Breeding Biology of Trees Swallows and House Wrens in a Gradient of Gamma Radiation

Abstract

In a gradient of gamma radiation ranging from 38.7 mC°kg—1°d—1 to background levels of 0.05 μC°kg—1°d—1, Tree Swallows (Iridoprocne bicolor) and House Wrens (Troglodytes aedon) avoided nesting in areas of high radiation. Nest boxes selected by swallows and wrens had a mean exposure rate of only 9.3 and 6.6 μC°kg—1°d—1, respectively. Lateral and canopy vegetation indices and nest hole height and direction could not explain the observed pattern of box selection. Of the boxes with low exposure rates, swallows selected those with little vegetation cover, whereas wrens chose boxes with heavy cover. It appears that the birds responded to radiation levels as low as 100 times background, but it is not clear whether they actually detected radiation or simply responded to secondary clues. The number of swallows and wrens fledged per box was unrelated to radiation exposure. The same was true for number eggs, hatching success, fledging success, incubation time, and nestling time. Breeding success was reduced because of infertile eggs, eggs with dead embryos, cracked eggs, predation, adverse weather, abandonment, and parasites. The logistic model was ideally suited for describing gains in mass in nestling swallows and wrens. Growth of nestlings was not related to radiation exposure as indicated by the growth rate constant, asymptotic mass, and the proportion of variation explained by the logistic model. Breeding and growth performance were similar in studies not involving experimental radiation. The data show that birds avoided adverse effects of radiation by judicious box selection. However, there were indications that at higher breeding densities birds may use high—exposure boxes, where breeding success or growth of nestlings may become reduced due to radiation.

Effects of ionizing radiation on breeding swallows at current radiation protection standards

“…During the 1991 breeding season, we were unable to locate any returning young. In a previous study [15], a single female acutely exposed to 0.9 Sv returned and laid five fertile eggs that resulted in apparently healthy young. This suggests that in the present experiment, which involved much lower doses, nestlings might not have suffered any detrimental long-term effects.…”

“…The study was carried out in 1990 with 120 nest boxes, randomly divided into four treatments with 30 boxes each. Three treatments corresponded to about 5 , 15, and 45 times the background radiation level. The control was at background level.…”

“…Tree swallows prefer established breeding areas over newly erected grids of boxes [15,19], and as a result there was a larger percentage of boxes with eggs in the old portion of the grid (Table 2). Because the four treatments were represented equally in the 32 24 56 old and new portions, this result does not represent a bias in the settlement, or choice of boxes, by the swallows in relation to treatment.…”

Gamma Radiation Effects on Nestling Tree Swallows

“…In the Gammacell, specimens are lowered into a ringshaped 6°C o source for uniform irradiation. The actual mean exposures for the three irradiated groups were 0.91 ± 0.00, 2.68 ± 0.02, and 4.48 ± 0.03 Gy (n = 40), as determined in separate runs with lithium fluoride thermoluminescent dosimeters (Zach and Mayoh 1982a).…”

“…In many environmental situations, chronic, low-level stress is more insidious than acute, high-level stress. Detailed data are not available on the effects of chronic radiation stress on growth in wild birds, but in 1982 we located a Tree Swallow nest with five eggs chronically exposed to ~ 1.0 Gy/d throughout the nestling period (Zach and Mayoh 1982a). The body-mass growth curve for the four young that fledged falls well below that for the nestlings acutely exposed to 4.5 Gy, suggesting that chronic radiation is more stressful (Fig.…”