Seasonal changes in mixed-species bird flocks and anti-predator information

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 nine 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.