Morné A. Du Plessis

Can species data only be appropriately used to conserve biodiversity

Despite widely acknowledged handicaps of the species approach to identifying priority conservation areas, many workers continue to use these flawed techniques as the backbone of their analyses. Species-based approaches address only a small part of biological diversity by ignoring different levels of organisation as well as the functional linkages among these levels. These data are often biased and incomplete and are often used in preference to data dealing with higher biological levels of organisation though the latter may be readily available. Within the framework of Nosss [(1990) Conservation Biology 4: 355–364] hierarchical definition of biodiversity (and Scott etal. [(1993) Wildlife Monographs 123: 1–31] gap analysis), we propose a top-down model dealing with broad organisational levels first, and finer-scale species distributions last. Note that we do not discard the latter approach, but merely argue for its use at a stage when, in our opinion, it adds most to the value of the prioritisation exercise. The model is flexible so that additional information, particularly those related to threats to biological diversity, can be added when they are available.

Helpers in colonial cooperatively breeding sociable weavers Philetairus socius contribute to buffer the effects of adverse breeding conditions

Some studies on the effects of helpers in cooperatively breeding vertebrates show a positive effect of helper presence on reproductive output whereas others find no effect. One possibility for this discrepancy is that helpers may have a positive effect when breeding conditions are adverse, while their effect might go unnoticed under good conditions. We investigate this hypothesis on sociable weavers Philetairus socius, a colonial cooperatively breeding passerine that inhabits a semi-arid region where breeding conditions vary markedly. We used multivariate mixed models to analyse the effect of helpers on reproduction under contrasting environmental and social conditions while controlling for parental and colony identity. We found that reproductive success in sociable weavers was primarily influenced by nest predation and rainfall. In addition, colony size was negatively associated with hatching and fledging success and number of young fledged per season. Helpers had a less prominent but significant influence on feeding rates and reproductive outcome. In agreement with expectations, the presence of helpers counteracted some of the negative effects of breeding in periods of low rainfall or in large colonies and was also associated with an increased number of young fledged per season. Our results illustrate that the effect of helpers might be detectable mostly under unfavourable conditions, but can contribute to improve reproductive performance in those situations.

Obligate cavity-roosting as a constraint on dispersal of green (red-billed) woodhoopoes: consequences for philopatry and the likelihood of inbreeding

SummaryI studied and compared life histories and dispersal patterns of two populations of green (red-billed) woodhoopoes Phoeniculus purpureus, occupying different habitats (with widely different vegetation, topography and climate), over a period of eight years (258 “flock years”) in the eastern Cape Province, South Africa. The birds are obligate cavity-roosters, and I performed an experiment in which several woodhoopoe territories were established after the introduction of artificial roost sites in an area which previously supported no woodhoopoes. The evidence supports the hypothesis that roost cavities are critical in determining the limits to woodhoopoe distribution. Group size, dispersal frequency and dispersal distance differed significantly between the two study sites. I suggest that where cavities are in short supply and unevenly distributed, long-distance scouting forays are undertaken at a high probability of not finding a safe roost site to sleep in. This may result in an increased probability of predation, or physiological conditions with which an individual in poor body condition cannot cope. I propose that among woodhoopoes a stay-and-foray dispersal strategy is preferred to a depart-and-search strategy for a number of reasons. First, there is no within-group competition for limiting resources, since roost cavities do not provide a situation in which within-group conflict could arise. Second, I show that competition for breeding vacancies is more severe at the inland than at the coastal site, further borne out by the fact that, compared to coastal birds, inland ones are proportionally more likely to attain breeding status in the natal group than elsewhere. Finally, I propose that such environmentally-induced differential dispersal patterns between the two study sites have indirectly affected the frequency with which apparent inbreeding occurs, and I suggest that since incestuous pairings are relatively common, the either have no deleterious consequences for breeder fitness, or the potential costs of inbreeding are counterbalanced by the risks associated with dispersal.

Unexpected sex ratio adjustment in a colonial cooperative bird: pairs with helpers produce more of the helping sex whereas pairs without helpers do not

For cooperatively breeding species, the most popular and intensely debated explanation for sex ratio variation is the helper repayment hypothesis. It predicts overall sex ratio bias towards the helping sex, and/or facultative adjustment towards the helping sex in absence of helpers. We tested these predictions in a colonial cooperative bird, the sociable weaver Philetairus socius. We quantified variation in overall sex ratio, and tested the effects of helping and local breeding conditions on individual sex ratio. To better understand the factors affecting sex ratio variation, we investigated sex-related differences in helping, dispersal behavior, and nestling morphology. Our results did not support the predictions of the helper repayment hypothesis. Indeed, although we found that males are the philopatric helping sex: (1) we did not detect a consistent bias in overall sex ratio; and (2), more importantly, it was pairs with helpers that produced additional males, not pairs without helpers. We propose and discuss different factors that could explain these results.

Territorial vocal rallying in the green woodhoopoe: factors affecting contest length and outcome

Most studies of asymmetric contests have focused on interactions between individuals. We examined territorial contests between groups of green woodhoopoes, Phoeniculus purpureus, which take the form of vocal rallying displays. The distribution of encounter durations was bimodal: interactions were generally either decided within 5 min (short contests) or took longer than 15 min to reach an outcome (extended contests). As short contests progressed, there was an escalation in the length of rallies, and these encounters were longer when the competing groups were more evenly matched in size. Residents won the majority of short contests, whereas the difference in the sizes of the competing groups was not a significant predictor of the outcome. The resident group appeared to match the rally length given by the intruding group, tending to lose the contest when it no longer achieved this. In extended contests, there was no further escalation in rally length after the first 5 min. There was also no resident advantage, but larger groups were more likely to win. These extended contests may be a test of stamina. To achieve a similar length of rallying, individuals in smaller groups must contribute more effort than those in larger groups. We discuss the possibility that physiological constraints prevent smaller groups from sustaining the same level of rallying as larger groups, and that this difference decides the outcome of extended contests. We consider the different possible functions of short and extended contests, and discuss our findings in relation to existing models of agonistic interactions.

Dual function of allopreening in the cooperatively breeding green woodhoopoe, Phoeniculus purpureus

Allopreening is a widespread but little-studied phenomenon in birds and is particularly prevalent in species where individuals are forced into close proximity. Such a situation facilitates the transfer of ectoparasites between individuals and allopreening has therefore been proposed to serve a hygienic function. In addition, allopreening might theoretically play a role in social communication. Green woodhoopoes (Phoeniculus purpureus) are cooperatively breeding birds that roost communally every night in a tree cavity and are thus susceptible to high ectoparasite loads. Our results suggest that allopreening of the head and neck (“head”), which cannot be efficiently self-preened, serves a primarily hygienic function: all individuals, irrespective of sex, dominance status, and group size, donated and received similar rates of head allopreening and terminated a similar proportion of bouts in which they were involved. Furthermore, there was a high occurrence of reciprocation and head allopreening occurred at a constant rate throughout the year. In contrast, allopreening of the rest of the body, which is accessible to the recipient itself, is likely to serve a primarily social function: body allopreening rates were higher in larger groups; dominant individuals received more body allopreening and terminated a significantly higher proportion of bouts than subordinates; and subordinates donated body allopreening at a higher rate than dominants. Moreover, bouts initiated by dominants were more likely to be reciprocated than those initiated by subordinates and body allopreening rates varied seasonally. Allopreening in the green woodhoopoe is therefore likely to serve a dual function, depending on the part of the body involved.

Delayed breeding affects lifetime reproductive success differently in male and female green woodhoopoes.

In cooperatively breeding species, many individuals only start breeding long after reaching physiological maturity [1], and this delay is expected to reduce lifetime reproductive success (LRS) [1-3]. Although many studies have investigated how nonbreeding helpers might mitigate the assumed cost of delayed breeding (reviewed in [3]), few have directly quantified the cost itself [4, 5] (but see [6, 7]). Moreover, although life-history tradeoffs frequently influence the sexes in profoundly different ways [8, 9], it has been generally assumed that males and females are similarly affected by a delayed start to breeding [7]. Here, we use 24 years of data to investigate the sex-specific cost of delayed breeding in the cooperatively breeding green woodhoopoe (Phoeniculus purpureus) and show that age at first breeding is related to LRS differently in males and females. As is traditionally expected, males that started to breed earlier in life had greater LRS than those that started later. However, females showed the opposite pattern: Those individuals that started to breed later in life actually had greater LRS than those that started earlier. In both sexes, the association between age at first breeding and LRS was driven by differences in breeding-career length, rather than per-season productivity. We hypothesize that the high mortality rate of young female breeders, and thus their short breeding careers, is related to a reduced ability to deal with the high physiological costs of reproduction in this species. These results demonstrate the importance of considering sex-specific reproductive costs when estimating the payoffs of life-history decisions and bring into question the long-held assumption that delayed breeding is necessarily costly.

The influence of nest-site characteristics on the nesting success of the Karoo Prinia (Prinia maculosa)

Choice of nest site has important consequences for nest survival. We examined nest-site characteristics relative to nest success in Karoo Prinias breeding in coastal dwarf shrubland, where high nest predation is the main cause of nest failure. Initially, we compared nests that failed during the building, laying, incubation and nestling stages and those from which young were successfully raised, to test whether nests that survived to progressive stages in the nesting cycle differed in their nest-site characteristics. Subsequently, we compared the characteristics of successful nests with those of unsuccessful nests. The nest-site characteristics considered included nest height, nest-plant height, nest-plant species, distance from lateral foliage edge, nest concealment, nest-patch heterogeneity and vegetation cover at four different heights. We were unable to distinguish between the nest-site characteristics of nests that failed during the various stages of the nesting cycle. Concealment was the main nest-site characteristic that differentiated successful nests from unsuccessful nests, with successful nests being located in more concealed sites. The other variables that contributed to the discrimination between successful and unsuccessful nests by discriminant function analysis included nest-plant type and distance from edge, which are also directly related to concealment. This suggests that nest concealment is the most important variable influencing nesting success at this site, which has a preponderance of visually-oriented predators.

Black and white under the South African sun: are juvenile Cape gannets heat stressed?

(1) We measured dorsal surface temperature (Td), skin temperature (Ts) and body temperature (Tb) of adult (white plumage) and juvenile (black plumage) Cape gannets, Morus capensis, subject to high ambient temperatures. (2) Mean Td and Tb of juveniles were significantly elevated compared to those of adults. (3) Mean Ts of older juveniles, which had down beneath their body feathers, were significantly lower than Ts of younger juveniles, indicating that the down provided insulation. (4) Juveniles spent a significantly greater proportion of time thermoregulating via evaporative cooling than adults. (5) The high thermal load of the dark plumage is presumably associated with major costs for the thermal and water balance of juveniles.

The effect of helpers on artificially increased brood size in sociable weavers (Philetairus socius)

In cooperatively breeding birds, the presence of helpers is expected to increase the reproductive success of the breeding pair. However, some studies fail to find this effect. A positive effect of helpers may be restricted to cases in which a breeding pair has a poor likelihood of raising the entire brood on its own, as would be the case under stressful environmental conditions or with enlarged brood sizes. We conducted brood size manipulations in a cooperative breeder, the sociable weaver, Philetairus socius, to investigate the relationship between the difficulty of raising nestlings and the effort and impact of helpers. Overall, sociable weavers did not work harder to raise the enlarged broods. However, the presence of helpers significantly increased the feeding rates at enlarged nests, but not controls. This was insufficient to prevent generalised brood reduction in enlarged broods, whether attended by pairs alone or with helpers. Nonetheless, the presence of helpers was associated with decreased nestling mortality and an increase in the numbers of young fledged. Our results suggest that groups are better able to respond to the needs of enlarged broods than pairs alone and that the presence of helpers has a beneficial effect on overall reproductive success.