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The biology of Field Sparrow nestlings was studied during two breeding seasons in central Illinois, on a 23.6-ha tract composed of mixed vegetation. Predominant nestling foods were lepidopteran larvae, orthopteran nymphs, homopterans, and spiders. Foods given to nestlings ranged in size from less than 0.005 cc to 0.38 cc, the upper size limit increasing with nestling age. The food size class most frequently fed to the young contained items smaller than 0.05 cc, although its importance decreased as the nestlings grew older. Girth rather than length limited the size of food the young successfully ingested. The number of feeding trips per hour to 6-day-old broods ranged from 0 to 21 and averaged 9, divided about equally between male and female. The feeding frequency peaked shortly after dawn and just before dusk, and was lowest near midday. There was a significant inverse relationship between the female feeding rate and brooding time. Ambient temperature significantly influenced brooding time, with the least brooding at 60° to 69°F. The rate of feeding individual nestlings decreased slightly (nonsignificant) as brood size increased. The overall nestling growth rate index (weight) averaged 0.568, and ranged 0.444 to 0.712. No nestling starvation was evident. The rate of weight gain in broods of four was significantly higher during early summer than late summer, while broods of three showed a significant opposite trend. The dependence of thermoregulatory maintenance costs upon brood size is discussed. Growth rates of the youngest nestlings were significantly less than those of their siblings. It was concluded that Field Sparrows forage opportunistically for their nestlings, and that food availability did not limit breeding success on the study tract.