Health of tree swallows (Tachycineta bicolor) nesting in pesticide-sprayedapple orchards in Ontario, Canada. II. Sex and thyroid hormone concentrations and testes development

To investigate the effects of pesticides on wild birds, sex (17 beta-estrad iol; testosterone) and thyroid (triiodothyronine (T3) hormone concentration s, body mass, and testes mass were measured and the development of testes w as evaluated in wild tree swallows (Tachycineta bicolor) nesting in four sp rayed apple orchards and three nonsprayed I sites in southern Ontario, Cana da, in 1995-1996. In orchards, birds were exposed to as many as I I individ ual spray events and five sprays of mixtures of chemicals. Residues of orga nochlorine pesticides, PCBs, lead, and arsenic concentrations were low and not variable among sites except p,p'-DDE concentrations, which ranged from 0.36 to 2.23 mu g/g wet weight in eggs. These persistent compounds were not correlated with any endocrine response measured in tree swallows. In 16-d- old male tree swallow chicks, body mass and concentrations of 17 beta-estra diol (estradiol), testosterone, and T3 in plasma showed no significant diff erences between sprayed and nonsprayed groups and among sites within those groups. However, T3 concentrations were slightly elevated in the sprayed gr oup compared to the nonsprayed group, and there was a significant and posit ive correlation between T3 and the number of mixtures of sprays applied dur ing egg incubation through chick rearing. In 16-d-old female chicks, there were no significant differences among spray treatments or sites and no corr elations with spray exposure for testosterone, estradiol, or T3 in plasma. Body mass was correlated positively with T3 and negatively with estradiol b ut showed no differences among spray exposure groups or sites. Histology of testes of 16-d-old male chicks indicated there were no significant differe nces among sprayed and nonsprayed birds in tester mass, area, or diameter, or the presence of Leydig cells in the interstitium, the distribution of th e Sertoli cells, or the occurrence of heterophils in the testicular interst itium. For the percentage of spermatogonia present on the basement membrane , there were significant differences among sites, but these differences wer e not specifically associated with spray exposure. However, there was a mar ginally significant trend between increasing occurrence of a disrupted Sert oli cell population on the seminiferous tubular basement membranes as the n umber of mixtures of pesticides sprayed during chick rearing increased. In adult male and female parent tree swallows, there were no differences in ho rmone concentrations between birds from sprayed and nonsprayed sites. Nor w ere there any significant correlations between the concentration of any hor mone and collection date, body mass, or any type of spray exposure for adul ts. The correlations between increasing pesticide exposure and abnormal thy roid hormone and testes development in male chicks indicate that further re ductions of pesticide use in orchards may benefit the health of birds that nest there. However, it is unclear which of these pesticides or spray mixtu res are responsible for these effects, and this needs to be examined in fut ure studies.