The Defence of First and Second Broods by Great Tit (Parus major) Parents: A Test of Predictive Sociobiology¹

Abstract

and Summary We examined the extent to which parental investment, as measured by brood defence, is determined by selection via life history in a short‐lived bird, the great tit (Parus major). Great tit parents tending 1st and 2nd broods of the season were used to test five predictions of a cost/benefit model of brood defence based on the species average demography. The benefit was envisaged as the brood's contribution to a parent's fitness, and the cost as the potential loss if the defender dies in the act of defence; this loss was mirrored by Residual Reproductive Value plus a fraction of the brood dying as a consequence of the defender's death. Univariate and multivariate procedures were applied to six measures of defence response in 221 experimentally naive great tit pairs with nestlings. Stimuli consisted of a live raptor and a multi‐species taped mobbing chorus (both of which triggered strong alarm), the latter alone, and a novel, visual stimulus inhibiting nest visits for some time. As predicted by the model, response strength and associated risk increased with (1) advancing time in the breeding season, (2) the age of young, and (3) the number of young in 2nd broods. The last finding, being non‐trivial, is the strongest and the only unequivocal support for the notion that life‐history has moulded parental investment also in a short‐lived species; findings (1) and (2) could be alternatively accounted for by simply assuming that parents tailor their defence to environmental conditions permitting a new breeding episode. The model failed to predict the change of defence behaviour between 1st and 2nd broods. Instead, proximate factors coupled to the precise breeding area proved to be of prime importance in determining defence level, thereby falsifying the idea of a rigidly preprogrammed change of response level based on population parameter means. Instead, parents might gauge their defence to quality of young, as also allowed for in the model, or they may sometimes be constrained to display the full defence. As a by‐product, the experiments permit us to narrow down the range of selective agents producing the male parent's stronger defence. The sex difference, being prevalent in 1st broods and abating in 2nd broods, indicates that the male, by its defence behaviour, not only invests in its offspring but also in its home range and/or its mate. The latter interpretation is supported most directly by the male's defence behaviour in 1st broods.

Temporal increase in nest defence intensity of the willow tit (Parus montanus): parental investment or methodological artifact?

“…In stonechats (Saxicola torquata) Greig-Smith (1980) found that the intensity of nest defence behavior, having increased during the first breeding attempt, dropped to a low level at the beginning and increased again during the course of the second brood. The same pattern has been found in barn swallows (Hirundo rustica) by Moller (1984), in great tits by Curio et al (1984), and in song sparrows by Weatherhead (1989). These findings support the PI hypothesis instead of the revisitation hypothesis.…”

“…Instead, the intensity was correlated with earlier visits at the nests, and the results were explained as positive reinforcement and loss of fear (Knight and Temple 1986b). Earlier, however, although not particularly testing the revisitation hypothesis, Regelmann and Curio (1983) and Curio et al (1984) had found that naive great tits (Parus major) increased their nest defence intensity towards a live predator model during the nestling cycle.…”

Parental risk management in relation to offspring defence: bad news for kids

“…It is already demonstrated that parental renesting potential may play a major role for risk taking. There is evidence that an adult that is more likely to nest again should risk less for its current brood than a bird with a lower renesting potential [60,73,74]. Unfortunately, we do not have any information about the age of our birds to draw any conclusions along this line.…”

Risk-taking by female ducks: intra- and interspecific tests of nest defense theory

“…flushing distance should decrease) with larger clutch size and advancing stage of incubation because both reflect increased clutch value (Montgomerie and Weatherhead 1988;Redondo and Carranza 1989). In addition, we test the prediction that risk taking should increase as renesting potential within the breeding season decreases (Barash 1975); this prediction has been tested several times with passerine birds, but thus far has received little support (Weatherhead 1982;Regelmann and Curio 1983;Curio et al 1984). We also assess the possibility of a methodological artifact: that our previous nest visits influenced risk taking by female ducks, as envisioned by Knight and Temple (1986 a, b).…”