Parasitoid wasps may act as hyperparasites and sometimes regulate the populations of their hosts by a top-down dynamic. Nasonia vitripennis (Walker, 1836) is a generalist gregarious parasitoid that parasitizes several host flies, including the blowfly Protocalliphora Hough, (Diptera: Calliphoridae), which in turn parasitizes bird nestlings. Nonetheless, the ecological factors underlying N. vitripennis prevalence and parasitoidism intensity on its hosts in natural populations are poorly understood. We have studied the prevalence of N. vitripennis in Protocalliphora azurea (Fallén, 1817) puparia parasitizing wild populations of pied flycatcher (Ficedula hypoleuca) and blue tit (Cyanistes caeruleus) birds in two Mediterranean areas in central and southern Spain.We found some evidence that the prevalence of N. vitripennis was higher in moist habitats in southern Spain. A host-dependent effect was found, since the greater the number of P. azurea puparia, the greater the probability and rate of parasitoidism by the wasp. Our results also suggest that N. vitripennis parasitizes more P. azurea puparia in blue tit nests than in pied flycatcher nests as a consequence of a higher load of these flies in the former. Based on the high prevalence of N. vitripennis in P. azurea puparia in nature, we propose that this wasp may regulate blowfly populations, with possible positive effects on the reproduction of both bird species.

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Avian growth is an important predictor of fitness due to reproductive and survival benefits associated with body size (Gebhardt-Henrich & Richner 1998). The rate at which asymptotic body mass is achieved by nestlings may affect their oxidative status (Stier et al. 2014), telomere dynamics (Geiger et al. 2012) and immune function (van der Most et al. 2011), with consequences for their survival at the postfledging stage (Merrill et al. 2021) or future reproductive success (Richner et al.1989). Altricial nestling growth has been traditionally examined using non-linear equations, such as the Gompertz (Gompertz 1825), von Bertalanffy (Bertalanffy 1957) and logistic (Ricklefs 1967) models. These models have been proven to describe well the increase in nestling mass, although recently other models have been developed to better fit growth data (Brown et al. 2007, Tjørve & Tjørve 2017. Depending on the species, nestlings may reach asymptotic mass

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Mixed-species broods have been reported among some passerine species, including Eurasian Nuthatches Sitta europaea and Flycatchers Ficedula spp. but mostly Tits (Paridae; Baucells 1990, Barrientos et al. 2015. The causes of these mixed broods are still poorly understood. In Tits, the frequency of mixed-species clutches increases as the nestbox occupation rate increases (Barrientos et al. 2015). Indeed, most studies suggest that the main ecological cause for the occurrence of mixed-species broods in secondary hole-nesting birds is a shortage of nestboxes (Petrassi et al. 1998, Samplonius & Both 2014, Barrientos et al. 2015, see also Vedder et al. 2007 for intraspecific brood parasitism in Blue Tits Cyanistes caeruleus). In this situation, mixed-species clutches appear to be the result of one of two possible strategies: interspecific brood parasitism or nest takeover.Avian brood parasites frequently impose high fitness costs on hosts, as parasitic nestlings usually outcompete their unrelated foster nestmates (Soler

Wild birds are hosts of Culicoides from as early on as the nesting stage when constrained to their nests. However, the environmental factors which determine the abundance and composition of Culicoides species within each bird nest are still understudied. We sampled Culicoides from Eurasian blue tit (Cyanistes caeruleus) nests found in 2 types of forests located in southern Spain. Firstly, we monitored the abundance of Culicoides species in bird nests from a dry Pyrenean oak deciduous forest and a humid mixed forest comprising Pyrenean and Holm oaks throughout 2 consecutive years. During the 3rd year, we performed a cross-fostering experiment between synchronous nests to differentiate the role of rearing environment conditions from that of the genetically determined or maternally transmitted cues released by nestlings from each forest. We found 147 female Culicoides from 5 different species in the birds' nests. The abundance of Culicoides was higher in the dry forest than in the humid forest. Culicoides abundance, species richness and prevalence were greater when the nestlings were hatched later in the season. The same pattern was observed in the cross-fostering experiment, but we did not find evidence that nestling's features determined by the forest of origin had any effect on the Culicoides collected. These results support the notion that habitat type has a strong influence on the Culicoides affecting birds in their nests, while some life history traits of birds, such as the timing of reproduction, also influence Culicoides abundance and species composition.