Climate, isolation and intraspecific competition affect morphological traits in an endangered steppe bird, the Dupont’s Lark Chersophilus duponti

ABSTRACT

Capsule: We assess biometric variation in the threatened Dupont’s Lark along a wide fraction of its distribution area analysing the largest data set for this bird species available to date, comprising a 28-year period and including birds captured in Spain, Morocco and Tunisia.

Aims: To analyse Dupont’s Lark morphology evaluating five potential sources of divergence: (I) sexual dimorphism, (II) macro-regional differences, (III) climate, (IV) isolation and (V) intraspecific competition.

Methods: Multivariate analysis was used to summarize biometric data. Sexual dimorphism and macro-regional divergence were assessed by generalized linear mixed models. Climate, isolation and intraspecific competition effects on phenotype were explored by means of model averaging.

Results: Sexes differed in wing shape suggesting a sexual selection pressure on males for aerial displays. Males showed longer bills after controlling for body size. We found an increasing Spain < Morocco < Tunisia trend in body size and bill length and volume. Tunisian populations showed more pointed wings than Moroccan and Iberian ones. Maximum temperature increased bill size as predicted by Allen’s rule. We found support for Bergmann’s rule in relation to temperature in the breeding season and water availability. Intraspecific competition was associated with more rounded wing shapes suggesting a pressure related to breeding display performance. Isolation reduced wingtip pointedness.

Conclusion: We found convincing results for wing morphology variation in relation to intrasexual competition, as males seem to be subjected to a sexual selection pressure for aerial display and this adaptation strengthens when intraspecific competition increases. We also found solid support for Allen’s rule, with results suggesting that bill size plays an important role in the bird’s thermoregulation, which does not occur in the case of tarsus. Although Bergmann’s rule is not supported in relation to minimum annual temperature, we found a relationship with thermal conditions in the breeding season, as well as water availability.

Acknowledgments

We wish to express our acknowledgement to Francisco Quico Suárez, who led the Dupont’s Lark research group until his early death in 2010. Many people collected data in the field; we especially wish to thank: A. Agirre, R. Aymí, M. Calero, E. Carriles, J. T. García, I. Hervás, J. H. Justribó, E. G. de la Morena, J. J. Oñate and J. Viñuela. We also want to thank two anonymous reviewers who notably improved the original manuscript with their comments and suggestions. All birds were captured and processed following the Wild Birds Ringing Manual (Pinilla 2000) and under the correspondent official licenses.

Additional information

Funding

Data used in this paper come from different projects, in particular, the II National Census (2004–2006), granted by the Spanish Ministry of Agriculture, Food and the Environment, and the Dupont’s Lark Monitoring Program in Medinaceli Region, funded by Fundación Patrimonio Natural de Castilla y León. This is a contribution to the Excellence Network Remedinal 3CM (S2013/MAE­2719), supported by Comunidad de Madrid, and to the projects ‘Scientific basis for the National Conservation for Dupont’s Lark in Spain’, supported by Fundación Biodiversidad, of the Ministry of Agriculture, Food and Environment, LIFE-Ricotí project (LIFE15-NAT-ES-000802), supported by the European Commission, and BBVA-Dron Ricotí, funded by the BBVA Foundation.