Data from: Context-dependent seed dispersal by a scatter-hoarding corvid

1. Corvids (crows, jays, magpies and nutcrackers) are important dispersers
of large-seeded plants. Studies on captive or supplemented birds suggest
that they flexibly adjust their scatter-hoarding behaviour to the context
of social dynamics and relative seed availability. Because many
corvid-dispersed trees show high annual variation in seed production,
context-dependent foraging can have strong effects on natural corvid
scatter-hoarding behaviour. 2. We investigated how seed availability and
social dynamics affected scatter-hoarding in the island scrub jays
(Aphelocoma insularis). We quantified rates of scatter-hoarding behaviour
and territorial defence of 26 colour-marked birds over a three-year period
with variable acorn crops. 3. We tested whether caching parameters were
correlated with variation in annual seed production of oaks as predicted
by the predator dispersal hypothesis, which states that caching rates and
distances should vary with seed abundance in ways that benefit tree
fitness. We also tested whether antagonistic interactions with
conspecifics would affect scatter-hoarding adversely, as found in
experimental studies. 4. Caching behaviour varied with acorn availability.
Caching distances correlated positively with annual acorn crop size,
increasing by as much as 40% between years. Caching rates declined over
time in years with small acorn crops, but increased when crops were large.
Acorn foraging and caching rates were also negatively correlated with
rates of territorial aggression. Overall foraging rates, however, were not
associated with aggression, suggesting that reduced dispersal rates were
not simply due to time constraints. 5. Our field results support
laboratory findings that caching rates and distances by scatter-hoarding
corvids are context-dependent. Furthermore, our results are consistent
with predictions of the predator dispersal hypothesis and suggest that
large seed crops and social interactions among scatter-hoarders affect
dispersal benefits for oaks and other masting tree species.