Abstract
The formation of multi-species feeding flocks (MSFFs) through visual recruitment is considered an important strategy for obtaining food in seabirds and its functionality has been ascribed to enhanced foraging efficiency. Its use has been demonstrated in much of the world's oceans and includes numerous species. However, there is scant information on the temporal stability of the composition and abundance of MSFFs as well as the effect of seasonal food availability on their dynamics. Between July 2006 and September 2014, we conducted monthly at-sea seabird counts at Valparaiso Bay (32°56′ to 33°01′S, 71°36′ to 71°46′W) within the area of influence of the Humboldt Current in central Chile. This area is characterized by a marked seasonality in primary and secondary production associated with upwelling, mainly during austral spring-summer. Based on studies that provide evidence that flocking is most frequent when food is both scarce and patchy, we hypothesized that seabird MSFF attributes (i.e. frequency of occurrence, abundance and composition) will be modified according to the seasonal availability of food. Using generalized linear models (GLMs), our results show that the contrasting seasonality in food availability of the study area (using chlorophyll-a concentration as a proxy) had no significant influence on MSFF attributes, sparsely explaining their variations (P>0.05). Rather than seasonal food availability, the observed pattern for MSFF attributes at Valparaiso Bay suggests a substantial influence of reproductive and migratory (boreal and austral migrants) habits of birds that modulates MSFF dynamics consistently throughout the whole year in this highly variable and patchy environment. We highlight the importance of visual recruitment as a mechanism by which migratory and resident birds interact. This would allow them to reduce resource unpredictability, which in turn has a major impact on structuring seabird’s MSFF dynamics.
Highlights
To obtain information on certain quality traits of the environment, animals usually rely on the presence or behavior of other individuals [1,2,3]
Neither chlorophyll-a concentration nor its interaction with seasonality had a significant effect on the variation of both multi-species feeding flocks (MSFFs) attributes (P>0.05; Table 3)
Our results showed that Valparaiso Bay presented a pronounced seasonality in primary production, with highest concentrations of chlorophyll-a in spring-summer and low concentrations of chlorophyll-a in autumn-winter seasons
Summary
To obtain information on certain quality traits of the environment, animals usually rely on the presence or behavior of other individuals [1,2,3]. Seabirds are predators that usually do not face high risk of predation at sea [7], so MSFFs function has been associated mostly to foraging efficiency [8]. A major mechanism used by seabirds to find their prey is to observe the behavior of other birds, a strategy called local enhancement [8,9,10,11,12]. This mechanism assumes that it is easier to detect other birds feeding than directly detecting prey (in the case of seabirds, usually fish, crustaceans and cephalopods [13]). Detectability of prey is determined by the species role and may include initiators or catalysts (e.g. gulls), joiners (e.g. shearwaters), divers (e.g. cormorants) and kleptoparasites (e.g. jaegers [8,15])
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