Egg burial in penduline tits, Remiz pendulinus: its role in mate desertion and female polyandry

One important component in the mating strategy of an already-matedindividual is the decision to remain with the partner and carefor the offspring or to desert. Almost all research on nestdesertion has focused on the costs and benefits of continuedparental care versus desertion of both parents. However, ifit pays both parents to desert, the timing of desertion is mostimportant. In birds, where all eggs are fertilized well beforethe last egg is laid, males should be the first to desert. Evenif females try to hide their fertile period, it is likely thatthe appearance of eggs acts as a cue that males can use to calculatethe timing of their desertion. Here we examine egg burial behaviorin penduline tits and its possible effects on parental behaviorand desertion. Penduline tits perform uniparental care fromthe earliest point of breeding, and both sexes try to becomepolygamous. We found that 36% of investigated males were polygynousand deserted as soon as the first egg appeared in their nest,and 12.5% of females became polyandrous. About 27% of the nestswere deserted by both sexes, which means high costs for femalesin terms of wasted energy in eggs and for males in terms ofwasted energy and time in building elaborate nests. Femalescover the eggs, and several facts indicate that egg coveringis a deceptive behavior of females: (1) females cover the eggsin the morning before leaving the nest for the first time, (2)females are more aggressive toward their mates during the layingperiod than before laying, (3) females try to prevent malesfrom entering the nest when eggs have been experimentally uncovered,and (4) females uncover the eggs as soon as males are experimentallyremoved. Finally, we found that a female can only desert thenest before the male deserts when she covers the eggs. We concludethat the higher the proportion of eggs a female can hide, thegreater her chance of becoming polyandrous