Parental care is costly since it takes time and energy, and whilst caring the parent may be predated. The benefits of care (i.e., viable offspring) however, are shared equally between the genetic parents: the male and the female. Thus a conflict occurs between the parents over care in many multiple-brooding animals, since each parent prefers the other to do the hard work of raising young ('sexual conflict over care'). One of the most striking examples of this conflict occurs in a small passerine bird, the penduline tit Remiz pendulinus in which both the male and the female may sequentially mate with several mates within a single breeding season. Incubation and brood-rearing are carried out by a single parent (either the male or the female). However, about 30% of clutches are abandoned by both parents. We investigated how body condition may influence parental behaviour of male and female penduline tits. We show that three measures of body condition (body mass, fat reserves and haematocrit value) are consistent with each other for males, although not for females. Nest building appears to be energetically more demanding than incubation in both sexes. In line with this, we found that males and females in good condition deserted their clutch more often than males and females in poor condition. Individuals in poor condition may care because incubation is energetically less expensive than nest building, and they cannot afford the energy requirement of building a new nest. We argue that understanding body condition in the context of parental care is both challenging and essential, since mathematical models (single-parent optimisation models and game-theory models) provide conflicting predictions. Future work, preferably by experimentally manipulating the body condition of penduline tits, is needed to test how body condition influences caring/deserting decisions in this puzzling avian system.