The Use of Food Samples from Sea Birds in the Study of Seasonal Variation in the Surface Fauna of Tropical Oceanic Areas

Many parts of the tropical oceans appear to be relatively seasonless, but, because of the difficulty of sampling mobile and patchily distributed animals and the cost of oceanographic investigations, few data are available on the extent of seasonal changes. By regularly collecting regurgitations from sea birds, and identifying and measuring the food items, seasonal data could be obtained on the availability, size classes, and perhaps reproductive cycles of the fish and squid characteristic of the surface layer of tropical seas. Flying fish (Exocoetidae), juvenile tunas (Scombridae), and squid of the family Ommastrephidae are especially easily obtainable. Experience gained during a recent study of the comparative feeding ecology of sea birds on Christmas Island (Pacific Ocean) makes it possible to assess the characteristics of bird species which affect their suitability for such study. Potentially useful species include terns (especially Sterna fuscata, Anous stolidas, A. tenuirostris, and Gygis alba) and boobies (especially Sula Sula). Samples could be obtained from several bird species in the same period, and a program could include sampling of inshore waters with the neuston net and making basic oceanographic observations. Investigations of this kind could be carried out economically on any of a large number of tropical oceanic islands

Control of the annual cycle in birds: endocrine constraints and plasticity in response to ecological variability

This paper reviews information from ecological and physiological studies to assess how extrinsic factors can modulate intrinsic physiological processes. The annual cycle of birds is made up of a sequence of life-history stages: breeding, moult and migration. Each stage has evolved to occur at the optimum time and to last for the whole duration of time available. Some species have predictable breeding seasons, others are more flexible and some breed opportunistically in response to unpredictable food availability. Photoperiod is the principal environmental cue used to time each stage, allowing birds to adapt their physiology in advance of predictable environmental changes. Physiological (neuroendocrine and endocrine) plasticity allows non-photoperiodic cues to modulate timing to enable individuals to cope with, and benefit from, short-term environmental variability. Although the timing and duration of the period of full gonadal maturation is principally controlled by photoperiod, non-photoperiodic cues, such as temperature, rainfall or food availability, could potentially modulate the exact time of breeding either by fine-tuning the time of egg-laying within the period of full gonadal maturity or, more fundamentally, by modulating gonadal maturation and/or regression. The timing of gonadal regression affects the time of the start of moult, which in turn may affect the duration of the moult. There are many areas of uncertainty. Future integrated studies are required to assess the scope for flexibility in life-history strategies as this will have a critical bearing on whether birds can adapt sufficiently rapidly to anthropogenic environmental changes, in particular climate change