Ann. Zool. Fennici 43: 118–130 ISSN 0003-455X Helsinki 28 April 2006 © Finnish Zoological and Botanical Publishing Board 2006 Reproductive decisions of boreal three-toed woodpeckers (Picoides tridactylus) in a warming world: from local responses to global population dynamics Philippe Fayt Department of Biology, University of Joensuu, P.O. Box 111, FI-80101 Joensuu, Finland; present address: Research Centre of Nature, Forests and Wood (DGRNE), Avenue Maréchal Juin 23, B-5030 Gembloux, Belgium (e-mail: Philippe.Fayt@joensuu.fi) Received 7 Oct. 2005, revised version received 6 Mar. 2006, accepted 6 Feb. 2006 Fayt, P. 2006: Reproductive decisions of boreal three-toed woodpeckers (Picoides tridactylus) in a warming world: from local responses to global population dynamics. — Ann. Zool. Fennici 43: 118–130. Here, I examine whether both nestling prey abundance and profitability affect the timing of reproduction and hence the number of fledglings of three-toed woodpeck- ers, Picoides tridactylus. Individuals living in eastern Finland reproduced earlier and reared larger broods in habitat patches where the bark-beetle community developed earlier and/or faster and where the wood-boring beetles, whose larvae account for the bulk of the nestlings’ diet, were more abundant. As expected, mean breeding success increased with spring temperature, if laying date was related to prey phenology, which is temperature-dependent. However, using a larger data set with more years, adults seemed to face increasing difficulties in optimising their reproductive effort above certain spring temperature threshold, with a dramatic reduction in natal dispersal and winter population size over Finland following the warmest springs. I suggest climate- mediated phenological disjunction between the predator and its prey to be a likely cause of further decline in the insect specialist three-toed woodpecker. Introduction 1973, Zandt et al. 1990, Blondel et al. 1992). Such a pattern seems to be especially prevalent Annual changes in nestling food supply are in populations in natural habitats, where the known to affect breeding initiation and limit pro- range of environmental conditions is similar to ductivity in birds (Lack 1968). Besides the over- those experienced in the past (i.e., in the environ- all abundance of food in the environment, there ment of selection). is now considerable evidence that prey availabil- Therefore, it is predicted that a sudden change ity acts as main selection pressure on the timing in the environment would promote a mismatch of the breeding season (Nager & van Noordwijk between offspring needs and food abundance 1995). For most forest insectivorous bird popula- and should negatively affect reproductive suc- tions, initiation of egg-laying is determined by a cess and perhaps population size (Winkler et al. synchronisation of the peak energy demand of 2002). Accordingly, in habitats recently modified nestlings with maximum food supply (van Balen by human activities, or in years with unusual ANN. ZOOL. FENNICI Vol. 43 • Fitness determinants of boreal three-toed woodpeckers 119 spring weather conditions, evidence suggests the comitant increase in the consumption of ener- timing of laying may be maladaptive, caused by getically more profitable longhorn-beetle larvae. phenological disjunction between predator and Among controlling factors, ambient temperature prey. Local mismatch between nestling require- has a major effect upon the duration and timing ments and food supply may result from regular of the different phases of development of the asymmetric dispersal in the breeding habitat of woodpecker insect prey (Annila 1969, Salonen individuals that are genetically programmed to 1973, Post 1984). As a general pattern, beetles breed at different times in their natal habitats, begin to move and develop again after hiber- preventing adapted response mechanisms from nation at a temperature threshold of 3–6 °C evolving (Dhondt et al. 1990, Blondel et al. (Chararas 1962, Annila 1969). Insect emergence 1992, 1993, Dias & Blondel 1996, Dias et al. or timing of natal dispersal depends on the time 1996, Lambrechts et al. 1997). Global change of reproduction (Annila 1969). The length of the in spring temperature, an important constraint life cycle depends also on the species of beetle on laying date (Stevenson & Bryant 2000), may and factors such as moisture content of tree tis- also promote mistimed phenotypic responses in sues and density of conspecifics (Post 1984). relation to a temperature-dependent food peak. Like other insectivorous woodpeckers, female In warmer springs, ectothermic insect prey tend three-toed woodpeckers that lay later in a breed- to hatch earlier and develop faster, tracking ear- ing season tend to produce smaller broods (Fayt lier blooming and leafing of host plants (Penue- 2003). This decline in breeding output parallels a las & Filella 2001). This, in turn, tends to disrupt seasonal decrease in the amount of wood-boring the evolved phenological synchrony between beetle larvae delivered to offspring (Fayt 2003), predatory birds and prey, with implications for supporting the view that, similar to birds in gen- population dynamics (Visser et al. 1998, Both & eral (e.g., Siikamäki 1995), breeding time and Visser 2001). In general, mismatched breeding productivity in woodpeckers is largely determined individuals suffer higher fitness costs than opti- by the amount of food available to nestlings. Nev- mally adjusted birds, whether directly in terms of ertheless, at a smaller spatial scale, it is unknown adult survival (Thomas et al. 2001) or indirectly whether variation in reproductive traits are caus- following a reduction in likelihood of double ally related to the timing and abundance of wood- brooding (Visser et al. 2003), nestling condition boring beetles, especially the longhorn beetle. and fledging recruitment (Buse et al. 1999, Sanz Earlier observations, that woodpeckers in habitats et al. 2003). where spruce bark-beetles develop earlier had The dietary preferences of the three-toed larger broods, suggest an additional role for prey woodpecker Picoides tridactylus change season- phenology as a cue in fine-tuning reproductive ally, but throughout the year it eats insects that decisions (Fayt 2003). On the other hand, given colonise dying and recently dead trees (Imbeau its particularly narrow diet during egg-laying and & Desrochers 2002). From late summer to early chick rearing, the three-toed woodpecker is likely spring, the woodpecker relies almost exclusively to be limited in its ability to cope with unusual on bark beetles (Col., Scolytidae), with a marked changes in prey phenology (Winkler et al. 2002). preference for the species that colonise spruce In this paper, I test general predictions for the (Picea spp.) (Hogstad 1970, Fayt 1999). During working hypothesis that the availability of profit- the late spring–summer months, wood-boring able longhorn-beetle larvae, the main food of the beetle larvae, and in particular longhorn beetle three-toed woodpecker nestlings, at the time of larvae (Col., Cerambycidae), contribute signifi- egg-laying is a key factor in explaining intraspe- cantly to the diet of individuals, including nest- cific variation in reproductive success. lings (Dement’ev 1966, Hogstad 1970, Pechacek & Krištín 1996). Pechacek and Krištín (2004) 1. Brood size and laying (here fledging) date, found that three-toed woodpeckers reduced their two fitness-related traits (Fayt 2003), will be bark beetle consumption with the onset of repro- related to local longhorn-beetle abundance duction, despite no apparent changes in beetle and timing of development. availability. Meanwhile, they observed a con- 2. Annual mean brood size will increase with 120 Fayt • ANN. ZOOL. FENNICI Vol. 43 spring (April–May) temperature if laying size required climbing the nest tree and using date is related to prey phenology, which is a small mirror and flashlight to investigate the temperature-dependent. cavity. In case the nestling count was uncertain, 3. As a result, the annual number of dispersing tree cavities were revisited during the following juveniles and the subsequent number of win- days. The initiation of laying was estimated from tering individuals should increase following the fledging date. Nests were visited daily once warmer springs (and so with an early nestling a nestling was seen extending its head from the prey development), assuming a positive rela- cavity to beg; fledging was considered to have tionship between the population density prior occured when the first nestling left the nest. In to dispersal and dispersal rate. this study, brood size referred to a single measure for an individual female in a given year. To avoid repeated brood size measurements from the same Material and methods females, woodpeckers were individually colour- ringed whenever possible. To catch birds a plastic Prediction 1: Nestling prey supply and tunnel prolonged with a hoop net was placed in breeding performance front of the nest hole. The birds were captured upon entering the nest cavity to feed the off- The woodpecker population study was conducted spring. In addition, un-trapped males and females from 1996 to 1999 in North Karelia, easternmost were considered as different birds when observed Finland (63°N, 31°E). The study area consisted in forest stands a year before the capture of the of a Norway spruce (Picea abies) dominated second calendar year individuals or after the patchwork of seven old-growth and one recently occurrence of colour-ringed