Top-down regression of the avian oviduct during late oviposition in a small passerine bird

Egg production in oviparous vertebrates is assumed to be costly but the physiological basis of any costs remains unknown. The avian oviduct is a highly differentiated linear organ, with five functionally specific regions. Here we show that the oviduct regresses rapidly 'from the top down' as soon as the more proximal regions have completed their function but while the distal regions still retain an oviductal egg. In zebra finches Taeniopygia guttata, oviduct mass did not differ between early laying birds at the 1-egg stage compared with late-laying birds (with one remaining yolky follicle; dry mass, 151-167 mg). However, in birds with no remaining yolky follicles but with an oviductal egg, oviduct mass decreased to 94 mg (44%). Regression occurred unequally among different regions of the oviduct, with significant decreases in the proximal infundibulum/magnum. and isthmus regions (59% and 40%, respectively), but no change in distal shell gland/vagina mass. The shell gland did not regress until after the last oviposition. Thus, the avian oviduct has a highly regulated size-function relationship consistent with a high maintenance energy cost for this organ. We suggest that oviduct function is a significant contributor to the physiological costs of egg production and might mediate individual variation in maternal effects associated with non-yolk components of egg quality (e.g. immunoglobulins, lysozyme).