Weight Recession in Nestling Birds

IT has been noted in many species of birds that the weight of growing young increases to a peak above normal adult weight and then decreases before fledging.This phenomenon of weight recession (Edson, 1930) has been attributed to various causes (e.g., drying out of the feathers, high energy demands with rapid feather growth, starvation periods, decrease in the size of the digestive organs) but little evidence has been sought to support any of these hypotheses.This paper analyzes in detail the phenomenon of weight recession in nestling Barn Swallows (Hirundo rustica) and presents a general survey of the literature to determine the distribution of the phenomenon among avian groups. MATERIALS AND METHODSDuring the latter part of their 20-day nestling period the weight of young Barn Swallows decreases from an average peak of 20.5 g to 17.0 g (Stoner, 1935), a loss of 3.5 g. Adult weights in this species average about 18 g.To determine the nature of this weight change, a series of nestlings was collected in the vicinity of Philadelphia, Pennsylvania, and analyzed for water, lipid, and lean dry constituents.The 38 specimens ra•ged from 3 to 19 days in age and, with some variation, bore the same ageweight relationship described by Stoner.Ages were determined primarily from average growth curves of linear measurements, especially wing length, although nests were visited at least every other day.This method usually gives a more accurate estimate of a nestling's age than can be obtained from daily visits to the •nest.Thirty of the specimens were analyzed whole with the stomach contents removed, but the remaining 8, aged 9 to 19 days, were dissected into integument (skin and feathers), pectoral muscle, heart, liver, and stomach components which were analyzed separately.The whole nestlings and the components were dried in a vacuum desiccator and then extracted in a 5:1 mixture of petroleum ether (30-60øC) and chloroform to determine water, lean dry, and lipid constituents.All weights were measured on an analytical balance accurate to 1 mg. RESULTSThe data on the constituents of whole birds (Figure 1) show that weight recession in nestling Barn Swallows can be accounted for entirely by a decrease in water content.The lean dry constituent exhibits no change after 12 days, when it levels off at about 4.8 g.Lipids vary considerably during the latter half of the nestling period but no decreasing trend is evident in the sample analyzed.