Juvenile photorefractoriness in starlings, Sturnus vulgaris , is not caused by long days after hatching

Abstract

Juvenile starlings, Sturnus vulgaris , do not become sexually mature until they are almost 12 months old because they are photorefractory, and therefore need to experience short days before they can respond to long days. In adults, photorefractoriness is caused by long days, but it is unclear whether juvenile photorefractoriness is also caused by long days, or if young are already photorefractory when they hatch. Earlier experiments involving hand-reared young starlings under different daylengths were inconclusive because it proved impossible to rear birds younger than 4 days old. This experiment adopted a different approach. Nestboxes used by free-living starlings were modified to exclude most external light. The interiors were illuminated by lights producing the same light intensity as in unmodified nestboxes. These lights were switched on for 8 h each day (8 h light : 16 h darkness) (8L : 16D) from the beginning of the breeding season. Four-day-old nestlings were taken from these boxes, hand-reared under 8L : 16D, and either kept under 8L : 16D or transferred to 16L : 8D at 3 weeks, 6 weeks or 9.5 weeks of age. Control nestlings from unmodified nest boxes were reared under 16L : 8D. In birds transferred to 16L : 8D at 3 weeks of age, when they were fully grown, there was a slight increase in circulating luteinizing hormone (LH) but no increase in testis size in males. In birds transferred at 6 weeks, LH increased and there was a rapid low-amplitude cycle of testicular growth and regression. In birds transferred at 9.5 weeks, the change in LH and the testicular cycle were as great as in photosensitive adults. Birds reared under continuous 16L : 8D showed no change in LH or testicular size. At 20 weeks of age, the response to exogenous gonadotrophin-releasing hormone (GnRH - I) varied with previous exposure to short days. These data show that starlings hatch and develop in a photorefractory state; juvenile photorefractoriness is not caused by long days after hatching.