Why do the young of cooperative breeders--species in which more than two individuals help raise offspring at a single nest--delay dispersal and live in groups? Answering this deceptively simple question involves examining the costs and benefits of three alternative strategies: (1) dispersal and attempting to breed, (2) dispersal and floating, and (3) delayed dispersal and helping. If, all other things being equal, the fitness of individuals that delay dispersal is greater than the fitness of individuals that disperse and breed on their own, intrinsic benefits are paramount to the current maintenance of delayed dispersal. Intrinsic benefits are directly due to living with others and may include enhanced foraging efficiency and reduced susceptibility to predation. However, if individuals that disperse and attempt to breed in high-quality habitat achieve the highest fitness, extrinsic constraints on the ability of offspring to obtain such high-quality breeding opportunities force offspring to either delay dispersal or float. The relevant constraint to independent reproduction has frequently been termed habitat saturation. This concept, of itself, fails to explain the evolution of delayed dispersal. Instead, we propose the delayed-dispersal threshold model as a guide for organizing and evaluating the ecological factors potentially responsible for this phenomenon. We identify five parameters critical to the probability of delayed dispersal: relative population density, the fitness differential between early dispersal/breeding and delayed dispersal, the observed or hypothetical fitness of floaters, the distribution of territory quality, and spatiotemporal environmental variability. A key conclusion from the model is that no one factor by itself causes delayed dispersal and cooperative breeding. However, a difference in the dispersal patterns between two closely related species or populations (or between individuals in the same population in different years) may be attributable to one or a small set of factors. Much remains to be done to pinpoint the relative importance of different ecological factors in promoting delayed dispersal. This is underscored by our current inability to explain satisfactorily several patterns including the relative significance of floating, geographic biases in the incidence of cooperative breeding, sexual asymmetries in delayed dispersal, the relationship between delayed dispersal leading to helping behavior and cooperative polygamy, and the rarity of the co-occurrence of helpers and floaters within the same population. Advances in this field remain to be made along several fronts.(ABSTRACT TRUNCATED AT 400 WORDS)

We measured acorn production by individual oaks of five different species at Hastings Reservation in central coastal California between 1980 and 1991. Variation in acorn production was considerable both within and among years and was generally uncorrelated between species. Compared to expected values, variance within years in the size of acorn crops was small, while variance among years was high. Crop failures occurred fairly frequently and large crops in successive years were observed, but not more than expected by chance. Individual trees masted at species—specific intervals, but these patterns did not result in regular masting cycles at the production level. We compared these patterns to predictions of four hypotheses for the evolution of seed production patterns. Observations did not support the hypotheses that production patterns track resource availability (the ”resource matching” hypothesis) or that they have evolved to attract seed dispersers (the “seed dispersal” hypothesis). However, they are generally consistent with two additional hypotheses, that masting in these wind—pollinated species evolved because of a proportional increase in fertilization and seed set during mast years (the “wind pollination” hypothesis) and that masting has evolved to maximize the probability of avoiding predation via predator satiation (the “predator satiation” hypothesis).

Mean annual acorn crop size of oaks in central coastal California correlated with weather for four of five species. For the deciduous Quercuslobata and Quercusdouglasii, larger crops were produced in years when April temperatures were warmest, while for the evergreen Quercusagrifalia and Quercuschrysolepis, crops were larger when there was more rain one and two years prior to the year when acorns were produced. No correlations were found between weather and mean annual acorn production by Quercuskelloggii. Within years, individual Q. douglasii that flowered during periods of greater solar radiation produced more acorns, and during 1 of 5 years individual Q. lobata and Q. agrifolia produced larger crops when they flowered during warm, dry periods. These results indicate that conditions favorable for pollination and fertilization strongly enhance mean annual acorn production in Q. lobata and Q. douglasii and may have an effect on differences in acorn crop size among individual Q. lobata, Q. douglasii, and Q. agrifolia. Mean annual acorn crops of these three species were significantly concordant among three sites in central coastal California 300 km apart. Weather conditions were also significantly correlated, especially annual rainfall and mean April temperatures. Thus, the environmental factors that have the strongest influence on mean annual acorn production within a site are also those that are the most geographically concordant. However, we found no relationship between environmental factors and differences in mean annual acorn production by these same three species at the three sites.

We describe a visual survey technique for evaluating acorn production. In contrast with previously proposed methods, our technique yields ratio-level data on annual productivity that are analyzable with standard statistics and, by sampling the same trees each year, data on the reproductive patterns of individual trees. We compared this technique with two independent sets of acorn-trap data acquired on oaks of three species at Hastings Reservation in central coastal California. Correlations between acorns counted by the visual surveys and collected from acorn traps under the same trees were significant for all three species. Most scatter in the data appeared to be attributable to three causes: (1) sampling error, especially among trees with very small crops, (2) finite counting speed, leading to a lack of discrimination among trees with very large crops by the visual surveys, and (3) arboreal acorn removal by animals. This latter factor can be particularly large, rendering visual surveys more reliable than the use of traps. Furthermore, only the high efficiency of visual surveys allows for the practical assessment of samples large enough to accommodate high within-population variation and detect widespread geographic variation in acorn production. Visual surveys offer a method of assessing the fruit or cone crops of many hardwood and conifer species that is not only more efficient but also more accurate than the use of traps.

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