This may imply that in rapid growth situations it is these two levels that require the largest increases in investment. A stronger but more tentative hypothesis is that there is a positive correlation between rates of growth and rates of return to both primary and university schooling. rapid economic growth in the 10-year period preceding the estimates-appear to have higher rates of return to primary and university students than do Chile and Colombia. The comparisons show that, although patterns of rates vary, Mexico and Venezuela-which experienced. The article presents a summary of the author's own results for Mexico and compares these results with the rates derived for the other three economies. from AuthorsĮstimates of the rate of return to schooling, by level, have now been made for four Latin American countries: Chile, Colombia, Mexico, and Venezuela. Although surface mining is providing new nesting habitat, inadequate reproductive success suggests that these man-made grasslands may not be of benefit to the sparrow populations.

Environmental cues are sufficient to elicit a settling response, but, due to predation, reproductive success is low. Adults are not replacing themselves and immigration is necessary to maintain a stable population. Variable nesting success reflects differential predation pressure. Northern black racers Coluber constrictor constrictor and common crows Corvus brachyrhynchos were the major predators. The study site, surrounded by woodlots and pastureland, concentrated predators. Predation was the major cause of egg and nestling losses. Only vesper sparrows showed significant annual differences in clutch size. Effects of edge predators in pastures are likely to extend beyond the 50 m suggested by other grassland passerine studies.Įxamines the reproductive success of grasshopper sparrows Ammodramus savannarum, savannah sparrows Passerculus sandwichensis, vesper sparrows Pooecetes gramineus and field sparrows Spizella pusilla. Predators known to prefer wooded edges traveled up to 190 m into pastures and up to 150 m from wooded areas. Those species usually depredated nests located closer to wooded edges than to any other type of edge, but there was no evidence that those species restricted their movements to depredate nests within a certain distance from wooded areas in the landscape compared to grassland specialist species (P = 0.28). Over one-third of documented predation events were caused by species that prefer wooded edges. Raccoon (Procyon lotor), thirteen-lined ground squirrel (Spermophilus tridecemlineatus), and snakes (Thamnophisspp.

Grassland passerine nests were depredated by at least 11 different species in that system, and the predator community differed from those documented in similar studies in other regions. Predation rate was high in the early incubation stage. There was limited evidence of differences in predation rates between nests with and without cameras. Trampling rates did not differ between nests with and without cameras. Abandonment rates were higher for nests with cameras than for nests without cameras (P = 0.04). In 1998–2000, we deployed cameras at 89 nests of Savannah Sparrows (Passerculus sandwichensis), Grasshopper Sparrows (Ammodramus savannarum), Song Sparrows (Melospiza melodia), meadowlarks (Sturnellaspp.), and Bobolinks (Dolichonyx oryzivorus) in southwestern Wisconsin pastures 16–169 ha in size. The purpose of our study was to evaluate the effects of using miniature video camera systems to document predation events, identify grassland passerine nest predators in grazed pastures, and determine whether predation patterns of nest predators known to prefer wooded edges differed from those of other nest predators. Studies that simultaneously document the nest predator community and associate predator species with edges created by fragmentation have not been conducted for grassland habitats.

Fragmentation of grassland habitat may increase predation rates on grassland passerine nests and contribute to population decline of several species.

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