This suggests that the arid belt of western Central Asia is a serious ecological barrier for Palaearctic–Indian passerine migrants in autumn, but much less of one in spring. In spring, Blyth’s Reed Warblers carried less fuel than in autumn, and less than African migrants such as Garden Warblers Sylvia borin in spring. However, another Indian migrant, the Red-breasted Flycatcher Ficedula parva, had low average fuel stores.
In autumn, fuel loads of Blyth’s Reed Warblers Acrocephalus dumetorum before their desert crossing were much greater than in Palaearctic–African migrants, which face a much narrower barrier, and also greater than in conspecifics captured during and after the desert crossing. We studied migratory stopovers of nocturnal passerine migrants migrating between the Urals and Siberia and the Near East and Africa (five species) and between European Russia and the Indian subcontinent (two species) in an oasis in the arid belt of the northwestern edge of western Central Asia. 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. 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. 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. 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.
Nest building appears to be energetically more demanding than incubation in both sexes. 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. We investigated how body condition may influence parental behaviour of male and female penduline tits. However, about 30 % of clutches are abandoned by both parents. Incubation and brood-rearing are carried out by a single parent (either the male or the female). 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. 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’). The benefits of care (i.e., viable offspring) however, are shared equally between the genetic parents: the male and the female.
Parental care is costly since it takes time and energy, and whilst caring the parent may be predated.
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