Criterios de exclusión

In temperate breeding birds, reproductive success is fundamentally determined by the product of brood productivity and the number of broods attempted (Crick et al. 1993). One strategy to maximise reproductive success, adopted by some species, is to attempt to successfully raise multiple broods per season. Multi-brooding can substantially increase annual and lifetime reproductive success, measured by the number of nestlings fledged (e.g. Ogden and Stutchbury 1996, Weggler 2006, Hoffmann et al. 2014, Cornell and Williams 2016) or the number of offspring recruited into the breeding population (Townsend

et al. 2013, Hoffmann et al. 2014). However, there are also potential costs to being multi-brooded such

as reduced parental care for first brood fledglings (Geupel and DeSante 1990, Verhulst and Hut 1996, Naef-Daenzer et al. 2011), delayed post breeding moult (Ogden and Stutchbury 1996, Klemp 2000, Morrison et al. 2015) and reduced adult survival to the following breeding season (Bryant 1979, Verhulst 1998, Brown et al. 2014). In addition, offspring raised from second broods do not necessarily contribute equally to parental fitness as those raised from first broods, as numerous studies have found that offspring raised in later broods are less likely to survive and recruit into the breeding population (e.g. Verboven and Visser 1998, Mallord et al. 2008, Hoffmann et al. 2014). The decision of whether to multi-brood or not represents a cost-benefit trade-off between current and future reproduction for individuals, in terms of both annual and lifetime fecundity, as predicted by life history theory (Reznick 1985, Stearns 1992). Within multi-brooded populations, there is individual variation in multi-brooding, with some individuals being single-brooded and others multi-brooded in a given breeding season (e.g. Verboven et al. 2001, Nagy and Holmes 2005b, Hoffmann et al. 2014). Understanding the factors which influence multi-brooding behaviour at the individual level is important, not only because multi-brooding influences population productivity, but also in the context of the extension in the length of the breeding season, which has been observed in some multi-brooded species in response to spring warming in recent decades (Møller et al. 2010, Halupka and Halupka 2017).

The timing of the first brood has been found to be the most important determinant of double brooding in most studies across a wide range of species (Geupel and DeSante 1990, Ogden and Stutchbury 1996, Verboven and Verhulst 1996, Brinkhof et al. 2002, Weggler 2006, Bulluck et al. 2013, O’Brien and Dawson 2013, Townsend et al. 2013, Carro et al. 2014, Hoffmann et al. 2014, Zając et al. 2015, Béziers and Roulin

25 2016, Jackson and Cresswell 2017), although a recent study of European Starlings, Sturnus vulgaris, proves an exception (Cornell and Williams 2016). Verboven and Verhulst (1996) dismissed the possibility that earlier breeding individuals are more likely to double-brood as a result of being higher quality, rather than as a direct result of timing, as their experimental manipulations of the timing of breeding in great tits,

Parus major, confirmed that double brooding probability increased when breeding attempts were

advanced and declined when they were delayed. Other potential predictors include parental factors (such as age or effort) or environmental factors (food availability or weather). Several studies have found female age to be important, with older individuals being more likely to double brood, whereas the age of males appears to be unimportant (Geupel and DeSante 1990, Weggler 2006, Bulluck et al. 2013, Hoffmann et al. 2014). Assessments of the effect of the number of offspring in the first brood on the probability of double brooding, representing a trade-off in parental effort between first and second broods, has revealed conflicting results. Smaller first broods increased the probability of attempting second broods in great tits and blue tits, Cyanistes caeruleus (Tinbergen 1987, Verboven and Verhulst 1996, Parejo and Danchin 2006), while the opposite effect was found in Eurasian hoopoes, Upupa epops (Hoffmann et al. 2014). Furthermore, several other studies have found no effect of first brood size on double brooding probability (Ogden and Stutchbury 1996, Brinkhof et al. 2002). Finally, male parental care may also contribute to the decision to double brood as a recent study of Japanese tits, Parus minor, found that the likelihood of double brooding increased with the proportion of provisioning undertaken by the male (Nomi et al. 2018). Food availability has been shown to be associated with the probability of double brooding in both observational studies (Nagy and Holmes 2005b, Husby et al. 2009, Jackson and Cresswell 2017) and food supplementation experiments (Nagy and Holmes 2005a, O’Brien and Dawson 2013). Weather variables such as temperature and rainfall have also been found to be related to the extent of double brooding in populations and the breeding season length of individuals as a result of their association with the timing of the start of breeding (Carro et al. 2014, Jankowiak et al. 2014, Jankowiak and Wysocki 2015), although the effect of weather after fledging a first brood on the probability of double brooding does not seem to have been previously assessed. Weather conditions at the time of potential double brooding decisions could, however, be important through potential influences of food availability and post-fledging mortality of fledged first broods.

Further to the ‘decision’ of whether to attempt an additional brood or not, there is further scope for variation in multi-brooding behaviour in terms of the length of the interval between attempts (hereafter ‘inter-brood interval’). A long-term study of barn swallows, Hirundo rustica, found that inter-brood

26 intervals between first and second broods have extended as the start of the species’ breeding season has advanced, and that longer inter-brood intervals have fitness benefits for adults (Møller 2007). Several studies have shown that inter-brood intervals are longer when first broods are larger, likely because larger first broods require more parental care (Verboven and Verhulst 1996, Møller 2007, Carro et al. 2014). Food availability has also been shown to influence inter-brood interval lengths; as food supplementation did not affect the probability of double brooding, but did shorten the inter-brood intervals of double- brooded great tits (Verboven et al. 2001). Post-fledging parental care of first broods is often cut short by multi-brooded individuals that attempt second broods and brood overlap may also be employed, whereby care of the first brood continues after the second brood has been initiated (Verhulst et al. 1997, Naef- Daenzer et al. 2011, Béziers and Roulin 2016, Stępniewski and Halupka 2018). A further fundamental factor determining inter-brood interval lengths is post-fledging mortality, as short intervals may be the result of mortality of part or all of the first brood soon after fledging (Verhulst and Hut 1996).

In this study, the factors influencing the probability of double brooding and the inter-brood interval length between first and second broods was assessed in Eurasian reed warblers, Acrocephalus scirpaceous (hereafter ‘reed warblers’). Reed warblers are a facultative double-brooded species in which females also frequently replace failed nesting attempts and have been known to lay up to five clutches in a season (Borowiec 1992, Schulze-Hagen et al. 1996, Halupka et al. 2008). Previous estimates of the frequency of double brooding range between 0-35 % of pairs (Schulze-Hagen et al. 1996, Calvert 2005) and the first cases of triple brooding have recently been documented in the population subject to the current study (Batey and Leech 2018/ Appendix 1). The reed warbler is a long-distance migrant whose spring arrival and timing of breeding have advanced significantly with climate change induced spring warming (Crick and Sparks 1999, Schaefer et al. 2006, Halupka et al. 2008) and there is also evidence of breeding season extension (Halupka et al. 2008). Reed warblers are generalist insectivores that breed in wetlands, where invertebrate prey is generally considered to be abundant throughout the breeding season (Bibby and Thomas 1985, Both et al. 2009, Dodson et al. 2016); however, the timing of first broods has been shown to be influenced by food availability (Vafidis et al. 2016).

Further to being an aspect of avian breeding ecology which is relatively understudied, multi-brooding behaviours are of further interest in the context of breeding season extension, which has been observed in some multi-brooded species in recent decades and in association with climate change (Møller et al. 2010, Halupka and Halupka 2017). An increased propensity to multi-brood (Halupka et al. 2008), increased failure rates resulting in more replacement nesting attempts, or extended intervals between broods

27 (Møller 2007) are all potential mechanisms behind such breeding season extensions . Reed warblers are a good model species for assessing the potential for increased double brooding to be driving breeding season extension, as increased reproductive output has been suggested as a driver of recent population increase (Woodward et al. 2018); therefore increased replacement nesting attempts as a result of increased failure rates is not a likely mechanism of season length extension in reed warblers (Halupka et

al. 2008, Woodward et al. 2018). Instead, it can be postulated that breeding season extension could result

from either longer intervals between broods or shorter intervals between broods allowing for an increase in the number of broods attempted. Assessing the relationship between multi-brooding and interval lengths with environmental conditions including wetland food availability, which is likely to have increased with climate warming in recent decades (Vafidis 2014), allows for inference of the likely mechanism. Using a four year data set from an intensively monitored breeding population, this study investigates the effects of first brood breeding performance (timing and number of nestlings fledged) and environmental factors (food availability, temperature and rainfall) on the decision to double brood and, for double- brooded pairs, the length of the inter-brood interval. This is to my knowledge the first formal analysis of the determinants of double brooding in reed warblers.

METHODS