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).

We examined the demographic consequences of road mortality in the cooperatively breeding Florida Scrub‐Jay (Aphelocoma coerulescens), a threatened species restricted to the oak scrub of peninsular Florida. Between May 1986 and July 1995 we monitored the survival and reproductive success of a color‐banded population of jays along a two‐lane highway at Archbold Biological Station. Annual mortality of breeding adults was 0.38 on road territories, significantly higher than the rate of 0.23 for breeders on nonroad territories. High mortality on road territories appeared to be a direct result of automobile traffic per se and not a consequence of road‐induced changes in habitat characteristics. Mortality was especially high for immigrants without previous experience living along the road: in their first two years as breeders on road territories, naive immigrants experienced annual mortality of 0.50 and 0.45. From year 3 onward, however, annual mortality dropped to 0.29, not significantly different from the rate for birds on nonroad territories. This experience‐dependent decline in road mortality could be caused either by surviving jays learning to avoid automobiles or by selective mortality operating through time (demographic heterogeneity). Proximity to the road had no effect on nesting success beyond its indirect effects on breeder experience and group size. Because the mortality of 30‐ to 90‐day‐old fledglings was significantly higher on road territories than on nonroad territories, however, breeder mortality greatly exceeded production of yearlings on road territories. Roadside territories therefore are sinks that can maintain populations of Florida Scrub‐Jays only via immigration. Because Florida Scrub‐Jays do not avoid roadside habitats and may even be attracted to them, road mortality presents a difficult challenge for the management and conservation of this threatened and declining species.

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