Why do fleshy-fruit plants of the mediterranean scrub intercept fall-but not spring-passage of seed-dispersing migratory birds?

Production of fleshy fruits by 8 tree and shrub species, and patterns of their utilization by 6 resident and 6 transient bird species, were assessed in an Israeli Mediterranean scrub. Whereas more than half the ripe crop of plants that fruit in one relatively short burst is not removed by birds, the greater part of the crop of species that fruit throughout a long period, is removed. Of two plant species that fruit simultaneously, the one with an inconspicuous fruit is utilized by a single, year-round resident frugivorous bird, whereas the species with bright fruits is utilized by several non-resident and omnivorous bird species. A further two simultaneously fruiting species differ in fat content and the color of their conspicuous ripe fruits; the low-fat fruit being taken by a resident species and the high-fat fruit by a transient congener, just prior to the desert-crossing portion of its Fall-passage.Though more migrants pass through in Spring than in Fall, none of the fleshy-fruit plants fruit in Spring, and most fruit during the Fall. Two non-exclusive explanations for this phenomenon are (a) Fall dispersal ensures immediate germination, with least exposure to desiccation and predation; (b) Spring transients approaching their breeding territories may either be reluctant to spend much time on feeding, or mostly require proteins, amply supplied by the Spring flush of insects, typical of the Mediterranean region. Fall transients approaching the desert require mostly fats and carbohydrates, supplied by fruits. Fruits attract birds easily in the Fall, when insects are scarce.