Elsevier

Animal Behaviour

Volume 198, April 2023, Pages 107-116
Animal Behaviour

Contrasting effects of cooperative group size and number of helpers on maternal investment in eggs and nestlings

https://doi.org/10.1016/j.anbehav.2023.01.013Get rights and content

Highlights

  • Maternal pre- and posthatching investment is associated with different social cues.

  • Maternal egg investment (egg size) increased with group size.

  • Maternal feeding rates decreased with number of helpers.

  • Fledging success increased with group size.

  • Wing length, but not condition, of fledglings increased with number of helpers.

Females are predicted to adjust their reproductive investment to optimize the trade-off between current and future reproduction. In many cooperatively breeding birds, females have been shown to reduce their investment both pre- and posthatching in response to the presence of food-provisioning helpers. However, in species where not all group members help during the posthatching stage, it is currently unclear to which social cues (i.e. group size versus number of helpers) females should tune their investment. Here, we used the cooperatively breeding placid greenbul, Phyllastrephus placidus, as a model species to examine whether and how group size and number of helpers affect female investment in eggs and food provisioning. We found that females used a contrasting strategy pre- and posthatching in response to different social cues: they laid larger eggs in larger groups while reducing their feeding rate when assisted by more helpers. We also found that fledging success increased with group size and that nestlings raised in groups with helpers fledged with longer wings but found no relation between condition of the young and number of helpers. Since the perceived contrasting investment strategies may have several underlying causes, we conclude that predicting the influence of social conditions on maternal investment may be more challenging than previously believed.

Section snippets

Study System

We conducted our study in the indigenous cloud forest archipelago of the Taita Hills, southeast Kenya (3°25′S, 38°20′E), where the medium-sized, insectivorous placid greenbul inhabits dense cloud forest understory. This habitat occurs in a highly scattered and fragmented pattern in the study area, where the landscape is dominated by exotic plantations and small-scale subsistence agriculture (Burgess et al., 2007; Lovett & Wasser, 1993; Pellikka et al., 2009). Cloud forest remnants vary in both

Prehatching Maternal Investment

When examining variation in egg size using the reduced data set for which the number of helpers could be determined (Table 1), the model with group size was better supported than the model including number of helpers (ΔAICc = 2.42). However, since the null model was as well supported as the model with group size (AIC weight: 0.43 and 0.44, respectively), evidence for group size as a predictor of egg size variation was inconclusive for this data set. Yet, when including all nests that reached the

Discussion

We have shown that females of a tropical cooperative breeder modulate both their pre- and posthatching reproductive investment in relation to social conditions. Contrary to our predictions, females with more potential helpers in the group did not systematically reduce their investment during the breeding event. Rather, they employed an additive strategy during the prehatching stage by producing larger eggs in larger groups, while they practised load lightening in the posthatching stage by

Author Contributions

D.V.L. and L.C. analysed the data and drafted the manuscript. D.V.L. and B.A. collected the data. M.G. facilitated data collection. L.L. and E.M. designed the study and helped draft the manuscript. All authors critically revised the manuscript and gave final approval for publication.

Data Availability

Code and data to reproduce all analyses and figures will be made available on request.

Declaration of Interest

The authors declare no competing interests.

Acknowledgments

We thank the Kenyan government, the Kenya Wildlife service and the Kenya Forest Service for permitting research in the Taita Hills and L. Chovu, P. Kafusi, J. Kiiru, A. Mwakulombe, O. Mwakesi, M. Makomba and V. Otieno Onyango for fieldwork. Kenya Forest Service kindly facilitated access to the forest fragments. We also thank Peter Njoroge and the National Museums of Kenya for support. This work was funded by the Fonds Wetenschappelijk Onderzoek (FWO) grant G.0308.13N, the Alexander von Humboldt

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