Abstract
Climate warming could lead to ‘mis-matching’ of birds’ breeding times with availability of their invertebrate food resources. Fluctuating spring temperatures could influence variation in the commencement of egg-laying, as well as, in phenology of their resources (e.g., tree bud burst, appearance of folivorous caterpillars). We studied timing of Dendrocopos leucotos, Dendrocoptes medius, Dendrocopos major, Dryobates minor, and Picoides tridactylus breeding in, free of direct human intervention, strictly protected fragments of Białowieża Forest (Poland) in 1975–2019. We related their onset of breeding to inter-year variation in spring weather, trees’ bud burst and timing of folivorous caterpillars’ availability. Individual species initiated breeding at dates differing by a month. In spite of intraseasonal differences in the absolute laying dates, sometimes exceeding three weeks, the sequence of laying by different species within a season was highly repeatable; D. leucotos commenced the first and P. tridactylus the last. Within a species and season, the females commenced laying synchronously; usually 50% of them begun laying ≤ 8 days of each other. All species bred earlier when spring temperatures got higher. Nestling time of D. medius and D. major broadly coincided with ‘caterpillar peaks’, while D. minor young appeared too late. All woodpeckers advanced egg-laying in Białowieża National Park over 45 years but the inter-specific sequence of laying has been retained. These could be responses to strong advance of snow disappearance and to warming of the second half of April. The early recent laying dates remained within the range of earlier recorded ones, the early springs just became more frequent. Thus, the phenological acclimatization could be achieved by plastic behavioural and physiological adjustments. Lack of data on types of food actually used and their seasonal variation hinders mechanistic understanding of how warming would affect the future food supply and breeding performance of birds.
Highlights
The timing of reproduction and synchronisation with varying environmental conditions constitutes a major problem for all organisms living in seasonal environments
Despite sharing the same forest patches and having to cope with identical fluctuations in environmental conditions, the five woodpeckers studied in Białowieża NP initiated breeding at vastly divergent dates with their egg-laying times differed by a month (Fig. 1)
Similar local spread of breeding dates was observed in North America (Drake and Martin 2018)
Summary
The timing of reproduction and synchronisation with varying environmental conditions constitutes a major problem for all organisms living in seasonal environments. Lack (1950) suggested that the variation in food resources was the most important factor and postulated that the breeding season of each species should be adapted, so that it coincided with the time of the year when there was abundant food for the young. For insectivorous birds in the North temperate and boreal forests, this period occurs generally in spring, when temperatures typically rise and invertebrates become more plentiful. The insectivorous birds (including woodpeckers, cf reviews in Glutz von Blotzheim and Bauer 1994; Pasinelli 2003; Michalek and Miettinen 2003) breed at this time of year. To test the ‘food for young’ hypothesis, one needs to collect much more specific data on both the types of food brought to the young of the studied species, and the temporal variation in the availability of these food types (Cholewa and Wesołowski 2011)
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