Climate change, migratory connctivity and changes in laying date and clutch size of the pied flycatcher

Abstract

We examined long‐term (1943–2003) variability in laying dates and clutch sizes in a Finnish population of the pied flycatcher Ficedula hypoleuca Pallas, and analysed whether potential changes were explained by changes in climatic factors at the wintering area in Africa, at migration route or at breeding grounds. Among‐year variation in both mean and skewness of laying dates increased, which for mean laying date appeared to be explained by variability of temperatures at the breeding grounds and for skewness by variable temperature trends along the migration route. Pied flycatchers bred earlier in warm springs, but despite a warming trend in pre‐laying temperatures, the laying dates tended to delay. Laying dates became continuously later in relation to the phenology of the environment. Mean clutch size decreased with time when mean laying date was controlled for, but the climatic factors did not appear to explain the decrease. The advancement of spring phenology may have shifted some food sources needed for egg‐laying, thus leading to later laying and smaller clutches. Variation in clutch size increased when wintering conditions were favourable so that clutch size distribution was skewed with a tail of small clutches when there had been lot of rainfall (more vegetation and insects) in the wintering area. We suggest that when ecological conditions during winter were good, the tail of small clutches represented low‐quality individuals that were not able to breed after bad winters. Our analyses demonstrate that measures of spread and symmetry give different information about population level changes than means, and thus complement the understanding of the potential influences of climate change on populations.

What prevents phenological adjustment to climate change in migrant bird species? Evidence against the “arrival constraint” hypothesis

“…Analysis of SD allowed consideration of the entire laying period, including outliers (Laaksonen et al 2006). Mean lay dates and between-year variability in the period 100 1990-2004 were compared with those at the same site during 1948-1963(Lack 1966) using a two-sample ttest and an F-test, respectively.…”

“…Laaksonen et al 2006). Although in some populations, particularly those in Scandinavia, phenological inertia is due to lack of local warming (i.e.…”

“…no stimulus for earlier laying) (Both et al 2004), it also occurs where co-occurring resident species are laying 28 earlier, seemingly in response to increasing spring temperatures (Crick and Sparks 1999). Even where migrant lay dates are advancing, as they are in 36% of European pied flycatcher populations (Both et al 2004), advancement often appears insufficient given the substantial shift in the timing of peak food sources, which is leading to phenological mismatches between trophic levels (Both et al 2006;Laaksonen et al 2006). Such 32 mismatching is seen in a range of bird species (e.g.…”

Climatic effects on breeding grounds are more important drivers of breeding phenology in migrant birds than carry-over effects from wintering grounds

“…Secondly, after unfavourable winters, individuals may prioritize investment in reproduction over self-maintenance, resulting in carry-over effects on survival [23], which were not examined. Thirdly, any increase in the survival of low-quality females during favourable winters may increase the proportion of small clutches laid, masking any potential carry-over benefit for fitter individuals [24]. Finally, females experiencing favourable climatic conditions may elect to nest earlier rather than lay more eggs.…”

Projected changes in thermal seasons and the growing season in Finland

“…For example, the cultivable limits of spring wheat are projected to shift substantially northward (Saarikko and Carter, 1996), and even the cultivation of maize may become profitable in Finland (Fronzek and Carter, 2007). Conversely, warming can be detrimental to migratory birds if earlier springs make them to breed in asynchrony with the availability of nourishment (Laaksonen et al, 2006).…”