Eggs, egg formation and the timing of breeding

The eggs of birds contain all the nutrients necessary for the developing embryo. The female has to obtain these nutrients, and while in some species these may be gathered and stored over a period of time, in many species they seem to be obtained on a daily basis. Obtaining the calcium necessary for the eggshell may pose special problems for some species, which may have to give up valuable feeding time in order to search for calcium.
Egg size varies considerably, even within species, and the reasons for this are not clear. In the Great Tit Pants major, large eggs produce nestlings which have a higher chance of hatching, surviving to fledging and surviving after fledging. However, large eggs require the female to acquire more nutrients to form them, and large chicks may sometimes be at a disadvantage. The amount of energy required to form a larger egg often seems small unless the metabolic costs of egg formation are taken into account.