Health of tree swallows (Tachycineta bicolor) nesting in pesticide-sprayedapple orchards in Ontario, Canada. I. Immunological parameters

The degree of pesticide exposure and its effects on the immune system and i ts development were determined in 16-d-old tree swallow (Tachycineta bicolo r) chicks from 4 sprayed apple orchards and three nonsprayed sites in south ern Ontario, Canada, during 1994-1995. Persistent contaminant residues were measured in tree swallow eggs and in each chick hepatic ethoxyresorufin O- deethylase (EROD) activity; body, immune organ, and liver masses; lymphocyt e blastogenesis response; respiratory burst and phagocytic responses; hemat ological evaluation; and histological development of thymus, bursa of fabri cius, and spleen were determined. Chemicals sprayed on apple orchards were mainly ethylene bisdithiocarbamate and myclobutanil fungicides and organoph osphorus, carbamate, and synthetic pyrethroid insecticides. During the peri od between oviposition of the first egg in each nest to d 16 after hatching , individual nests in orchards were exposed to between 4 and I I individual chemical applications and up to 3 mixtures of pesticide sprays. Concentrat ions of pesticides, polychlorinated biphenyls (PCBs), and lead and arsenic residues in tree swallow eggs and liver were low and not variable among sit es except p,p'-DDE, which was as high as 2.29 mu g/g wet weight in eggs. ER OD activity was not different among sites. Organochlorine and trace metal r esidues and EROD activity were not correlated with any immune parameter. in sprayed birds, we found a significantly increased blastogenic response to pokeweed mitogen (12.5 mu g/ml). However, nests were initiated over a perio d of several weeks and we also found changes in other tree swallow immune p arameters that were related to the date of chick collection. Hematological parameters, bursal and thymic masses, phagocytic response, and thymic devel opment were all correlated with the day the chicks were 16 d of age. After accounting for the collection date of birds from each nest, we found cell p roliferation in the cortex and delayed thymic involution correlated positiv ely with increasing spray exposure. We also found that birds in sprayed orc hards were slightly anemic compared to birds from nonsprayed sites, and the re were smaller bursal masses and an increase in relative heterophil concen trations in the sprayed orchard birds. The local inflammation may have been caused by trematode parasite infections, although pesticide exposure also correlated positively with these parameters. This is the first study of the immunology and effects of current pesticide exposures in wild passerines; therefore it is difficult to predict the longterm consequences of the appar ent stimulated immune systems in sprayed birds. However, some environmental contaminants that overtly stimulate the immune system in mammals have indu ced hypersensitivity and/or autoimmunity. Therefore we speculate that these effects are possible in tree swallows.