# What is the relationship between lay date, mismatch, and overall fitness for migratory birds?

Food availability at the breeding site often has a peaked temporal profile such that food is very abundant for a short period of time. Therefore, migratory birds must also time their arrival such that there is adequate time to gather the resources needed for egg production, and time their nesting and laying so that nestlings can take advantage of these food peaks. Failure to do so leads to a mismatch’, and presumably a suboptimal nestling health and fledging rate.

The benefits of early laying include match to the food peak, a more favourable environment for nestlings (Verhulst & Nilsson 2008), and the possibility of raising a second brood (Tinbergen & Daan 1990, Brinkhof et al. 2002). However the laying date that maximises reproductive success is typically earlier than the laying date in most birds (Lack 1968; Perrins 1970; Perrins & Moss 1975) (Drent 2006). This suggests that there must be some counter-acting disadvantage to early breeding that predates climate change effects. There is a growing literature suggesting that phenological mismatch may be adaptive (Singer & Parmesan 2010, Visser et al. 2012, Johansson & Jonzen 2012).

McNamara & Houston (2008) recommends that trade-offs be framed in terms of their physiological basis. One example is the individual optimisation hypothesis’ (Drent & Daan 1980, Rowe et al. 1994), which proposes that the trade-off between laying time and reproductive output is mediated by individual body condition.

It is known that females with better body condition lay earlier (Bety et al. 2003, Descamps et al. 2011), that decreasing body condition delays lay date (Bety et al. 2003), and that delayed breeding improves body condition (Descamps et al. 2011). By assuming that high body condition allows for a larger clutch size, and that body condition can be improved by extending the pre-laying period, key patterns in laying date and clutch size can be explained Rowe et al. (1994). Further experimental evidence for the theory is that fitness is highest when brood size and laying date is chosen by female, and that the clutch size is dependent upon female condition Pettifor et al. (2001).

Body condition also plays a role in the relationship between laying cost and reproduction; experiments where laying costs were increased result in reduced condition and delayed reproduction the following year (Visser & Lessells 2001, Kalmbach et al. 2004) (Nager et al. 2001). There is also likely to be a condition-mediated relationship between early laying and adult survival (Brinkhof et al. 2002, Nilsson 1994). Experimentally increased laying costs also decrease adult survival (Visser & Lessells 2001) (Nager et al. 2001). The question of trade-offs between current reproduction and adult survival/future reproduction is also key topic in life history theory (Stearns 1992).

The individual optimisation hypothesis is usually described with condition gain as a linear function of time, however it is recognised that food availability is lower earlier in the season (Perrins 1970), birds expend more energy during cold conditions (te Marvelde et al. 2012), resource availability during the pre-laying period is variable (Gauthier 1993), the rate of body-condition gain may constrain the laying date (Lepage et al. 2000). Key nutrient constrains may still apply to `capital breeders’; for example, female birds cannot store sufficient calcium in the legbones to meet the needs of a clutch (Pahl et al. 1997), and so depend upon calcium rich sources like snails during the prelaying period (reviewed by Perrins 1996 and Nager 2006).

A restricted maximum-likelihood animal model revealed that adult survival in collared flycatchers is traded- off against laying date, and that clutch size covaries with laying date and is not under direct selection (Sheldon et al. 2003).