Island-dwelling birds may maximize their reproductive output by investing more in larger and fewer eggs than their close mainland counterparts (Lack’s predictions). Less predictable environments could lead to increased intra- and inter-clutch egg size variability (environmental predictability hypothesis, EPH). Because these predictions have only seldom been tested, especially in raptors, we analysed egg size variability of the Common Kestrel (Falco tinnunculus) inhabiting a xeric scrub on Tenerife Island (Spain) over a twelve-year period. We also examined patterns of geographical variations in egg volume within the Western Palaearctic. In Tenerife, clutches of three, four, and five eggs had a higher volume than the six-egg clutches. Although egg volume decreased throughout the breeding season, this pattern was not constant over the years. Egg volume was positively correlated with hatching success, but not so with fledging success; it was inversely associated with rainfall in the previous autumn and temperature in March. Tenerife kestrels had a smaller mean egg volume in comparison to continental counterparts. However, after controlling for the effect of female body mass, the eggs were significantly larger, which seems to support Lack’s predictions. Also, egg volume in the Western Palaearctic kestrel populations increased with latitude. Tenerife kestrels had low intra-clutch and high inter-clutch egg volume variability, supporting only the predictions on inter-clutch variability of the EPH. Breeding strategies of Tenerife kestrels seem to be adapted to unpredictable rainfall fluctuations over the years. The effects of environmental factors on life-history traits provide support for changes in breeding strategies of island-dwelling birds, highlighting a great investment per egg/offspring produced.