Abstract
The thymus of adult mallards and house sparrows of both sexes and of female robins shows a marked enlargement following the annual breeding season. In male robins the thymus begins to enlarge earlier, almost synchronously with the testes. Histologically the enlarged thymus becomes indistinguishable from the uninvoluted thymus of immature birds, showing lobulation and development of a cortex and medulla. In the atrophic thymus found in adults for most of the year these two zones are not recognizable. The bursa Fabricii of adults does not enlarge at any time. Evidence of a similar thymus enlargement in some other species of birds is presented. The distribution of thymus tissue in passerine birds and hawks differs from the well known arrangement in the chicken; the passerine pattern is illustrated. It was not possible to bring about thymic enlargement out of season by castration or unilateral adrenalectomy in mallards.
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