势种类组成季节更替明显。3—5月优势种为八斑唇腕水母;6月为嵊山秀氏水母和贝氏真囊水母;7月为贝氏真囊水母、9月有细颈和平水母;10月为细颈和平水母和大西洋五角水母,8月和11月无明显的优势种,但8月细颈和平水母和曲膝薮枝螅水母数量较多。莱州湾小型水母聚集结构主要呈现季节性时间格局上的变动。春季有八斑唇腕水母为代表、在湾内分布的聚集组;夏季有以嵊山秀氏水母和贝氏真囊水母为代表、在湾口西侧和湾中部分布的聚集组,以及细颈和平水母为代表、在湾中底部分布的聚集组;秋季有四枝管水母和大西洋五角水母为代表、在湾口和中部分布的聚集组。水温对小型水母聚集结构分布具有较为重要的影响作用。大型水母在本调查出现较少。海蜇和沙海蜇出现于6—8月个别站位;海月水母7—10月均有出现,但高密集区出现在10月紧邻莱州湾的湾底水域。     

渤海湾浮游纤毛虫丰度和生物量的周年变化

为了解渤海湾浮游纤毛虫丰度、生物量和种类组成的周年变化,于2019年7月-2020年6月在渤海湾一个站位进行每月1次共12个航次浮游纤毛虫样品的采集。样品按照Utermöhl方法进行分析,通过倒置显微镜镜检,计算纤毛虫的丰度和生物量。无壳纤毛虫和砂壳纤毛虫出现峰值的时间不同,无壳纤毛虫丰度和生物量均在4月和8月呈现双峰值,砂壳纤毛虫丰度和生物量均在7月出现单峰值。周年砂壳纤毛虫丰度占纤毛虫总丰度的比例平均为(28.6±32.6)%,5-7月较高,均超过50%。共鉴定砂壳纤毛虫6属21种,其中拟铃虫属(Tintinnopsis)种类最多,6-8月砂壳纤毛虫种类数最高。砂壳纤毛虫种类组成呈现明显的周年变化,温度是驱动砂壳纤毛虫群落周年变化的主要环境因子。砂壳纤毛虫群落Shannon指数的平均值为0.95±0.78,Pielou指数的平均值为0.52±0.34,均在6-8月较高。除无壳纤毛虫外,砂壳纤毛虫丰度、纤毛虫总丰度和砂壳纤毛虫丰度占纤毛虫总丰度的比例均与温度、Chl a浓度呈显著的正相关,与盐度呈显著的负相关。     

Sufficiency of horseshoe crab eggs for red knots during spring migration stopover in Delaware Bay USA

Horseshoe crab (Limulus polyphemus) eggs are a dietary staple of the red knot (Calidris canutus) during its spring stopover on the Delaware Bay. Numbers of knots stopping in Delaware Bay declined in the 1990s concurrent with a decline in horseshoe crabs, leading to the hypothesis that reduced horseshoe crab egg abundance limited the red knot population. Management efforts, including a seasonal harvest moratorium in the Delaware Bay, have been instituted to restore crab populations to levels of sustainable use by multiple users, including migratory birds. Our objective was to evaluate the sufficiency of horseshoe crab eggs in Delaware Bay in May–June 2004 and 2005 for knots to refuel for their migratory flight to the Arctic breeding grounds. We examined egg counts to determine if there were fewer high egg-density sites later than earlier in the day and season, as migrating birds might deplete this resource. We studied foraging rates at red knot locations to determine if foraging probes increased with time of day and season as birds depleted surface eggs by pecking, then began probing for subsurface eggs. Finally, we experimentally tested whether red knots and their competitors depleted horseshoe crab eggs. Crab egg numbers at knot foraging sites did not decline throughout the day or season in 2004. In both years, we found no evidence that knots switched from pecking to probing with increases in time since sunrise or start of the stopover. Egg numbers were similar in exclosed and accessible plots on crab nesting depressions and in areas of open intertidal zone, but were significantly lower in accessible than in exclosed plots in the wrack line. Our results indicate that horseshoe crab eggs in Delaware Bay were sufficient to support the refueling of the present-day stopover population of red knots. If an increase in the availability of crab eggs to foraging birds does not result in an increase in knot numbers, managers must prioritize mitigation of limiting factors at other historically important spring stopovers and on the poorly understood breeding and wintering grounds in addition to the Delaware Bay. © 2011 The Wildlife Society.     

Risk Assessment of Rotavirus Infection in Surface Seawater from Bohai Bay,China

Dose-response data indicate that rotavirus (RV) may be one of the more infective agents among enteric viruses. The major limitation at present in the assessment of infection from rotavirus is lack of quantitative data on viral infectivity. In this work, an integrated cell culture and real-time quantitative polymerase chain reaction (ICC-qPCR) method and a Beta-Poisson model for risk assessment were employed. A set of 28 surface seawater samples was collected from December 2010 to September 2011 in Bohai Bay, China, to enable a seasonal risk assessment of infective RV at recreational beaches. Thirty-two percent of the samples were positive for rotavirus, and the estimated concentration range of infectious human rotavirus was 1 to 279 PFU/L. We further confirmed that the contamination of seawater with rotavirus was higher in autumn and winter, which was in reasonable agreement with the trend observed in a prior epidemiological study. Our preliminary risk assessment indicated the daily risk of illness at almost all the contaminated sites exceeded an acceptable threshold of marine recreational water quality (19 illnesses per 1000 swimmers). The detection method and dose-response model in the current work appear useful for evaluating pathogenic risks of seawater to vacationers and can inform management actions.     

初夏渤海湾初级生产力分布特征及影响因素

根据2016年初夏在渤海湾周边重要经济开发区外海域所进行的调查和研究数据进行分析,利用稳定同位素¹³C示踪技术估算海域内初级生产力（Primary Productivity,PP）等,同时结合叶绿素a（Chlorophyll-a,Chl a）,营养盐以及透明度等水文环境参数,深入分析和探讨初夏渤海湾环境特征对浮游植物生物量（Chl a）和初级生产力的影响。结果表明：受陆地径流和渤海中部冷流输入等因素的影响,调查海域呈现3个温盐特征差异显著的海区,即近岸高温低盐海区、中部高温高盐海区和湾口低温高盐海区。Chl a受温度和营养盐等因素影响整体呈现近岸高湾口低、表层高底层低的分布特征,含量变化范围为1.27-20.82 mg/m³;在近岸营养盐含量充足,温度适宜,浮游植物生产旺盛,Chl a平均含量达（8.37±2.90）mg/m³,其中表层近27.5%水样中含量超10 mg/m³,存在发生赤潮的风险。而在中部和湾口区域受营养盐限制和温度的影响,Chl a含量远低于近岸。PP整体水平在44.79-792.73 mg C m^-3 d^-1之间,平均为（144.13±137.79）mg C m^-3 d^-1。其中近岸营养盐含量充足浮游植物生长旺盛,初级生产力水平较高,而中部和湾口海域受营养盐和温度的限制,初级生产力水平较低。初级生产力指数I（同化系数）变化范围在0.79-5.90 mg C/（mg Chl a·h）之间,平均为（3.40±1.33）mg C/（mg Chl a·h）。利用标准深度积分模型对水柱初级生产力（Depth-integrated primary productivity,∑PP）进行估算,结果表明其范围在56.88-772.31 mg C m^-2 d^-1之间,平均为（232.26±126.47）mg C m^-2 d^-1,近岸受陆源输入影响Chl a较高,在天津海河口和黄骅市排污河外出现高值点,受透明度的影响,中部和湾口部分站点出现高生产力区。     

Niche dynamics of shorebirds in Delaware Bay: Foraging behavior,habitat choice and migration timing

Niche differentiation through resource partitioning is seen as one of the most important mechanisms of diversity maintenance contributing to stable coexistence of different species within communities. In this study, I examined whether four species of migrating shorebirds, dunlins (Calidris alpina), semipalmated sandpipers (Calidris pusilla), least sandpipers (Calidris minutilla) and short-billed dowitchers (Limnodromus griseus), segregate by time of passage, habitat use and foraging behavior at their major stopover in Delaware Bay during spring migration. I tested the prediction that most of the separation between morphologically similar species will be achieved by differential migration timing. Despite the high level of overlap along observed niche dimensions, this study demonstrates a certain level of ecological separation between migrating shorebirds. The results of analyses suggest that differential timing of spring migration might be the most important dimension along which shorebird species segregate while at stopover in Delaware Bay. Besides differences in time of passage, species exhibited differences in habitat use, particularly least sandpipers that foraged in vegetated areas of tidal marshes more frequently than other species, as well as short-billed dowitchers that foraged in deeper water more often than small sandpipers did. Partitioning along foraging techniques was less prominent than segregation along temporal or microhabitat dimensions. Such ranking of niche dimensions emphasizes significance of temporal segregation of migratory species – separation of species by time of passage may reduce the opportunity for interspecific aggressive encounters, which in turn can have positive effects on birds' time and energy budget during stopover period.