THE NESTING SEASONS OF CENTRAL AMERICAN BIRDS IN RELATION TO CLIMATE AND FOOD SUPPLY

Summary. This study is based upon records of about 1900 nests, mostly in Costa Rica and Guatemala. For Central America as a whole, and the birds as a whole, the peak of the nesting season falls between the vernal equinox and the summer solstice, with April the principal month for laying. An important, although minor, number of nests is found in March and in July and August, especially at lower altitudes. Relatively very few birds nest during the half of the year from September to February, inclusive. At high altitudes the concentration of nesting in April, May and June is more pronounced than in the lowlands. In southern Central America the number of birds nesting increases more gradually during the early months of the year than in northern Central America, where the rise of nesting activity in late March is more abrupt. On the whole, the number of species singing and the volume of song they produce is a good indication of the amount of nesting. The song of many birds is almost restricted to their season of reproduction, though some birds sing freely at other limes of the year. Certain groups of birds depart radically from the general mass in their time of breeding. Nectar‐drinkers, including hummingbirds and the honeycreepers Diglossa and Coereba , nest chiefly at the beginning of the dry season—from November to February, according to the locality—when flowers are most abundant. Grass‐seed‐eaters nest later than the majority of the birds, waiting for the seeds of the grasses which spring up with the rains. Where the dry season continues into May, birds that hunt food in the ground litter wait until this has been moistened by the rains, and so nest later than other species. In numerous other instances the special dietary habits of a bird appear to explain its distinct nesting season. April to June seems to be the principal nesting season because this is when, for the majority of birds, food, both vegetable and insect, is most abundant. But if for any species with peculiar diet food is more plentiful at some other time of year, it may nest then in spite of apparently inimical weather. Thus in the high mountains hummingbirds rear their young when nights are frosty, and small ground‐feeders nest beneath chilly, drenching rains. Minor factors which make the second quarter of the year peculiarly favourable for nesting in Central America are (1) the withdrawal of the countless migrants to North America, (2) the circumstance that the destruction of the birds' habitats by the cutting and burning of brush and woodland, to clear ground for crops, is largely completed before this time. In the highlands of western Guatemala, where the rains do not come until mid‐May, most of the birds breed during the driest period of the year; but in southern Costa Rica, where wet weather returns in March or early April, they nest during the first three months of the rainy season. No single climatic feature, such as length or intensity of daylight, alteration of wet and dry seasons, fluctuations of temperature, etc., appears to regulate the initiation of breeding among the birds as a whole. A number of species begin to nest earlier or later as the rains come earlier or later; others start at about the same date each year, in spite of annual fluctuations in the rain. For the former it is suggested that the regulation of the date of reproduction is both internal and external. Internally controlled processes carry organic development to a certain stage at which it pauses, awaiting some stimulus associated with the return of the rains, as with the resting flower‐buds of certain plants. For birds whose date of nesting is independent of the weather, the timing mechanism is more difficult to visualize.