AbstractOther workers have shown that gonadal hormones can stimulate avian nest building and that there are species differences concerning the identity of the efficacious hormones. Nest building may be stimulated by estrogenic but not by androgenic material in one species. In another species, the converse is true. Budgerigars do not build nests. Their eggs are laid in tree-cavities. The female performs an easily quantified behaviour that is oriented to her prospective egg-laying site (a nestbox in the laboratory). This nestbox-oriented behaviour (NBOB) consists of remaining within the nestbox for various intervals throughout the day. This occurs daily, and she spends progressively more time within the nestbox as the day of initial oviposition approaches. Thus, NBOB is temporally and situationally related to nest building. A male rarely enters a nestbox unless he is engaging in courtship activities oriented to his mate. This report concerns laboratory studies that were conducted to determine: (1) the effects of different quantities of exogenous testosterone, estradiol and progesterone upon the NBOB of male and female budgerigars when they were individually-caged and unable to see or to hear members of the opposite sex, and (2) the effects of prior breeding experience(s) upon hormonally-induced NBOB. Four experiental types were studied: (V) virgins of either sex which, since fledging, had been visually isolated from the opposite sex and nestboxes; (Ex) males and females which had participated in at least one successful breeding cycle prior to this study; (V1) virgin females which were induced to perform NBOB but to maintain undeveloped ovaries; and (V2) virgin females which were induced to undergo full ovarian development and oviposition in the absence of nestboxes. Both intact and castrated males were studied. Ovarian hormones were given only to castrated males. No ovariectomized females were studied. Birds were injected thrice weekly and observed for 3 weeks. 1) Sexual identity and hormonal factors. Estradiol with or without progesterone stimulated NBOB by V and Ex males and females. The presence or absense of testicular androgens did not induce any male to perform NBOB. Larger (1.0 mg) quantities of testosterone induced females to perform NBOB, but such NBOB was atypically erratic. Testosterone-induced NBOB by females may have been a more direct manifestation of a testosterone-increased ovarian activity; however, the oviducts and ovaries of females receiving either 0.5 mg of 1.0 mg quantities of testosterone were not significantly heavier or larger than those of controls receiving only oil. Progesterone, alone, was just as ineffective as was the oil placebo: neither promoted any significant NBOB by males or females. These findings suggest that NBOB and nest building are not only related in temporal and situational ways, but share a common endocrinological denominator as well. Since NBOB appears to be primarily influenced by increased plasma levels of estrogenic material rather than by decreased levels of androgenic material, the NBOB of burgerigars is similar to the nest building of canaries and ring doves and diametrical to the androgen-stimulated nest building of black-crowned night herons. Estradiol with or without progesterone prompted females but not males to perform advanced phases of NBOB. Also females performed many phases of NBOB sooner than did males. Thus, males seem to be (genetically) less responsive to hormonal stimuli prompting NBOB than are females. 2) Experiential factors. In general, V birds of both sexes began to perform each phase of NBOB later and spent less time in nestbox occupation than did Ex birds receiving identical treatments. A previous study showed that Ex females, stimulated by either visual or vocal male courtship displays performed NBOB sooner than did V females. This prompted me to compare the ovarian follicle sizes and oviductal weights between Ex and V females receiving identical treatments and to examine the hormonally induced NBOB of V1 and V2 females. Since there were no significant differences in the ovarian and oviductal measurements between V and Ex females receiving identical injections, the differential response in the NBOB of V and Ex females does not seem to be solely due to a difference in the development of their reproductive tracts. Accordingly, we cannot say that male courtship more readily promotes NBOB with Ex than with V females because V females require more male stimulation in order to attain a given endogenous hormonal level or physiological state than do Ex females. Indeed prior experience may affect neural thresholds for response to given endogenous hormonal states without altering the response of reproductive organs. Perhaps Ex females are more readily induced to perform NBOB due to some factor involved in previous NBOB or a general familiarity with nestboxes. Data on V1 and V2 females supports this latter hypothesis. The onsets of each phase of NBOB and the amount of nestbox occupation were both potentiated by prior cxperience(s) concerning nestboxes. Prior experience(s) concerning heterosexual interactions or full ovarian activity and oviposition did not significantly affect hormonally induced NBOB.
Factors affecting individual variation in nest-defense intensity in colonially breeding Black-tailed Gulls (Larus crassirostris)