Selective Pressures That Lead to Asynchrony in Hatching
Student essay by Micha Lederman
Introduction
Among many species of birds, the young within the clutch do not hatch simultaneously. Instead, hatching can extend over a period of a several days.
The time of hatching is determined by the time of the onset of incubation. If incubation begins only after the last egg is laid, the eggs will hatch within a few hours of each other.
The question that then arises is: Why do some birds choose synchronous hatching as a survival strategy, while others choose asynchrony?
This paper will examine some of the reasons why asynchrony became the evolutionary selection for certain types of birds.
Methods
A library search produced articles on the subject in the magazines "Condor" and "Oak." Additional material was found through an Internet search.
Results
In general, there was a lot of material on bird migration and on the correct diet for birds. There was less material on the subject of the influence of habitat on survival. Remarkably, though, the least-studied field appeared to be that dealing with environmental pressures exerted on the nest immediately prior to hatching and up to the time of first flight.
Discussion
The selective pressures leading to the evolution of synchronous or asynchronous strategies would appear from the literature to be the product of a great number of factors. Stoleson and Beisinger (1995) list no fewer than 18 different hypotheses. Not all have been tested rigorously. This paper, therefore, will concentrate solely on those propositions for which there is currently adequate scientific evidence, or consensual agreement on the logical viability of the hypothesis to warrant reasoned discussion, comparison and further research; or where conflicting hypotheses have created significant disputes. The factors discussed below include incubation-onset, the availability of food, predation, the presence of parasites in the nest, the rate of growth of the hatchlings, the degree of sibling rivalry, the energy expended on feeding and in consuming food—and combinations of these elements.
Nowhere in the literature is there a theory ranking which of these factors is more or less important. Therefore, I have chosen, as much as possible, to treat the material chronologically, from the moment the egg is fertilized until the chick first flies.
It would appear that individual habitats play a significant role in the ranking of priorities for each species. For example, in habitats where the temperature or rainfall is more or less constant throughout the year, climate would appear to be a factor of less importance, while it may be of very great significance where there are wide seasonal swings in precipitation or temperature. The same is true for other variables such as the seasonal availability of certain preferred foods.
Although there are some very notable exceptions that will be discussed below, in general, observation has shown that birds which favour synchronous hatching as a strategy have more hatchlings.
In synchronously-hatched broods, since all of the chicks are about the same age, no single individual has an advantage in size or strength over any of the other chicks within the nest. This would appear to maximize the chances for species' survival. Each chick would consume the same amount of food, grow at roughly the same rate, be equal in size to every other, and relieve the parents of their duties at the same time. At first glance the strategy of synchronous, large-clutch hatching would appear to be the more advantageous one, since it would provide for the survival of the species through numbers of individuals. However, when all the environmental factors are taken into account, this simple game of addition may, in fact, be disadvantageous under many circumstances.
Some factors appear to be strictly related to the hen's physiology. Valkama et al (2002) found that the age and physical maturity of the hen can influence the rate of the asynchronous hatching. The younger it is the higher the rate of asynchronous hatching when compared to older and more experienced birds.
Even prior to hatching, the
ambient temperature can play a critical role in determining egg viability.
Veiga (1993) found that the eggs of house sparrows remained viable even when
exposed for short periods to temperatures below physiological zero (the
temperature at which the embryo begins to develop, 24-28 oC; Webb
1987). However, as the temperature rises to, but does not reach, the optimal
temperature for incubation (35-38oC; Webb 1987), the
eggs begin to lose their viability. It would seem, therefore, that eggs which
are exposed to this "danger zone," because they have been left alone
pending the arrival of the rest of the full clutch, would be under considerable
threat of not hatching.
In habitats where the surrounding climate is not stable and changes often, asynchrony may provide a better survival strategy. In synchronous clutches, the first-laid eggs may lie unincubated for as long as four days. They may thus be exposed to wide variants in temperature—both below the physiological zero and within the "danger zone." Vinuela (2000), in a controlled experiment on the synchronous clutches of black kites, in which the ambient temperature was varied, found that first-laid eggs had significantly lower hatching success. The author also notes that there may be a physiological limit to the length of time an egg may be maintained unincubated.
A conclusion that can arise from these findings is that if a bird lays her eggs and incubates them continuously over a longer period of time, there is a better chance for one of the chicks to catch a brief period of appropriate climate to enable it to survive should the parent be forced for one reason or another—such as attempts to draw predators away-to leave the nest unattended.
The subject of predation and brood reduction (nestling mortality by starvation or siblicide) has been one of considerable interest to researchers. Obviously, the presence of a single egg in the nest, with the probability of it being replaced by another shortly after it predation, would be a significant reason for asynchrony.
Clark and Wilson (1981) and Magrath (1990) also suggest that since, in asynchronous hatching, siblings spend less time in the nest together, the chances that one chick may survive—by not yet being hatched or by leaving the nest first—is greater. However, Valkana et al (2002) in their specific study of Tengmalm's owls, dispute this assertion.
It is generally agreed that there is greater hatching asynchrony among raptor species that practice obligate siblicide than those that practise facultative siblicide (Meyburg 1974; Edwards and Collopy 1983). Siblicide would appear to be a waste of energy and limited resource. Eggs are laid, hatched with care and then killed. However, when egg viability is taken into consideration, the cost may not be as extreme as it seems at first sight. If the first-laid egg does not hatch, there will be a second chance with a second egg. However, if the first does hatch, the chick, by now stronger and larger, need only kill its newly-arrived sibling to ensure its own food supply through prior monopolization of the resources provided by the parent.
Researchers clearly differentiate between obligate siblicide and facultative siblicide. In the former, the older chick kills the younger without any stress being placed upon it. In the latter, siblicide only occurs when there is a food shortage and one sibling kills another in order to assure itself of a continuous supply of nourishment in time of want.
Lack (1966, 1968), who studied the Great Tit, makes an important point: Parents are unable to feed the nestlings of synchronous broods differentially. Amundson and Slagsvold (1991b) in a review of 30 studies on 25 different species found that the actual frequencies for nestling survival were higher in synchronous than in asynchronous broods. However, the mean nestling body mass at fledgling time was generally higher in asynchronous than in synchronous broods. Eight different studies (Perrins 1965, 1988, Dhondt 1971, Garnett 1981, Drent 1984, Smith et al. 1989, Linden 1990, Tinbergen and Boerlijst 1990) have shown that there is a positive relationship between body mass, recorded just before fledging, and the subsequent survival rate. Amundson and Slagsvold (1998) suggest that synchronous hatching reduces brood reduction. When this occurs when food is scarce, while there may be more chicks, they fledge at very low body masses, and, consequently, have poor chances of survival. If hatching is synchronous and conditions are poor, and the brood size is reduced before all the nestlings have suffered severely, the reduced brood normally fledges in far better condition than is the case for non-reduced synchronous broods.
This would explain the rationale behind facultative siblicide during periods of want, but not obligate siblicide during periods when nesting takes place in good territory, predators are few, and the parasite load is low. Amundson and Slagsvold 1991a, Pijanowski 1992, and Weiebe and Bortolotti 1995 note, though, that, in any case, late-hatching offspring may suffer severely due to sibling competition—even when food is plentiful. Thus, partial feeding of a younger and weaker chick would be a waste of resources because the chick would be unable to reach optimal fledgling body mass; and its future survival would be in doubt.
The whole issue of food availability and its distribution within the family is a complex one. Valkama et al (2002) found that there is a direct correlation between the female's body mass and the rate of asynchronous hatching. Body size, is directly influenced by the quality of the territory in which nesting occurs. Poor territories produce less food, not only for the chick, but also for the hen prior to egg-laying.
Evolutionary strategies develop to cope with uncertainties. Parental resources required to nourish offspring may be unpredictable at the time fertilization occurs. Lack (1947) suggests that the best strategy might be to lay an optimistic clutch size and to begin incubation before laying is complete. This would lead to asynchronous hatching among a relatively large clutch. In times of food shortage, there would be a reduction of the brood to an optimum size through the elimination, by starvation or siblicide, of the youngest and weakest nestlings. Slatkin (1974) supports this view, especially for environments that are temporarily and spatially variable.
One area being investigated by some researchers is
whether parents can predict the availability of future food supplies and adjust
their nesting strategies accordingly. Valkama et al (2002) suggest that this
may be the case for the Tengmalm's owl, which feeds on voles. Vole populations
in northern
Synchronized hatching under these conditions of food scarcity would obviously lead to more fighting between the chicks for the available food. As mentioned above, all of the chicks would be more or less the same size. Lacking clear hierarchies, the ensuing occurrence of long and intense fights (Vinuela 1999) would only worsen the problem. The energy expenses of this competition would further exacerbate the already-slower nestling growth rate (Seddon and Van Heezik 1991; Wiebe and Bortolotti 1994b).
The means by which parents may determine clutch size and whether to hatch synchronously or asynchronously may be their own body condition, including the parasite loads in their individual nests ( Wiebe and Bortolotti 1994a; Wiebe 1995). Amundson (1998) points out that in his study some pairs fledged full broods of high quality, while others suffered brood reduction and/or severe reduction in fledgling quality. He attributes this primarily to the difference in the quality of parenting within the same species in the same study area.
Conclusion
It would appear that, as a strategy, asynchronous hatching works best in territories where there is a greater level of uncertainty with regard to dangers such as food supplies, ambient temperatures and other factors outside the immediate control of the parent birds. Synchroneity is a better strategy in more stable environments where the threat level is more predictable. Synchroneity, when combined with facultative siblicide, may be the best strategy for birds living in areas where the dangers are cyclical and only partially predictable.
However, the data available for making these judgments is tentative at best. Far more research over a broader spectrum of species and environments is necessary in order to make more accurate statistically-based judgments. Moreover, some critical factors have been ignored. For example, none of the studies reviewed here examined issues such as the cyclical rise and fall of predator populations and its impact on hatching strategies, bird population densities, fledgling time spent in the nest prior to first flight, the size of the territory necessary to provide the quantity of food needed and the time needed to search within that territory, a bird's breeding lifespan, and man's influence through the reduction in nesting areas.
References:
Amundsen T, Slagsvold T, (1991a) Asynchronous hatching in the Pied Flycatcher: an experiment. Ecology 92: 797- 804
Amundsen T, Slagsvold T, (1991b) Hatching asynchrony: facilitating adaptive or
maladaptive brood reduction. Acta Internatonal Ornithological Congress,
Christchurch, New Zealand XX: 1707-1719
Amundsen T, Slagsvold T, (1998) hatching asynchrony in great tits: a bet – hedging strategy. Ecology Jan.
Dhont A, (1971) The regulation of numbers in Belgian populations of Great Tits. Pages 532-547 in P.J. den Boer and J. R. Gradwell, editors. Dynamics of populations. Advanced study Institute, Pudoc, Wageningen, The Netherlands.
Drent, P. J, (1984) Mortality and dispersal in summer and its consequences for the density of Great Tits Parus major at the onset of autumn. Ardea 72:127-162
Edwards TC, Collopy MW, (1983) Obligate and facultative brood reduction in eagles: an examination of factors that influence fratricide. Auk 100:630-635
Eikenaar, C., Berg M. L. and Komdeur J, (2003) Experimental evidence for the influence of food availability on incubation attendance and hatching asynchrony in the Australian reed warbler Acrocephalus australis – J. Avian Biol. 34: 419-427
Garnett M. C, (1981) Body size, its heritability and influence on juvenile survival among great tits, Parus major. Ibis 123:31-41
Laaksonen Toni, (2004) Hatching asynchrony as a bet-hedging strategy – an offspring diversity hypothesis.
Lack D, (1947) The significance of clutch size. Ibis
89: 302352.
Lack D, (1966) Population studies of birds. Clarendon Press, Oxford,
UK.
Lack D, (1968) Ecological adaptations for breeding in birds. Methuen, London, UK.
Linden M. (1990) Quality or quantity? Fitness consequences of investment in individual offspring in the Great Tit Parus major. Chapter 2 in M. Linden, Reproductive investment and its fitness consequences in the Great Tit Parus major. Dissertation. Uppsala University, Uppsala, Sweden.
Magrath Robert, (1990) Hatching asynchrony and reproductive success in the black bird. Nature 339:536-538
Magrath Robert, (1990) Hatching asynchrony in atricial birds. Biol Rev 65:587-622
Meyburg B-U, (1974) Sibling aggression and mortality among nesting eagles. Ibis 116:224-228
Perrins
C.M, (1965) Population
fluctuations and clutch size in the Great Tit, Parus major L. journal of animal ecology 34:601-647
Perrins C.M, (1988) Survival of young Great Tits: relationships with weight. Acta International Ornithalogical congre, Ottawa,, Canada XIX: 892-898
Pijanowski, B. C, (1992) A
revision of Lack's brood reduction hypothesis. American Naturalist 139:
1270-1292
SeddonOJ, Van Heezik YM, (1991) Hatching asynchrony and brood
reduction in the jackass penguin: an experimental study. Animal behavior
42:347-356
Siegel Rodney, Weathers Wesley, Beissenger Steven, (1999) Hatching asynchrony reduces the duration, not magnitude, of peak load in breeding green rumped parrotlets (Forpus passerinus). Behavior Ecology Sociobiology 45:444-450
Slatkin M, (1974) Hedging one's
evolutionary bets. Nature 250: 704-705.
Smith, H. G., Kallander H., and Nilsson J.-A.. (1989) The trade-off between offspring number and quality in the Great Tit Parus major. Journal of Animal Ecology 58: 383-401.
Stoleson SH, Beissinger SR, (1995) Hatching asynchrony and the onset of incubation in birds, revisited. Curr Ornithology 12:191-270
Tinbergen, J. M., and Boerlijst M. C, (1990) Nestling weight and survival in individual Great Tits (Parus major). Journal of Animal Ecology 59:1113-1128
Valkama Jari, Korpimaki Erkki, Holm Aki, (2002) Hatching asynchrony and brood reduction in Tengmalm’s owl. Population ecology 133:334-341
Veiga JP, (1993) Does brood heat loss influence seasonal patterns of brood size and hatching asynchrony in the house sparrow? Ardeola 40:163-168
Vinuela Javier, (1999) Sibling aggression, hatching asynchrony, and nestling mortality in the black kite (Milvus migrans ). Behavior Ecol Sociobiology 45:33-45
Vinuela Javier, (2000) Opposing selective pressures on hatching asynchrony: egg viability brood reduction, and nestling growth. Behavior Ecology Sociobiology 48:333-343
Wiebe, K. L. (1995) Intraspecific variation
in hatching asynchrony: should birds manipulate hatching spans according to
food supply? Oikos 74: 453-462.
Wiebe,
K. L., and Bortolotti G. R., (1994a) Food supply and hatching spans of birds:
energy constraints or facultative manipulation? Ecology 75: 813-823.
Wiebe, K. L., and Bortolotti G. R., (1994b) Energetic efficiency of reprodution: the beneffits of asynchronous hatching for american Kestrels. Animal Ecology 63:551-560
Wiebe, K. L., and Bortolotti G. R, (1995) Food-dependent benefits of hatching asynchrony in American Kestrels Falco sparverius. Behavioral Ecology and Sociobiology 36:4957.