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.

1. 鸦科鸟类（Corvids，包括乌鸦、松鸦、鹊与星鸦）是大型种子植物的重要传播媒介。针对圈养或经人工补给食物的鸟类的研究表明，它们可根据社会动态与种子相对可获得性的情境灵活调整分散贮食（scatter-hoarding）行为。由于多数由鸦科传播的树木的种子产量存在显著年际波动，依赖情境的觅食行为会对自然状态下鸦科鸟类的分散贮食行为产生强烈影响。
2. 本研究以岛丛松鸦（Aphelocoma insularis）为研究对象，探究了种子可获得性与社会动态如何影响其分散贮食行为。我们在为期三年、橡子结实量存在差异的研究周期内，对26只带有彩色标记的个体的分散贮食行为速率与领地防御行为进行了量化分析。
3. 我们检验了贮食参数是否如捕食者传播假说（predator dispersal hypothesis）所预测的那样，与橡树的年度种子产量变化相关——该假说提出，贮食速率与贮食距离会随种子丰度发生适应性变化，以提升树木的适合度。我们同时验证了同种个体间的对抗性互动是否会如已有实验研究所示，对分散贮食行为产生不利影响。
4. 研究结果显示，贮食行为随橡子可获得性发生变化：贮食距离与年度橡子结实量呈正相关，不同年份间的贮食距离最大可增加40%；在橡子结实量较少的年份，贮食速率随时间推移逐渐下降，而在结实量较大的年份，贮食速率则呈上升趋势。橡子觅食与贮食速率同时与领地攻击行为速率呈负相关。但整体觅食速率并未与攻击行为产生关联，这表明贮食速率的降低并非单纯由时间限制所致。
5. 本项野外研究结果支持了实验室研究结论，即分散贮食鸦科鸟类的贮食速率与贮食距离均具有情境依赖性。此外，我们的研究结果与捕食者传播假说的预测相符，同时表明大型种子结实量以及分散贮食者之间的社会互动，会对橡树及其他大年结实树种的传播效益产生影响。