Parents invest in their progeny to increase offspring survival, ultimately maximizing their own fitness. However, this investment is limited and this leads to the emergence of evolutionary conflicts of interest (i.e., sexual, parent-offspring and sibling conflict), as the different family members may disagree on how resources are distributed. Historically, theoretical models assumed that parental care strategies are fixed and that they can only change over evolutionary timescales. Yet more recent models rather specified rules for responding to the behaviour of partners and offspring in behavioural timescales. These models assume that individuals may negotiate until reaching a settlement, which implies that their fitness-related decisions are flexible and can be modulated according to the information on offer. The acquisition of information then again requires communication, which could be facilitated through informative signals that can be either static (e.g., plumage colouration) or plastic (e.g., offspring begging behaviour). However, we currently know too little about how flexible parental response rules are and to what extent individual responses and the flexibility in behavioural strategies are affected by signaling traits, in a context with multiple senders and receivers – the family. The main objective of this thesis was therefore to study (a) flexibility in care strategies and how this relates to resource allocation trade-offs and the environmental context, and (b) how optimal care strategies are modulated by signals as expressed by the family. Using blue tits (Cyanistes caeruleus) as a model system, we tested the hypothesis that signals expressed by family members facilitate flexible response rules (“negotiation”) over parental care in different contexts (e.g., physical or social environment). First, I investigated how the trade-off between current reproduction and self-maintenance is modulated by the availability of a key resource, lutein, during egg laying. Here I found that enhanced lutein availability increased female egg laying capacity, but at the cost a thinner egg shell. Possibly because the supplementation of this carotenoid pigment interfered with the calcium acquisition and deposition. Then, I investigated whether blue tit parents adjust their feeding strategies to the expression of an offspring signalling trait, and how changes in parental strategies depend on the quality of the environment (i.e., the manipulation of carotenoid availability; or family size). Parents should invest equally in all their offspring under good conditions (i.e., enhanced female intrinsic capacity, or once the initial brood size is reduced), when their rearing capacity allow them to raise all the brood. In contrast, they should shift their investment towards the offspring with the highest probability of survival when parental capacities are too limited (i.e., when the initial brood size is enlarged). Such parental favouritism could be facilitated through the expression of condition-dependent signalling traits that may inform parents about offspring quality (i.e., ultraviolet (UV)/yellow breast feathers of blue tit nestlings). Offspring UV colouration mediated intra-family interactions, and this effect depended on the quality of the prenatal environment and the brood demand. Fathers favoured UV-blocked nestlings when their female partner was lutein-supplemented during egg laying, and hence possibly in a better condition. Parents preferentially fed offspring signalling lower quality (UV-blocked) when their rearing capacities were sufficient to raise all nestlings reduced broods. Then, I investigated the function of the same colouration in blue tit adults. Parental feedings increased in nests in which the UV reflectance was experimentally reduced in one of the parents. This increase was due to the fact that partners of UV-blocked adults enhanced the number of feedings, presumably to compensate the lower-quality appearance of their mates. UV-blocked adults did not change their own rate of nestling provisioning. However, offspring did not respond to the parental manipulated signal, which contradicted my hypothesis that parental signals would simultaneously matter for multiple intra-family contexts. Ultimately, if parental feeding decisions are guided by the expression of signals, the offspring plumage colouration could be a condition-dependent trait that allows parents to distinguish their offspring quality. Here, I found that the total reflectance of yellow breast feathers could act as an honest signal during intra-family interactions, since it varied with body mass within broods. Interestingly, this colour parameter, and the reflectance in the UV region reflected brood quality (e.g., rearing capacity, genetic or environmental effects). The function of this pattern is as yet unclear. These results reveal a significant flexibility in behavioural strategies within the context of parental care. Family signals would facilitate flexible behavioural responses, which are dependent on the context. All family members responded (although to a different extent) to the experimental manipulation of different environmental factors or signalling traits that determine the information provided in the family environment. Yet signalling may facilitate the flexibility and hence diversity of fitness-related decisions taken by the different family members, which may allow them to reach a settlement that optimises trade-offs along a negotiation continuum.