Brood patch

Abstract: The time spent each day on the nest and the rate of formation of the brood patch before the onset of incubation were measured in bantams (Gallus domesticus) and related to changes in the concentrations of plasma LH and prolactin. The hens spent progressively more time on the nest in the 5 days before the onset of incubation so that by the first day of incubation they were spending more than 90% of their time in this way. The concentration of plasma prolactin increased while that of LH fell on successive days before the onset of incubation: the increase in plasma prolactin preceded the fall in plasma LH by 2 days. The formation of the brood patch closely followed the increase in the concentration of plasma prolactin. In four out of five bantams the increase in nesting behavior was preceded by an increase in the secretion of prolactin. An injection of chicken prolactin antiserum into bantams incubating eggs resulted in a significant (P less than 0.05) increase in the plasma concentration of LH. The observations suggest that, in the bantam, the onset of incubation is initiated by an increase in the secretion of prolactin which also suppresses the secretion of LH.

Abstract: A general model of animal heat exchange is modified for the special case of an incubating bird and applied to data describing the microclimate and nest/clutch/parent complex of the mountain white-crowned sparrow in southeastern Oregon. Heat exchange is mainly dependent upon meteorological factors and the measured thermal resistances of the nest and the bird's body. It is relatively insensitive to variation in a number of less exactly quantified factors, such as the thermal resistances of the eggs and the brood patch. Resting energy expenditure is estimated for a typical 24-h cycle and found to average 15% lower in an incubating female than in a bird perching outside of the nest but exposed to the same microclimate.

Abstract: Incubation conditions for eggs influence offspring quality and reproductive success. One way in which parents regulate brooding conditions is by balancing the thermal requirements of embryos with time spent away from the nest for self-maintenance. Age related changes in embryo thermal tolerance would thus be expected to shape parental incubation behavior. We use data from unmanipulated Black-capped Chickadee (Poecile atricapillus) nests to examine the temporal dynamics of incubation, testing the prediction that increased heat flux from eggs as embryos age influences female incubation behavior and/or physiology to minimize temperature fluctuations. We found that the rate of heat loss from eggs increased with embryo age. Females responded to increased egg cooling rates by altering incubation rhythms (more frequent, shorter on- and off- bouts), but not brood patch temperature. Consequently, as embryos aged, females were able to increase mean egg temperature and decrease variation in temperature. Our findings highlight the need to view full incubation as more than a static rhythm; rather, it is a temporally dynamic and finely adjustable parental behavior. Furthermore, from a methodological perspective, intra- and inter-specific comparisons of incubation rhythms and average egg temperatures should control for the stage of incubation.

Abstract: The control of clutch size is examined in several taxa of birds and the concept that species can be classified as determinate or indeterminate layers is reevaluated. Determinate layers are defined as species in which extrinsic factors perceived by the female, such as eggs present in the nest, are not involved in determining either the number of largeyolky follicles produced by the ovary or the number offollicles ovulated (e.g., albatrosses, auks and pigeons). In some species, the number of largeyolkyfollicles produced is unrelated to extrinsicfactors, but contact between thefemale and her eggs usually reduces the number offollicles ovulated and hence the clutch size. These species are classified as semideterminate layers (penguins). Among indeterminate layers, both the number of largeyolky follicles and the number offollicles ovulated depend upon extrinsic factors. Three categories of indeterminate layers are recognized, depending upon the nature of the stimulus that stops egg laying. The first is tactile contact between the female's brood patch and the eggs; it is the most common. This category may be subdivided according to the number of eggs required to stimulate the brood patch. In some species egg removal leaving no egg in the nest is requiredfor the female to lay extra eggs (contact with a Single egg sufficient: type S, woodpeckers, plovers); others may lay extra eggs even if an egg is left in the nest (contact with Multiple eggs required: type M, kestrels, hawks, grebes). The second category of stimuli causing laying to cease, observed in parasitic birds, is probably the lack of suitable host nests. These visual cues trigger either reabsorption of all largeyolky follicles (Discontinuous production of largeyolky follicles: type D, parasitic cuckoos) or simply reabsortion of one or two largeyolkyfollicles (Continuous production of largeyolky follicles: type C, parasitic cowbirds). The third category, used by megapodes, could be thermal information derivedfrom their nest mounds. Each category can be discriminated by observations on egg laying and appropriate egg removal experiments. Anatomical data from laying females are also a valuable source of information. The endocrine mechanism of clutch-size determination is still poorly understood; it has been linked to the onset of incubation behavior and in particular to a rise in prolactin levels. I argue that the rise in prolactin levels associated with incubation behavior might play a role in the determination of clutch size in tactile indeterminate layers, but not in determinate layers. Inhibition of gonadotrophin secretion constitutes another hypothesis that might be relevant whatever the mode of clutch-size control. Finally, a new hypothesis is proposed to account for the evolution of the control of clutch size in birds that relates determinate and indeterminate laying patterns tofood availability at the time of eggformation, and variability of the best time for raisingyoung.