Dr Rob Simmons & Prof. Phil Hockey
Dr Susan Cunningham (PFIAO)
Dr Jim Dale
Dr Thomas Martin (University of Montana)
Dr Rowan Martin
Dr Âkos Pogány (Eotvos Lorand University, Hungary)
Dr Claire Spottiswoode (University of Cambridge)
Dr Tamas Szekely (Bath University, UK)
Dr René van Dijk (Bath University, UK)
Life-history strategies describe the anatomical, physiological and behavioural adaptations control how individuals invest in reproduction and self-maintenance in response to their environmental conditions (see also Co-operative Breeding and Sociality in Birds). A major challenge in evolutionary biology is to explain why life-history strategies vary among species along a slow-fast continuum. Species at the slow end of the spectrum are characterised by slow metabolism and development, delayed reproduction, low reproductive investment, long life, and long-term pair bonds, with the opposite expression at the fast end. The South African south-temperate avifauna comprises species with life-history strategies that span much of the slow-fast continuum, making it an ideal region in which to study environmental influences on life-history strategies. Koeberg Nature Reserve, on the West Coast north of Cape Town, is the site of a long-term research programme examining the life history strategies of a community of southern hemisphere bird species.
Life-history and ecological correlates of reproductive investment
The first major publication arising from the Koeberg study (Martin et al. 2006) examined potential causes of geographical variation in investment in egg mass and clutch size among 74 passerine bird species from four different regions of the world. A central prediction of life-history theory, that egg mass increases as clutch size decreases, was supported. However, there was considerable variation in this relationship between geographic regions. A further prediction that increased egg predation can directly favour reduced reproductive investment was also supported. South African birds, which are exposed to substantially higher nest predation risk, produce smaller eggs than birds of North and South America with an equivalent clutch size. Also, investment in total clutch mass decreased with increased nest predation risk within all four regions. Finally, we found that species with increased parental care, where the male and female share incubation, produced larger eggs than species with female-only incubation.
A central focus of the Koeberg study, to determine annual adult mortality rates of a variety of species, is now nearing completion. With the invaluable help of a group of volunteer bird ringers from the Tygerberg Bird Club, over 2,000 adults of 20 species have been ringed with individually unique colour-ring combinations since 2001. The annual re-sighting and territory-mapping of all colour-ringed birds over the past six years now allows us to reliably estimate annual adult survival for 18 species. These data are being used to examine the influence of adult mortality risk on life history strategies, particularly investment in reproduction and mating strategies.
Greater Kestrel responses to increased nest predation risk
Dr Rob Simmons continues with long-term monitoring of a Greater Kestrel population nesting on telephone poles in other-wise tree-less habitat in the Northern Cape. The kestrels use old crow nests as nest sites, yet crows also prey on kestrel eggs. One focus of the study is testing the prediction from life-history theory that kestrels nesting close to crows should decrease clutch size in response to increased predation risk from crows. The study is also monitoring the impact of a range expansion by Pied Crows into the area, joining the resident Black Crows, which is expected to further increase nest predation risk to kestrels.
Environmental and parental influences on embryonic development
Variation in embryonic development rate, measured as incubation period length, among passerine birds is thought to be influenced largely by body mass (slower development in larger species) and intrinsic physiological constraints linked to offspring quality (slower development for higher offspring quality). The potential additional influences of embryonic mortality rate (higher mortality risk selecting for faster development) and parental influences on embryonic temperature (lower parental nest attentiveness and incubation temperature leading to slower development) remain controversial. Research at Koeberg led by Thomas Martin (University of Montana) is examining variation in incubation parameters among 18 species in the community in relation to environmental temperature, nest attentiveness, and age-specific mortality to better understand the relative importance of these different influences.
Strategies of maternal investment in egg constituents
Bird mothers can tailor the future developmental trajectories of their offspring to expected environmental conditions by varying their investment in egg nutrients, immunoglobulins, yolk hormones and carotenoids. After a spell of maternity leave at the beginning of the year, Dr Corine Eising continued her work on a variety of projects investigating the relationship between environmental factors and maternal effects. First, she has been testing a prediction of sexual selection theory that females paired with good quality males invest more heavily in their offspring, as they have a greater chance of success. Preliminary results show that female Bar-throated Apalis in better condition lay larger eggs, larger clutches, and are paired with males with larger chest bands. Yolk hormone levels vary with laying order, but after controlling for maternal body size do not seem to be related to male chest-band size as a measure of quality. Second, she finalised two papers with collaborators at the University of Antwerp (Belgium), University of Groningen (Netherlands), and Washington State University (USA) describing studies that experimentally manipulated breeding density of European Starlings and Black-headed Gulls, to test the prediction that females would invest more testosterone in their eggs when breeding at higher density, to prime their young for a more competitive social environment once they hatch. These predictions were supported. Finally, she has been working on a further two collaborative papers, one (with Claire Spottiswoode) examining maternal investment in yolk hormones and carotenoids by Sociable Weaver females nesting in colonies of varying size, and the second examining patterns of maternal investment in yolk hormones in the Australian Brush Turkey.
Hormonal control of plumage expression in Red-billed Queleas
Male Red-billed Queleas exhibit one of several discrete plumage types when in breeding condition. These different morphs do not appear to correlate with male quality, the traditional explanation. Instead, father-son correlations of plumage morphology suggest a strong genetic basis. Indeed the slopes of the father-son regressions are so high they strongly suggest that females mate assortatively – females carrying the genes of a particular morph mate preferentially with a male of that morph, even though females do not express the plumage polymorphism. One possible mechanism to explain this is that daughters might imprint on the plumage morph of their father, and use this as a basis for selecting a future mate. Dr Jim Dale and Dr Corine Eising have initiated a new study this year to investigate the mechanism of polymorphism maintenance in this species.
Sexual conflict in birds: comparative behavioural analyses of north- and south-temperate penduline tits
Sexual conflict, which stems from the antagonistic interests of males and females during breeding, is a powerful evolutionary process that is thought to be important in the evolution of body size, appearance, and behaviour. As part of a new bilateral Hungary/South Africa project, Dr Tamas Szekely (Bath University, UK), together with PhD students René van Dijk (Bath University, UK) and Âkos Pogány (Eotvos Lorand University, Hungary) spent a month at Koeberg Nature Reserve. This project aims to test predictions of sexual conflict theory by comparing the behaviour, plumage and ecology of the Eurasian Penduline Tit (Remiz pendulinus) in Hungary and Cape Penduline Tit (Anthoscopus minutus) in South Africa. Male and female Eurasian penduline tits co-operate in building the nest, but either the male or the female deserts at the end of egg-laying to pursue additional mating opportunities, leaving the other parent to incubate the eggs and raise the chicks alone. The Cape Penduline Tit, on the other hand, is monogamous, and both parents share the duties of incubation and chick rearing. The project is testing the prediction that greater sexual conflict in the Eurasian Penduline Tit leads to greater sexual selection (plumage elaboration and more complex song in the male). It will also investigate how sexual conflict is mediated by differences in life history and the environments (particularly food availability) that these two species experience.