Likewise, early hatching chicks are the first to develop begging calls. This strategy clearly confers an advantage for the early hatching chicks ( Leonard and Horn, 1996 Glassey and Forbes, 2002) as they are bigger than their younger nestmates and may more easily receive food from their parents because of their bigger beaks and greater ability to compete for the best places during feeding. Starting incubation before the last egg is laid leads to asynchronous hatching of the offspring, the amount of asynchrony depending on the time of the incubation start. This may have a direct advantage for parents because the period where they have to care for food is shorter compared with the strategy of most avian species ( Lack, 1968) to begin incubation after laying the first egg or at least before laying the last one. By this strategy, the offspring hatch synchroneously because the embryo cannot start to develop without incubation. Many birds, including nearly all precocial species, wait to the start of incubation until the last egg is laid. These strategies can be seen as to install some rank order between the chicks (some of the chicks are more fit than the others and have a higher chance to survive) and to optimize the parental investments in some way, to guarantee the survival of the maximum number of offspring and to keep the costs of rearing to a minimum ( Lack, 1968 Slagsvold, 1986 Williams, 1994 Adkins-Regan et al., 2013 Deeming and Reynolds, 2015).Ĭontrol of the offspring development continues after hatching. There are also cases where the last egg layed is the biggest one. It has also been shown that in some cases the size of the eggs depends on the order of laying, the first egg being bigger than the following, etc. In some species, during egg production mothers are able to control, e.g., the volume of the egg yolk, which is necessary for nutrition of the embryo, or the hormone level of individual eggs. The development of avian chicks is controlled by the parents in a variety of ways. Our observations suggest that the amount of care given to each chick may be equated with such factors as a camouflage effect of the down feathers, and that the low illumination within the nest also complicates the determination of the hatching order by the parents. We found that there were effects of asynchronous hatching, but these were smaller than expected and mostly not significant. To determine whether such processes in fact occur in the zebra finch, we observed chick development in 18 clutches of zebra finches. This may result in differences in parental care and in sibling competition based on body size differences among older and younger chicks, which in turn might produce asynchronous development among siblings favoring the first hatchling, and further affect the development and fitness of the chicks after fledging. In some songbirds and some other species, however, incubation starts immediately after the first egg is laid, and the chicks thus hatch asynchronously. Birds with multiple eggs in a single clutch often begin incubating when most eggs are laid, thereby reducing time of incubation, nursing burden, and sibling competition. The mode of hatching in birds has important impacts on both parents and chicks, including the costs and risks of breeding for parents, and sibling competition in a clutch.
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