**Abstract** from the paper, published in Behavioural Ecology and Sociobiology:
Sexual conflict has long been perceived as a solid background for the evolution of parental care. Recent studies, however, highlight the importance of cooperation between the parents, especially in socially monogamous systems. Here, we examined parental performance of a small monogamous passerine with bi-parental care, the Eurasian Reed Warbler (Acrocephalus scirpaceus, RW), looking at the issue from the perspective of parents’ cooperation. Using accurate and non-invasive video data on parent chick provisioning from 78 nests varying in brood size, we found some evidence of parents cooperation: both parents similarly adjusted their feeding rate to brood size, and the higher was their both contribution, the better was chicks condition (expressed by defecation rate). However, contrary to our expectations (based on premises from similar studies on other bird species) we did not find evidence for frequent and active synchronization of nest visits. Importantly, we found a negative relationship between synchronization level and chick body condition, suggesting that synchronization may actually not be that beneficial in the study system. The results are surprising, and highlight the importance of studying various species to understand mechanisms behind cooperation of partners in the bi-parental care system.
