Influence of breeding time, nest size, water depth, and egg size on the breeding success of the little bittern Ixobrychus minutus

Understanding the factors that influence avian reproductive output is critical for bird conservation as they reveal key considerations that directly impact a species’ long term survival and should be integrated into management plans. To better this understanding for the little bittern Ixobrychus minutus, we investigated how their breeding success relates to breeding time, nest size, water depth, and egg size in a man-made wetland (Ab-bandan) during the 2008 breeding season in Mazandaran Province, northern Iran. For each nest, the dates of the first egg laying, the first hatching, and the first fledging were recorded as well as clutch sizes. Nest size parameters (height, depth, and diameter), water depth under the nest, and egg characteristics (shape index and weight) were measured. The first egg was laid on March 21 and the last chick left the nest on June 24. From 140 eggs in 25 nests, 127 eggs hatched (91%) and 121 fledglings left their nests (87%). Generalized linear modeling revealed that nest diameter positively affected breeding success (p = 0.001) meaning larger nests were more likely to produce fledglings. Further studies will be necessary to elucidate the underlying mechanisms.