Does Nest Predation Risk Induce Parent Birds to Eat Nestlings' Fecal Sacs? An Experimental Study

Adult birds are frequently observed consuming nestling feces. However, fecal sac ingestion has received little attention, including a lack of experimental tests to understand such behavior. Traditionally, it has been explained by the “parental nutrition hypothesis” (parents acquire nutrients and energy) or the “economic disposal hypothesis” (parents save time to do other tasks). Here, we propose a third explanation, the “nest predation hypothesis”: parents ingest fecal sacs to reduce nest detectability in the presence of a nest predator. We experimentally manipulated the nest-predation risk perceived by adult common blackbirds (Turdus merula). We detected an effect of brood size in the removed and ingested feces while our experiment did not increase fecal sac comsumption. However, we found other evidences supporting that nest predation could play a more subtle role in this parental care behavior: (1) both sexes contributed equally to fecal ingestion; (2) adults ingested more feces in the population with t...