Seasonality in a temperate zone bird can be entrained by near equatorial photoperiods

Abstract

Birds use photoperiod to control the time of breeding and moult. However, it is unclear whether responses are dependent on absolute photoperiod, the direction and rate of change in photoperiod, or if photoperiod entrains a circannual clock. If starlings (Sturnus vulgaris) are kept on a constant photoperiod of 12h light:12h darkness per day (12L:12D), then they can show repeated cycles of gonadal maturation, regression and moult, which is evidence for a circannual clock. In this study, starlings kept on constant 11.5L:12.5D for 4 years or 12.5L:11.5D for 3 years showed no circannual cycles in gonadal maturation or moult. So, if there is a circannual clock, it is overridden by a modest deviation in photoperiod from 12L:12D. The responses to 11.5L:12.5D and 12.5L:11.5D were very different, the former perceived as a short photoperiod (birds were photosensitive for most of the time) and the latter as a long photoperiod (birds remained permanently photorefractory). Starlings were then kept on a schedule which ranged from 11.5L:12.5D in mid-winter to 12.5L:11.5D in mid-summer (simulating the annual cycle at 9 degrees N) for 3 years. These birds entrained precisely to calendar time and changes in testicular size and moult were similar to those of birds under a simulated cycle at 52 degrees N. These data show that birds are very sensitive to changes in photoperiod but that they do not simply respond to absolute photoperiod nor can they rely on a circannual clock. Instead, birds appear to respond to the shape of the annual change in photoperiod. This proximate control could operate from near equatorial latitudes and would account for similar seasonal timing in individuals of a species over a wide range of latitudes.

Figure 1

Changes in testis volume (open circles, left axis) and the occurrence of moult (solid circles, right axis) under constant 11.5 L : 12.5 D. Ten starlings were moved from an outdoor aviary in January 2000 to a constant photoperiod of 11.5 h light : 12.5 h darkness per day for 4 years. The birds are shown in the order of the number of moults undertaken during the 4 years, from 4 for bird no. 1 to 1 for bird nos. 9 and 10.

Figure 2

Changes in testis volume (open circles, left axis) and the occurrence of moult (solid circles, right axis) under constant 12.5 L : 11.5 D. Ten starlings were moved from an outdoor aviary in January 2002 to a constant photoperiod of 12.5 h light per day for 3 years.

Figure 3

Changes in testis volume (open circles, left axis) and the occurrence of moult (solid circles, right axis) in photoperiodic cycles simulating 9 °N. Twelve starlings were moved from an outdoor aviary in December 2002 to a photoperiodic regime that followed the natural annual pattern in photoperiod, but with an amplitude of 1 h, i.e. a mid-winter photoperiod of 11.5 L : 12.5 D and a mid-summer photoperiod of 12.5 L : 11.5 D, for 3 years. All the birds showed repeated cycles of testicular maturation, followed by complete regression and moult. Mean cycle length (inter-moult period) was 12.1±0.6 (s.d.) months.

Figure 4

Changes in testis volume (open circles, left axis) and the occurrence of moult (solid circles, right axis) in photoperiodic cycles simulating 9 °N and 52 °N. Mean±s.e. values for the 12 individual birds shown in figure 3 that were held on a simulated 9 °N photoperiodic regime (solid lines) superimposed on values for 12 control birds (broken lines) held on a photoperiodic regime simulating 52 °N (an amplitude of 9 h, i.e. a mid-winter photoperiod of 7.5 L : 16.5 D and a mid-summer photoperiod of 16.5 L : 7.5 D). Mean and s.e. for moult were calculated by regressing day on moult score (Dawson & Newton 2004). The timing of testicular maturation was slightly advanced in the second two cycles in the 9 °N birds, but the time of regression and moult was exactly the same as in the 52 °N birds.