Predation on breeding western toads (Bufo boreas)

Predation by ravens (Corvus corax) on western toads (Bufo boreas) was observed at three of 15 toad explosive breeding aggregations. At one aggregation, over 20% of the toad annual breeding population was killed and found eviscerated near the communal breeding site. Predation was observed when toads were breeding in shallow water, 5-25 cm deep, but not when toads remained in deeper water. During censusing of toads, paired males released their mates before oviposition more often at aggregations with predation. There were some body size differences between toads in mating pairs that remained together and pairs in which males released their mates. At one aggregation, releasing males were larger than non-releasing males, and at a second aggregation, males that released their mates were clasping smaller females than females of pairs remaining together. In breeding western toads, predation risk may be reduced by remaining in deeper water, the unclasping behavior of paired males, and the spatial and temporal synchronization of breeding activities.