Although various mysteries still shroud our knowledge of the social behavior of this species in North America (139), by far the largest gap in our knowledge concerns its behavior and ecology outside the United States. A recent review of work on populations outside the U.S. found a reduction in the importance of acorn storage associated with a greater variety of foraging behaviors, but no change in social organization (66). Recent molecular work on oaks indicates that the genus has a high-latitude origin and that diversification leading to the high species diversity seen today in Mexico and Central America came later (171). Thus, it is possible that the unique food storage behavior found in this species evolved in North America and has been subsequently reduced or lost in some more tropical populations. Such findings call into question the relationship between granaries, food storage, and the complex social organization of this species that will only be answered with future studies on non-food-storing populations.
At Hastings Reservation, Acorn Woodpeckers eat and store acorns of all oak species but are particularly dependent on blue oak (Quercus douglasii) and valley oak (Q. lobata) for granaries and nesting cavities. These are two oak species that appear to be regenerating poorly throughout much of their range in California (172, 173); the implication of this problem for Acorn Woodpeckers and other oak-dependent California birds over the long term is difficult to assess. If populations of these oak species decline, the degree to which Acorn Woodpeckers will be able to switch to other trees like coast live oak (Q. agrifolia) or California sycamore (Platanus racemosa) remains to be determined.
Acorn Woodpeckers remain a model, if difficult to study, system for testing several basic concepts in behavioral ecology dependent on highly complex social structure. The fact that cobreeding males are generally close relatives in California, but possibly not in the southwestern U.S., opens possibilities for comparative work on the role of kinship to various forms of social behavior; whereas the diversity in mating group composition, both within and between populations, offers the opportunity to compare the costs and benefits of group living in ways that have as yet gone unexploited. Incest avoidance raises questions about kin recognition and the costs and benefits of inbreeding, and new molecular methods will allow detailed analysis of the genetic relationships within and among social groups that have thus far often been inferred indirectly. Finally, the ecological factors leading to mate sharing and joint nesting remain unresolved, although a recent study supports the hypothesis that joint nesting by females at Hastings Reservation is driven by habitat saturation (107), as is, at least to a large extent, delayed dispersal and helping behavior in the same population (94, 174).
Other questions concerning the ecological factors leading to cooperative breeding in this species are unlikely to be resolved until detailed comparative studies have been performed on other Melanerpes, including species that are, like the Acorn Woodpecker, cooperative breeders [White-fronted Woodpecker (M. cactorum), White Woodpecker (M. candidus), Golden-naped Woodpecker (M. chrysauchen), Yellow-tufted Woodpecker (M. cruentatus), Yellow-fronted Woodpecker (M. flavifrons), Gray-breasted Woodpecker (M. hypopolius), and Hispaniolan Woodpecker (M. striatus)] and those that are not.