Physiological, morphological, and behavioural effects of developmental exposure to Aroclor 1254 in nestling and juvenile songbirds

Over the past several decades, there has been growing concern among the public and scientific community regarding adverse health effects resulting from exposure to natural and synthetic compounds that act as endocrine disrupters. The structural similarity of many of these compounds to natural hormones and receptors, as well as their ubiquity in the environment, can result in the potential for interference with the endocrine system of wildlife and humans. Much of the research examining the adverse effects of wildlife exposure to endocrine disrupting chemicals (EDCs) has focused on effects on reproduction or short-term changes in hormone physiology. However, organisms exposed to low levels of EDCs at early life stages could also be susceptible to developmental effects, including neurological and other physiological changes affecting later life stages. In birds, migration can be an important component of the annual life cycle and it can be vulnerable to disruption given that it is under endogenous hormonal and neurological control. Previous studies have shown that developmental exposure of birds to thyroid hormone disruptors, such as polychlorinated biphenyls (PCBs), have resulted in reduced hatching success, lower growth rates, teratogenicity, impaired development, and immunotoxicity. In this thesis, I aimed to supplement what is currently known regarding the effects of developmental exposure to low levels of a mixture of endocrine disrupting chemicals in songbirds, as well as further investigate the latent consequences of such an exposure on migratory life stages. I initially investigated the potential physiological and developmental effects of early exposure to Aroclor 1254, a PCB mixture, in two passerine songbird species: European starlings (Sturnus vulgaris) and red-winged blackbirds (Agelaius phoeniceus) during the critical nestling period. In 2011, European starlings and red-winged blackbirds were orally administered Aroclor 1254 over the period of nestling development, which was repeated in 2012 with only European starlings. For both years, morphological parameters (body mass, tarsus, wing-chord and bill-lengths) were measured daily and plasma thyroid hormones were measured. Additional measurements of wing chord and tarsus length fluctuating asymmetry (FA) were taken in the second year, to further assess contaminant-induced alterations in developmental stability. I found that treatment with environmentally-relevant levels of Aroclor 1254 caused increasing liver residues above the controls but did not result in overt effects on morphological growth parameters during the nestling period in either starlings or red-winged blackbirds. However, we did observe significant differences in 2012 starling’s wing chord FA at day 10 and 13, and tarsus length FA between all treatment groups and controls, indicating the potential for PCB-induced stress. Nestling thyroid hormone profiles (T3) sampled throughout the nestling period supported developmental changes but did not reveal any differences among treatment groups. Starlings were subsequently reared in captivity and further tested during a simulated autumn migration. Migratory activity and orientation were tested using Emlen funnel trials over 6 consecutive weeks. Across treatment groups, we found a significant increase in mass, fat, and feather moult, and decreasing plasma thyroid hormones over time. At 12L:12D, control birds showed a peak in activity and a directional preference for 155.95° (South-southeast), while high-dosed birds did not. High-dosed birds showed a delayed directional preference for 197.48° (South-southwest) under 10L:14D, concomitant with apparent delays in moult. These findings link alterations in avian migratory behaviour to contaminant-specific mechanisms. Exposure to a ubiquitous environmental endocrine disruptor exerted only subtle short-term effects during the period of exposure but importantly, latent effects may be far more relevant for individual fitness. We discuss how the impacts of exposure during early stages of development were not significant for short-term nest success, but can still give rise to longer time-scale effects that are potentially relevant for survival and population stability for migratory birds