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  • We conducted the study in the Yancheng National Natural Reserve for Coastal Rare Birds (32°36′–34°28′ N, 119°51′–121°5′ E), which is located on the coastline of central Jiangsu Province, China. The reserve is divided into three zones of differing conservation priorities, including a core zone of 219 km2, with highest conservation priorities, a buffer zone of 557 km2, with medium priority, and an experimental zone of 2066 km2, with lower priority. In the buffer zone, human intrusions can be frequent as farmers plant crops and people visit the reserve. In contrast, the core area has restrictions on the number of visitors and farming is not allowed, thus reducing the level of disturbance caused by human activities in this zone. The study was conducted in the core and buffer zones from the winter of 2009 to the spring of 2011. We focused on two winter migratory crane species in the reserve: red-crowned crane and common crane. The red crowned crane, which is listed as a First-Grade State Protection animal in China and as an Endangered Species in IUCN Red List, usually gathers in small family flocks consisting of two adult birds and between 0–2 juvenile birds. The common crane, listed as a Second-Grade State Protection animal in China and as a species of least concern in IUCN Red List, usually form much larger flocks, but the basic unit is still a family of two adults and between 0–2 juveniles. The red-crowned crane shows a preference for grassland, which is commonly found in the core zone [29], while the common crane usually forages in farmland that occurs in the buffer zone. In these habitats, the two crane species feed on left-over grains or naturally-occurring seeds as well as small insects. When foraging, cranes occur mostly in family units. We have noted very few instances of aggression within crane families but families may interact with one another. Density of birds in red-crowned cranes was quite low and families were often spread out in the habitat. In common cranes, density in agricultural fields can be quite high. To control for density effects, we focused on families that were foraging apart from the others minimising any potential influence of the rest of the flock. We focused on discrete family flocks consisting of two adult cranes and between 0–2 juveniles. In the winter, juvenile cranes are almost adult size but can still be identified through plumage characteristics. We could not distinguish between males and females because they have similar body size and plumage. Crane families were located during regular surveys of the reserve. The same route was not used more than once on the same day to avoid sampling the same flocks. Observations were not carried out on days with rain, snow or strong winds to lessen bias caused by extreme weather. We conducted observations from early morning to late afternoon mostly in January and February. Once a foraging crane family was located, we used focal sampling to record behaviour. Using binoculars or a spotting scope, we determined whether each bird in the flock was vigilant or not every minute for 30 min or until the flock flew away or changed in size. Vigilance refers to a crane stretching the head upwards while standing erect or scanning around. We could keep track of the identity of each bird during sampling given that individuals did not move to a large extent while foraging. Typically, observations were carried 100–300 m away to reduce potential disturbances by observers. We carried out observations during periods where birds were not disturbed by people. In family flocks, an adult bird typically scans once every minute for about 20 s [28], which makes the timing of our sampling procedure reasonable. The percentage of scans with vigilance served as our metric of vigilance and was arcsine square-root transformed prior to statistical analyses. First, we established whether individual vigilance in red-crowned cranes varied as a function of age, zone and family size. In common cranes, zone was not included since most birds foraged in the buffer zone. We used a linear mixed model with flock id as a random factor. We then performed an analysis restricted to the buffer zone to compare vigilance in the two species. Using the sequence of behavioural observations, we established collective vigilance considering adults only since vigilance levels by the juveniles were quite low (see below). Collective vigilance considering adults only was measured as the percentage of scans in which at least one of the two adults was vigilant. For each species separately, we examined whether collective vigilance varied as a function of zone and family size using linear models. We compared observed and predicted collective vigilance separately for each species. The predicted level of collective vigilance was calculated assuming that each individual scans for threats independently of the others. The predicted collective vigilance is given by 100-[(100-P1)*(100-P2)], where P represents the percentage of scans where adult 1 or adult 2 is vigilant. We calculated the contrast between observed and predicted values for each flock. A positive value indicates that individuals tend to coordinate their vigilance bouts (too few sequences where individuals scan at the same time), while a negative value indicates synchronisation (too many instances where individuals scan at the same time). To examine synchrony and coordination between adults and juveniles, we also calculated observed and predicted collective vigilance considering one parent and one juvenile randomly selected from each flock with at least one juvenile. In all cases, we used a paired t-test to determine whether observed and predicted values differed significantly from one another. Sexual differences have been noted in other crane species for vigilance levels [30]. Since we could not identify sex for our focal subjects, the random selection procedure ensured that at the very least we did not choose one sex more often than the other. However, we cannot tell whether patterns of collective vigilance are similar for males and females.
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