The limits of olfactory perception in black vultures: a field experiment

New World vultures have been considerably studied regarding their sense of smell. <i>Cathartes</i> species present a remarkable development of their olfactory apparatus, and experiments conducted with the turkey vulture (<i>Cathartes aura</i>) demonstrated that they can locate carrion exclusively by smell at considerable distances. Black vultures (<i>Coragyps atratus</i>), because of their less developed olfactory apparatus, have been compared with the turkey vulture as a phylogenetically related species lacking the sense of smell. However, little evidence from behavioural studies supports the lack of olfaction of black vultures. We conducted a field experiment where we presented urban black vultures a binary choice of decaying fish and sand (serving as control) inside plastic bags. Birds chose the fish bait in 81% of the times they approached the plastic bags. The probability of reaction decreased significantly with the distance between the birds and the fish bait, being virtually null at 40–50 m away. Furthermore, birds clearly reacted more when the fish presented higher decaying time (7 compared to 2 days). These results show that olfaction is not only used by black vultures but is probably the most relevant sense when they search for food hidden in plastic bags and in piles of unsorted waste, which is the most common foraging environment for this species in urban areas where they are very abundant.

针对新大陆秃鹫（New World vultures）的嗅觉感知，学界已开展了大量研究工作。美洲鹫属（Cathartes）物种的嗅觉器官发育极为完善；针对红头美洲鹫（Cathartes aura）开展的实验证实，它们可在较远的距离范围内仅依靠嗅觉定位腐肉。黑美洲鹫（Coragyps atratus）因嗅觉器官发育相对滞后，曾被归类为与红头美洲鹫亲缘关系相近但缺乏嗅觉的物种。然而，行为学研究中几乎没有证据能支持黑美洲鹫缺乏嗅觉这一论断。本研究针对城市黑美洲鹫开展了一项野外实验：向其提供置于塑料袋内的腐鱼与沙土（作为对照），供其进行二选一选择。在接触塑料袋的个体中，有81%选择了腐鱼诱饵。黑美洲鹫作出反应的概率随其与腐鱼诱饵的距离增加而显著降低，在距离达到40~50米时，反应概率几乎为零。此外，当腐鱼的腐烂时长更长（7天相较于2天）时，黑美洲鹫的反应明显更为强烈。上述研究结果表明，黑美洲鹫不仅具备嗅觉功能，在其觅食藏于塑料袋内或混杂垃圾堆中的食物时——这是该物种在城市高密度栖息环境下最常见的觅食场景——嗅觉或许是最为关键的感知方式。