Timing of breeding in subarctic passerines in relation to food availability

We studied timing of breeding in four species of hole-nesting passerines, the Siberian tit (Poecile cinctus), great tit (Parus major), pied flycatcher (Ficedula hypoleuca), and redstart (Phoenicurus phoenicurus), and the abundance of the arthropods in their diet (ground-dwelling spiders, winged insects, foliage-feeding lepidopteran and hymenopteran larvae) in Finnish Lapland for 6-12 years. Densities of the invertebrate groups varied considerably, both seasonally and annually. All the bird species started to breed in early summer, when weather conditions were often poor. At the beginning of the breeding season all species relied on ground-dwelling spiders, whose abundances peaked early and were less variable over the years than those of insects. Abundances and timing of emergence of caterpillars in birch and pine canopies (the major food sources for foliage-gleaners) were highly variable, but each year their abundances peaked in July or August, after the nestling period of the birds. This contrasts with reported cases in deciduous forests at midlatitudes and shows that at our site the birds did not time their nestling period ultimately to coincide with peak food availability. Mean clutch sizes declined seasonally in the tit species and the pied flycatcher but not in the redstart. The number of fledglings did not depend on laying date, except in the Siberian tit, for which dependence of fledgling number on laying date varied among years. Although early broods were not more productive (i.e., greater number of fledglings) than later ones, early breeding is ultimately a necessity for recruitment because of the short subarctic summer. Birds cannot delay breeding in the north because late breeding would shorten the time available for moulting, food-hoarding, and preparation for migration or wintering. Our results show that at the time of egg laying, passerines may receive proximate cues to allow them to predict food conditions during the nestling period. Still, exact prediction of food availability during later phases of nesting is constrained by an inherent discrepancy between warm-blooded birds and poikilothermic invertebrates: once a bird has started egg laying, the subsequent phases follow nearly automatically on certain calendar dates, whereas the development of the arthropods depends on the rate of temperature increase.