Abstract
Animals acquire information produced by other species to reduce uncertainty and avoid predators. Mixed‐species flocks (MSFs) of birds are ubiquitous in forest ecosystems and structured, in part, around interspecific information transfer, with “nuclear” species providing information that other species eavesdrop on. We hypothesized that in a seasonal tropical forest, the amount of information produced by birds about predation would be dynamic and particularly would decrease inside MSFs when the nuclear species leave MSFs to breed. We obtained baseline information on MSF encounter rate and species composition along established sampling routes over 9 months near the Sino‐Vietnamese border. We also conducted three experiments to quantify information produced by different species in response to typical predator encounters, including a moving predator stimulus presented inside of MSFs, and a stationary predator model presented both inside and outside of MSFs. MSFs were much less frequent in the breeding season with fewer individuals of the nuclear species, David's Fulvetta (Alcippe davidi), participating, though the diversity of other species remained stable. Fulvettas were the dominant producer of alarm‐related information both to the moving and stationary stimuli in MSFs and were also among the most active mobbers to stimuli presented outside of MSFs. In the breeding season, they tended to call less to the moving stimulus, and substantially fewer individuals responded to the in‐flock stationary stimulus. Other species increased their own information production at stationary predator stimuli (inside and outside of MSFs) during the breeding season, perhaps due to their increased investment in offspring during this time. Yet even during the breeding season, David's Fulvetta remained the highest producer of information about predators in MSFs. Hence, while we show that information production in MSFs can be somewhat dynamic, we describe a continually asymmetric communication system, in which a nuclear species is important to the whole community.
Highlights
Social information is often obtained from conspecifics, information from heterospecifics can increase animals’ fitness, and under some circumstances, it may be as or more valuable than that obtained from conspecifics (Seppänen, Forsman, Mönkkönen, & Thomson, 2007; Sridhar & Guttal, 2018)
While many signals may often be directed toward conspecifics, nuclear species may sometimes direct their signals toward heterospecifics, such as when alarm calls could provoke a scattering response that would be confusing to a predator (Charnov & Krebs, 1975), when recruiting other individuals to a food resource to dilute predation risk without greatly increasing competition (Farine et al, 2015; Hillemann, Cole, Keen, Sheldon, & Farine, 2019; Seppänen et al, 2007), or manipulating other species (Flower, Gribble, & Ridley, 2014; Goodale & Kotagama, 2006)
We predicted that: (a) Mixed-species flocks of birds (MSFs) encounter rate would decrease in the breeding season, with the participation of nuclear species decreasing as it engages in breeding activities, (b) behavioral responses to predator stimuli would be dominated by nuclear species both inside and outside of MSF, (c) nuclear species would decrease the information provision during the breeding season in MSF as they participate less in MSF, and (d) other species would increase their information production in the breeding season, either in compensation from the loss of information from the nuclear species, or because of their own investment in offspring
Summary
Animals acquire information produced by other individuals to reduce uncertainty and avoid predators (Dall, Giraldeau, Olsson, McNamara, & Stephens, 2005; Danchin, Giraldeau, Valone, & Wagner, 2004; Schmidt, Dall, & van Gils, 2010). The importance of nuclear species may extend well beyond MSF systems ( see Mönkkönen & Forsman, 2002), especially among species (birds and mammals) that readily utilize heterospecific information produced vocally by nuclear species (Jones & Sieving, 2019; Schmidt, Lee, Ostfeld, & Sieving, 2008) Both MSF systems and mobbing assemblages can be strongly seasonal. To simulate a surprise attack by a flying hawk (generating alarm calls) inside of MSF, we used a moving object (Goodale & Kotagama, 2005) and we stimulated mobbing responses with a small owl model (Hua & Sieving, 2016), presented both inside and outside of MSFs. We hypothesized that while the nuclear species would serve as the central information producer of MSFs, the marked seasonality of our system could cause fluctuations in both flocking and in the availability of social information. We predicted that: (a) MSF encounter rate would decrease in the breeding season, with the participation of nuclear species decreasing as it engages in breeding activities, (b) behavioral responses to predator stimuli would be dominated by nuclear species both inside and outside of MSF, (c) nuclear species would decrease the information provision during the breeding season in MSF as they participate less in MSF, and (d) other species would increase their information production in the breeding season, either in compensation from the loss of information from the nuclear species, or because of their own investment in offspring
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