Because they have to keep track of a lot of rapidly-changing information:
BN: How do Bay Area species like scrub-jays fit into the bird intelligence picture?
VP: First of all, the main premise for me was not just intelligence: it was survival skills like memory, which of course brings up intelligence. Scrub-jays in particular have been considered not to be that good at food caching because they live in mild climates, like here in Davis, in the Central Valley [where there’s less of a need to store large quantities of food in anticipation of a long, cold winter]. So, traditionally people thought that scrub-jays didn’t have very good memories. There was a lot of research trying to prove that scrub-jays do not have a very good memory compared to Clark’s nutcracker, for example. But what is interesting is that scrub-jays don’t cache intensely once a year; rather, they cache every day throughout the year. They also cache different kinds of foods; they cache acorns, yes, but they also cache insects, lizards, all different types of food. Potentially, this is very interesting because you’re caching perishable and nonperishable [food items] all over the place, overlapping in different areas, and some will last longer and some not. So scrub-jays have to actually remember not only where they’ve cached but what they’ve cached and when they’ve cached it. They keep track of time and content. Because if you cache an insect it will spoil within two or three days, while an acorn can last for a few weeks. If you don’t remember, it’s going to be a waste of energy; if you’re there two days late, it’s useless.