Veasey, J.S., Houston, D.C. & Metcalfe, N.B. 1998. Egg production, flight performance and predation risk in small birds. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 219.

Take off speed and manoeuvrability are of critical importance to the escape response of most bird species. Such parameters are likely to be affected by changes in weight and muscle condition occurring during egg formation. We investigated the effects of reproductive effort upon alarmed flight performance, and subsequently predation risk in the Zebra Finch Taeniopygia guttata. Using a technique to assess pectoral muscle condition in live birds, we demonstrated that female muscle loss during breeding was dependant upon clutch size and protein content of the bird's pre-breeding diet. Changes in muscle condition were found to positively correlate with changes in flight performance, whereby trials with the greatest declines in muscle condition were associated with declines in flight performance toward the end of a breeding attempt, where as those with the least decline in muscle condition, were associated with improvements in flight performance, despite similar changes in mass. This finding confirms previous work which suggested that the pectoral muscles of females may contain a protein storage component for egg production. Only after approximately 8% of muscle tissue is lost are declines in flight performance seen, suggesting that muscle stores represent a pre-breeding cost of reproduction, in terms of reduced pre-breeding flight performance. This experiment also suggests the existence of a trade-off between reproduction and predation risk, such that clutches laid may be below the Lack clutch.

Key words: clutch, Zebra Finch, Taeniopygia guttata, trade-off, pectoral muscle

Westerdahl, H., Bensch, S., Hansson, B., Hasselquist, D. & von Schantz, T. 1998. Primary females invest in a higher proportion of sons in the polygynous Great Reed Warbler. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 220.

The Great Reed Warbler Acrocephalus arundinaceus population at Lake Kvismaren, Sweden is small and relatively isolated. Natal fidelity is high for a small songbird wintering in Africa. If females can record the operational sex ratio (OSR) in the population it would be advantageous for them to respond to it and adjust the sex ratio of their broods accordingly. The sex of 1027 fledglings (from 243 broods) was determined using single primer PCR. Contrary to the expectation, females that arrived late in the season and had the possibility to assess the population OSR, produced a higher proportion of sons when the OSR was male biased than when the population OSR was female biased. The Great Reed Warbler is a polygynous species where non-primary females usually get limited feeding assistance from the male. This results in chick starvation and sons are particularly vulnerable, as they are larger. In contrast primary females are usually assisted by the male and they produce a higher proportion of sons in their broods. When the population is female-biased a larger proportion of the females will be of non-primary mating status and this may explain why females invest in more daughters when the population is female-biased.

Key words: polygyny, sex ratio, female status, operational sex ratios

Brooke, M. de L., Davies, N.B. & Noble, D.G. 1998. Short-term variation in discrimination by Eurasian Reed-Warblers against cuckoo eggs. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 220 - 221.

Because the activities of parasites usually reduce the reproductive success of their hosts, selection will favour those host responses that minimise the reduction in success. As brood parasites change in abundance, the ideal host response may therefore alter. This ideal response may be determined genetically or facultatively. Prior studies of variation in host response, over time or between geographically distant sites, have often assumed the variation is genetically based. Using experiments involving placing model cuckoo Cuculus canorus eggs in Eurasian Reed-Warbler Acrocephalus scirpaceus nests in the Fens of England, we show that the Eurasian Reed-Warblers have become significantly less discriminating in the period between two series of experiments, in 1985 to 1986 and 1997. Since the extent of the change is so great as to preclude a wholly genetic explanation, we suggest the warblers have reset their discrimination threshold, a facultative response that is advantageous as parasitism rates have declined from 1985 to 1997.

Key words: brood parasitism, cuckoo, reed-warbler, discrimination threshold, egg mimicry

Gardarsson, A. & Einarsson, A. 1998. A comparative study of fluctuations in numbers of breeding ducks at Lake Myvatn, Iceland. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 221.

Several thousand ducks of fifteen species breed at Myvatn, Iceland. They fluctuate widely in annual numbers and reproductive output. The relationship of production and density of several duck species to food and weather has been monitored annually for 24 years. Our initial question was whether population changes were caused by events at Myvatn or by outside factors (most of the populations are migratory). Local factors prevailed, but responses of different species varied. In this paper we compare population limitation in several species. Reproductive success of Eurasian Wigeon Anas penelope, Tufted Duck Aythya fuligula, Greater Scaup Aythya marila, Black Scoter Melanitta nigra and Harlequin Duck Histrionicus histrionicus was largely explained by variation in food availability. Food in the previous summer and production of young up to four years earlier mainly determined change in numbers of most species. Current food apparently determined numbers of Red-breasted Merganser Mergus serrator. Despite large short-term fluctuations, most species did not show any trend in numbers through the period 1975 to 1998, however, Harlequin Ducks and Red-breasted Mergansers increased significantly and Barrow's Goldeneye Bucephala islandica decreased. Again, long-term trends appear to be determined largely by local resources.

Key words: waterfowl, Anatidae, production, population limitation, food

McCleery, R.H. & Woodburn, R.W. 1998. Global warming, food supply and breeding success in Great and Blue Tits in Wytham woods. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 222.

There have been many previous correlational and manipulative studies of the relationship between food supply, temperature and the timing of breeding, and reproductive success, but few have looked in detail at the food supply. The Wytham tit populations have bred earlier since 1970 as spring temperatures have increased. Average daily temperature may increase by 3 °C in the UK by 2010, due to global warming. We used data from nest-balances, gut contents, nest cameras, behavioural observations and canopy sampling to look at the detailed mechanisms involved. Caterpillars are available long before laying, but resources for eggs are gathered rapidly once a critical prey density is reached. Rates of food delivery increase, and adults lose body weight, up to day eight after hatching. Experimentally delayed broods were fed more pupae, and were lighter at fledging. Blue tits had a competitive advantage later in the season. Rainfall during the nestling period reduced fledging weights. A 3 °C increase means earlier caterpillar emergence and a shorter breeding season. We calculate that these species could breed earlier but might have to cope with a shorter season by reducing clutch sizes, starting incubation before clutch completion or laying earlier relative to the food supply.

Key words: timing, global warming, food supply, breeding success, tits

Guentert, M., Eyer, D. & Lueps, P. 1998. The Mute Swan's knob: An indicator of body condition and parasite load?. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 222.

Ornamental structures in animals may signal genetic quality, body condition or resistance to parasites (Zahavi, A. 1977. J. Theor. Biol. 67: 603-605; Hamilton, W.D. & Zuk, M. 1982. Science 218: 384-387). Knob size in male Mute Swans Cygnus olor correlates with body mass, but not with body size. It is therefore a potential indicator of body condition or healthiness and is predicted to be negatively correlated with parasite load. To test this hypothesis we measured body size, organ weights and fat scores of 43 free-living swans and determined their infestation with ecto- and endo-parasites. Parasite loads were negatively correlated with knob size and with body condition. According to signal theory (Zahavi, A. 1991. Anim. Behav. 42: 501-503) reliable signals must be costly. Knob size is assumed to be mediated by testosterone; the cost might consist of increased sex-hormone levels needed to maintain it. Since steroid hormones reduce immunocompetence (Folstad, I. & Karter, A.J. 1992. Amer. Nat. 193: 603-622), a trade-off between producing a big knob and dampening the immune system is likely. Only high quality individuals resistant against prevailing parasites can afford high testosterone levels. Whether Mute Swans actually use knob size as a cue to assess the quality of conspecifics remains to be shown.

Key words: signalling, handicap, sexual selection, ornament, Cygnus olor