S11.Summary: Hatching asynchrony and the onset of incubation: Beyond brood reduction
Steven R. Beissinger1 & Tore Slagsvold2
1Department of Environmental Science, Policy & Management, 151 Hilgard Hall #3110, University of California, Berkeley, CA 94720, USA, fax 510 643 3946, e-mail beis@nature.berkeley.edu; 2Division of Zoology, Department of Biology, University of Oslo, PO Box 1050, Blindern, N-0316, Oslo, Norway.
Beissinger, S.R. & Slagsvold, T. 1999. Hatching asynchrony and the onset of incubation: Beyond brood reduction. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 582-583. Johannesburg: BirdLife South Africa.
In most animals the behaviour of parents often has little effect on the time between emergence of their first and last young. Usually offspring from a reproductive bout emerge relatively synchronously within a relatively short time of each other compared to the time required for their development. Synchronous reproduction tends to occur in vertebrates with internal fertilisation and development (e.g. some fishes, snakes, and most mammals), and with external fertilisation or development, where all zygotes are subject to the same environmental conditions (e.g. many anurans and fishes).
Birds, in contrast, are unusual because parental behaviour is thought to control hatching intervals. Avian parents may influence the onset of development of eggs by determining when to warm them by initiating incubation (White, F.N. & Kinney, J.L. 1974. Science 186:107-115; Drent, R.H. 1975. In: Farner, D.S. & King, J.R. (eds) Avian Biology, vol. 5: 333-420. New York, Academic Press). Birds can only lay one egg daily so if incubation is initiated before the last egg is laid, a clutch may hatch over a few days to several weeks (Lack, D. 1968. Ecological adaptations for breeding birds. Methuen & Co., London; O'Connor, R.J. 1984. The growth and development of birds. Chichester, UK; Wiley & Sons; Beissinger, S.R. & Waltman, J.R. 1991. Auk 108: 863-871.). Early incubation results in offspring of different sizes and this size hierarchy often contributes directly to the death of the last-hatched young (O'Connor 1978, Mock, D.W et al. 1990. American Scientist 78: 438-449.). This illustrates the ‘Paradox of Hatching Asynchrony’: avian parents are unique in having some ability to control birthing intervals but many opt for a strategy that seems maladaptive (Stoleson, S.H. & Beissinger, S.R. 1995. In: Power, D.M. (ed.) Current Ornithology. Vol. 12; Plenum Press: 191-270.).
The Brood Reduction Hypothesis (Lack 1968) has dominated the way in which ornithologists interpreted hatching asynchrony, but it has not been particularly successful at explaining hatching patterns. Most studies of hatching asynchrony have focused on identifying an adaptive function for the nestling size disparities that result from asynchronous hatching, with equivocal success. Synchronously hatched nests often produce as many or more offspring as asynchronously hatched nests, and in many cases the nestlings are of similar quality (Amundsen, T. & Slagsvold, T. 1991. Ecology 72: 797-804; Nilsson, J.-Å . 1995. Journal of Avian Biology 26: 255-259.; Stoleson, S.H. & Beissinger, S.R. 1997. Ecological Monographs 67: 131-154). Recent debate has questioned whether asynchronous hatching confers survival advantages for parents or selected offspring after hatching, or is a result of physiological constraints, resource limitations, and behavioral factors that affect the onset of incubation. Perhaps due to the lack of explanatory success of any single hypothesis, more than 17 hypotheses have been proposed to explain early incubation in birds (Stoleson & Beissinger 1995).
The five papers presented in this symposium summarise the new directions and multifaceted approaches that characterise recent studies of hatching asynchrony. Viñuela and Carrascal present a preliminary analysis of hatching patterns in birds and the factors that affect them using modern comparative approaches that partition variability into phylogenetic and nonphylogenetic components. Our understanding of the effects of conflict between the sexes over the onset of incubation and the need to begin incubation to maintain the viability of eggs are summarised by Amundsen and Stoleson, respectively. Murray uses his nontraditional approach to evolution to examine the conditions that should promote different hatching patterns. Finally, Beissinger examines how three dominant processes (egg viability, brood reduction, nest failure) interact to affect the onset of incubation.