Experimental evidence for parental, but not parentally biased, favouritism in relation to offspring size inBlueTitsCyanistes caeruleus

Abstract

In species with biparental care, males and females share the benefits of investing in offspring but pay the costs individually. As a result of these evolutionary conflicts of interest between the sexes, it is expected that the two parents should follow different behavioural rules when providing food to the young. Such a discrepancy may be accentuated when parents have to choose between different subsets of offspring (e.g. large and small nestlings). We manipulated the degree of hatching asynchrony in Blue Tits Cyanistes caeruleus and quantified male and female feeding behaviour when nestlings were 7 and 10 days old. First, we tested for a difference in the role of the sexes during the nestling rearing period between experimentally asynchronous and synchronous control broods. We then used experimentally asynchronous broods to assess differences between the sexes in the pattern of food distribution in terms of number of feedings and prey types, between junior and senior siblings. When nestlings in experimental nests were 7 days old, females fed young more often than did males despite facing a trade‐off between brooding the smallest nestlings and bringing food to the nest. At this age, there was also a skew in food delivery in favour of senior siblings, whereas food was more evenly distributed across the brood when nestlings were 10 days old. We found no difference in how male and female Blue Tits distributed feeding visits among junior and senior nestlings. However, females fed the smallest nestlings with more spiders in comparison with their senior siblings. This could be related to their more suitable size relative to other prey types, their high content of essential nutrients, or both, and may represent a more cryptic form of parentally biased favouritism. We compare these findings with previous work on other species and discuss why parents did not feed junior siblings more frequently.

Delivery rates and prey use of Mountain Bluebirds in grassland and clear-cut habitats

“…We also found that males generally brought larger prey than females, which is the same sex-bias documented in some other passerines such as Pied Flycatchers (Ficedula hypoleuca) and Blue Tits (Bańbura et al 2001, Wiebe andSlagsvold 2014). Females are probably more attuned to the needs of their nestlings (Budden and Beissinger 2009, García-Navas et al 2014, Liu et al 2014 and may therefore bring smaller prey suitable for young nestlings to ensure they get sufficient food. Alternatively, females may not be intentionally searching for small prey but only bring it because they may be spending more time looking after the nestlings in the box, i.e., brooding, than males and may have less time to find larger prey (Wiebe and Slagsvold 2009).…”

Facultative interspecific brood parasitism in tits: a last resort to coping with nest-hole shortage

“…Furthermore, great tit chicks showed larger fledging rates, probably because they hatched earlier and were larger (Slagsvold 1998). Larger chick size is advantageous because tit parents prioritise the feeding of larger chicks during early development stages (Dickens et al 2008;García-Navas et al 2014b). Thus, when food is a limiting factor, smaller chicks (i.e.…”

“…We expected that (i) the prevalence of facultative IBP would be greater in plots with higher nest-box occupation rates (Yom-Tov 1980, because nest-hole shortage could prevent all breeding birds from having their own nests, once food availability is not a limiting factor in plots of smaller size (Nour et al 1998). As the food provisioning by tit parents is mediated by chick size, with larger chicks receiving more supplies, at least in the early development stages (Dickens et al 2008;García-Navas et al 2014b), we expected that (ii) fledging rate of blue tits in mixed-species broods would be lower than that of their great tit broodmates. Additionally, (iii) it would be more advantageous for great tits to dump into blue tit nests than for blue tits to dump into great tit nests, because the larger size of great tit chicks makes them better competitors for parents' food (Slagsvold 1998).…”

Offspring plumage coloration as a condition‐dependent signal in the blue tit

“…Thus, parents could potentially rely on both carotenoid-chroma and total brightness to discriminate offspring sex while adjusting their feeding strategies. Indeed, in other study populations, blue tit males and females receive different food items (García-Navas et al, 2014) or the total amount of investment (Dickens & Hartley, 2007). In addition, fledging yellow plumage could play a signaling role in family flocks that are formed immediately after fledging (Stenning, 2018) and during social interactions within flocks (Tschirren et al, 2005).…”