Reproductive potential in hole-nesting birds such as cockatiels is often limited by nest-site availability. Thus, reproductive activity in captive cockatiels can be stimulated or discouraged by presenting or withdrawing nest-boxes. Whether environmental manipulation can be used to augment the reproductive stimulus of the nest-box has been studied by only a few investigators (Woodard and Morrissey, 1980, referred to by Grau and Roudybush, 1984). In diverse species environmental factors such as day length, light intensity, temperature, humidity and availability of nutrients, whether providing initial predictive cues or essential supplementary information can influence the progression of the reproductive cycle (Wingfield, 1983). Following a period of adverse weather or inadequate food supplies, increases in these factors tend to accelerate or strengthen the reproductive effort, as measured by date of first nest occupancy, date of first egg, clutch length, and egg size.
To increase the reproductive efficiency of our cockatiel colony we devised a program of environmental changes so the presentation of the nest-box would follow a series of changes, each having the potential of strengthening reproductive effort. Lacking clues as to which factor(s) might stimulate reproductive effort and lacking resources to test each factor separately and with proper controls, we adopted a shot-gun approach and simultaneously altered several parameters. It must be remembered that the results reported below were obtained without satisfactory controls and should be interpreted with appropriate caution.
Materials and Methods
Thirty-six pairs of cockatiels were housed in wire cages (30 x 30 x 60 cm) in a room provided with artificial lighting, heating, and evaporative cooling. In 1982-83 the diet consisted of mixed seed, mineral blocks, and vitamin supplementation in the drinking water. No environmental changes were made. Reproduction was elicited by presenting nest-boxes constructed from stainless steel or plywood. The photoperiod provided a minimum of 14 hr light, but was supplemented by natural daylight through ventilation screens in each end of the room. In 1984 the diet was changed to a crumbled diet (Roudybush et al., 1984) and the room was made light proof during the cooler portions of the year (November through March). The effect of the change in diet on the results reported below is not known.
The program of environmental changes we devised and tested in 1984, Trial #4, is presented in Table 1. Birds were first exposed to a 10-12 week period of time with no minimum temperature control, or humidity supplementation and with environmental conditions shown in Table 1, Day 0 (“winter”). Five factors were then changed over a two-week period as indicated.
In 1984, in Trial #5, the same procedure was followed with the exception that during the “winter” one half of the pairs received only millet, while the other half received the conventional crumbled diet.
A summary of the results of five trials - three trials before environmental manipulation was used and two trials after - is presented in Table 2. We observed a striking increase in three of the parameters measured after environmental manipulation: (1) percent of all pairs laying increased about two to four times; (2) mean clutch size increased by three to four times; and (3) the number of eggs laid within 21 days of nest-box presentation increased by three to seven times. Chick weight at hatch also increased by more than 15%. In contrast, latency to first egg, fertility, and hatchability were similar in the five trials. No effect of "winter" diet (millet vs. crumbles) was detected in 1984, Trial #5.
As these results were obtained without suitable controls, we must consider other factors that might account for the success we observed. First, the flock aged by two years during the course of this work and experience is known to improve reproductive performance. Second, in 1984 each of the pairs in both trials were proven breeders; in 1982-83 the history of numerous pairs was unknown. Third, the diet changed between 1982-83 and 1984. While both diets were considered to be nutritionally adequate, they had not been tested for effects on reproduction. Fourth, these factors may have interacted to produce these results. Breeding in an outdoor aviary at our facility was also substantially improved in 1984 compared to previous years, although the improvement was not as great as that reported above. In addition, 1984 trials at the outdoor aviary took place at a time when environmental conditions were changing from the extremes of winter and summer to milder conditions of spring and fall.
In summary, while these results may have resulted from an interaction of the uncontrolled factors mentioned above, the uniform and dramatic increase observed in two successive trials suggests that environmental manipulation per se may substantially improve reproductive performance in captive cockatiels.
1. Roudybush, T.E., C.R. Grau, T. Jerinin and D. Nearenberg. 1984. Pelleted and
crumbled diets for cockatiels. Feedstuffs 56:18-20.
2. Grau, C.R. and T.E. Roudybush. 1983. Cage-bird research at Davis. California
3. Wingfield, J.C. 1983. Environmental and endocrine control of avian reproduction: an
ecological approach. In “Avian Endocrinology - Environmental and Ecological
Perspectives,” S. Mikami, K. Homnia and H. Wada, eds. Japan Sci. Soc. Press,
Springer-Verlag, New York., pp. 265-288.