Much of the research on sublethal, adverse effects of mercury (Hg) has focused on impairment of neurological function and reproduction in fish and fish-eating vertebrates. Here we examined the associations between Hg and endocrine function (adrenocortical responses and plasma thyroid hormone concentrations) of insectivorous tree swallow nestlings adjacent to a Hg-contaminated river and nearby reference rivers in Virginia. Nestlings from the contaminated sites had blood Hg concentrations that exceeded those from the reference sites by more than an order of magnitude (354 ± 22 vs 17 ± 1 ppb wet weight). A regression of age and Hg concentrations suggested dietary Hg at the contaminated sites exceeded the nestlings' capacity to eliminate Hg through deposition into growing feathers. Although blood Hg concentrations among nestlings at the contaminated sites were lower than those typically associated with abnormal behavior or altered physiology in young birds, adrenocortical responses, plasma triiodothyronine, and thyroxin concentrations were suppressed, relative to reference levels, by the end of the nestling period. These results suggest that (1) Hg may disrupt endocrine systems of terrestrial avian young and (2) adverse effects of Hg on endocrine systems may be most evident once endocrine axes are fully developed.
We assessed adult reproductive and nesting success, and developmental instability, survival, and condition of nestling eastern bluebirds (Sialia sialis) breeding in nestboxes on and off golf courses. With this information we tested hypotheses predicting that nestboxes on golf-course habitat provided poor nesting and chick-rearing environments due to pesticide pollution or other factors. Contrary to our prediction, nestboxes on golf courses received 28% more bluebird eggs and produced 17% more fledglings than nestboxes on nongolf sites. Individual nests on golf courses appeared to have slightly reduced probabilities of survival during laying and incubation, but we found no depression in the overall Mayfield survival probabilities. Surprisingly, nestling bluebirds raised in golf-course boxes exhibited 39% less fluctuating asymmetry of their tarsus bones, possibly an indicator that nestlings that survived to fledge had been under less stress during development. This may have resulted from stronger selection against asymmetrical nestlings on golf courses, or it may indicate that golf-course nestboxes provide higher-quality habitat for bluebirds. Our results suggest that for this bird species, nestboxes on the golf-course habitat we studied can provide high-quality rearing environments and may serve as population sources. This may not apply to other species, including most birds of conservation concern, which do not nest in protected artificial cavities or forage directly on turfgrass habitat.
Juvenile birds are generally less efficient foragers than adults, and this can lead to reduced survival or delayed reproduction. Feeding on hard-shelled nuts and mollusks after breaking them open by dropping is particularly challenging. To determine whether juvenile birds are as proficient as adults at this complex foraging task, we compared juvenile and adult Herring Gulls (Larus argentatus) feeding at low tide by dropping clams (Rangia cuneata) at a coastal estuary in Virginia, USA. Juvenile (first-winter) gulls were less successful than adult (2 years old) birds at finding clams to drop. Moreover, the clams they selected were of a wider range of sizes, more often being difficult to handle or of lower nutritional value. Juveniles did, however, drop clams with the same degree of success as adult birds. Together these results suggest that learning to find and recognize profitable clams is the developmental bottleneck in this complex behavior.
Complex and energetically expensive foraging tasks should be shaped by natural selection to be efficient. Many species of birds open hard-shelled prey by dropping the prey repeatedly onto the ground from considerable heights. Urban-dwelling American crows (Corvus brachyrhynchos) forage in this way on two species of walnuts in central California, USA. As predicted from a theoretical model, crows dropped nuts with harder shells from greater heights and dropped them from greater heights when over softer substrates. The height selected for dropping nuts decreased in the presence of numerous nearby conspecifics, indicating that crows were sensitive to the risk of kleptoparasitism when selecting drop heights. Drop height decreased with repeated drops of the same walnut, suggesting that crows adjusted for the increasing likelihood that a repeatedly-dropped nut would break on subsequent drops. Crows did not alter height of drop in accordance with differences in the mass of the prey. When faced with multiple prey types and dropping substrates, and high rates of attempted kleptoparasitism, crows adjusted the height from which they dropped nuts in ways that decreased the likelihood of kleptoparasitism and increased the energy obtained from each nut.
