Testosterone and corticosterone during the breeding cycle of equatorial and European stonechats (Saxicola torquata axillaris and S. t. rubicola).

Northern-temperate male birds show seasonal changes in testosterone concentrations with a peak during the breeding season. Many tropical birds express much lower concentrations of testosterone with slight elevations during breeding. Here we describe testosterone and corticosterone concentrations of male stonechats from equatorial Kenya during different substages of breeding and molt. This tropical species has a short breeding season of approximately 3 months. We compare their hormone concentrations to previously published data of males of a northern-temperate relative, the European stonechat, also a seasonal breeder but with a breeding season of approximately 5 months. Equatorial stonechats show a pronounced peak of testosterone during the nest-building and laying stage. During all other stages, testosterone concentrations are low, similar to other year-round territorial tropical bird species. Corticosterone concentrations peak also during the nest-building and laying stage suggesting that this period of maximum female fecundity is a demanding period for the male. Equatorial stonechats have significantly lower concentrations of testosterone than European stonechats during all stages, except during the nest-building and laying stage. During this stage of maximum female fertility, testosterone levels tend to be higher in equatorial than in European stonechats. Our results suggest that equatorial stonechats belong to a group of tropical bird species that are characterized by a short breeding season and a brief high peak of testosterone during the female's fertile period. Such brief, but substantial peaks of testosterone may be common in tropical birds, but they may easily be missed if the exact breeding stage of individual birds is not known.