Seasonal Variation in the Reproductive Success of Blue Tits: An Experimental Study

In blue tit (Parus caeruleus L.) populations the numbers of surviving young from a clutch generally declines the later in the season it hatches, but hatching too early also reduces productivity. The extent to which seasonal changes in offspring survival were caused by environmental changes was assessed experimentally by cross-fostering clutches of differing age between pairs of nests in Bagley Wood, Oxfordshire. This manipulation had the effect of advancing or delaying the hatch date experienced by parents. Productivity of experimental nests could then be compared with the productivity of control pairs breeding at the same time, and with seasonal trends in long-term data from a blue tit population nesting in a similar habitat at Wytham, Oxfordshire