How do tropical birds know when to breed?

In most temperate zone areas birds breed in spring, when food resources critical for offspring survival are optimal. Favorable environmental conditions occur in most temperate habitats at a similar time each year and are preceded by an increase in daylength (photoperiod). Correspondingly, most temperate birds have evolved a strong response to photoperiodic cues for the timing of reproduction.

Seasonal changes in environmental conditions in tropical habitats can be very different from those in the temperate zones: The year-to-year variation in the occurrence of favorable environmental conditions for breeding can be high, and changes in daylength very small. How can tropical birds anticipate and prepare for the breeding season?

We study these questions using two avian model species from contrasting environments: Spotted Antbirds (Hylophylax n. naevioides) from the rainforest understory in Panama (latitude: 9°N) and Darwin's Finches (Geospiza spp.) from the arid Galápagos archipelago in Ecuador (latitude: 0°N).

Spotted Antbirds: timing of breeding in a seasonal rainforest

Since 1995, we have been studying the proximate mechanisms of seasonal breeding and year-round territoriality in Spotted Antbirds in Panama. We conduct our field work in Sobernia National Park in central Panama, at the Smithsonian Tropical Research Institute.

Our data indicate that Spotted Antbirds are regular seasonal breeders, much like typical temperate zone species. Furthermore, these birds appear capable of using the slight changes in the tropical photoperiod to initiate reproduction, which suggests an amazing sensory adaptation to life in the tropics.

Our field and experimental data suggest that Spotted Antbirds use photoperiodic information to activate their reproductive system in advance of the rainy season. Furthermore, the birds also respond to short-term environmental signals such as food availability to fine tune reproductive activity with local environmental conditions. In wet years with an early onset of the rainy season and an overall higher food supply Spotted Antbirds grow their gonads faster and breed earlier than in dry years with low food abundance.

Food cues also have a very interesting rapid effect on the behavior of male Spotted Antbirds. When the birds are allowed to feed on live crickets (in addition to a standard diet and live mealworms) their song activity is significantly increased compared to if live crickets are omitted from their diet. It remains to be tested whether this dramatic behavioral response to crickets and the increased rate of gonadal development under conditions of high food abundance are regulated by similar mechanisms.

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Darwin's Finches: timing of breeding in an unpredictable environment.

Since 1997, we have been investigating the temporal control of reproduction in four species of Darwin's Finches from two climatically different islands. We study Small ground finches (Geospiza fuliginosa), Cactus finches (G. scandens), Small tree finches (Camarhynchus parvulus) and Warbler finches (Certhidea olivacea).

Darwin's Finches live in an arid climate directly at the equator, where the unpredictable seasonal rainfall stands in sharp contrast to the more predictable rainforest habitat of Spotted Antbirds in Panama. This project is a collaboration with Martin Wikelski and Eberhard and Helga Gwinner (Research Centre for Ornithology, Max-Planck Institute, Germany). In Galápagos, we work at the Charles-Darwin-Research-Station.

One fascinating finding is that all four species of finches show strong opportunistic breeding capabilities and adjust breeding times quite flexibly to climatic (arid versus wet) conditions. Our current experimental studies on Darwin's Finches will allow us to understand how 'opportunistic' breeding is physiologically regulated and how it differs from the seasonal breeding of other tropical and temperate zone birds.

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Captive Spotted Antbirds increased testis volume, follicle diameter (mean ± SE) and song activity in response to photostimulation with an increase in photoperiod by one hour. The control group (open symbols) remained on 12 h light per day throughout the experiment, while the second group (colored symbols) was exposed to an abrupt increase in photoperiod by one hour (start of the photoperiodic treatment indicated by vertical line; from Hau et al., 1998).



Male Spotted Antbirds had significantly higher song activity when they had access to live crickets in their diet, which always included a standard insectivore diet and live mealworms. Song activity was very low when live crickets were omitted from their diet, as well as when dead crickets were given. The latter result suggests that nutritional cues do not stimulate song activity in these birds. However, when birds were given live crickets in a dish covered with clear plastic foil ('see cricket' treatment), song activity increased to intermediate levels, suggesting that visual cues stimulate song activity in these birds. From Hau et al. 2000.