A NEW EQUATION FOR CALCULATING REPRODUCTIVE SUCCESS OF CLUTCHES AS A FUNCTION OF THE DAY ON WHICH INCUBATION STARTS: SOME IMPLICATIONS

Auk, The, Jul 2006 by Murray, Bertram G Jr

ABSTRACT.-

Many species of birds start incubation before the last egg of a clutch is laid, which often results in asynchronous hatching and fledging. The incubation pattern favored by natural selection should lead to greater reproductive success than alternative patterns. Equations for calculating the reproductive success of a clutch as a function of the start of incubation have been proposed by Clark and Wilson (1981) and Hussell (1985). Both equations predicted a greater frequency of an earlier start of incubation than occurs in nature. Here, I present a new equation in which the relevant parameters determining reproductive success as a function of the start of incubation are (1) the probability of any young leaving a nest and (2) the post-first-fledging daily survival rates of the remaining nestlings. This equation is universally applicable and accounts for the greater frequency of incubation starting later, rather than earlier, in the laying sequence of most birds. I use the equation to evaluate various hypotheses regarding the evolution of incubation patterns.

Received 13 December 2004, accepted 23 September 2005.

Key words: asynchronous hatching, incubation behavior, successful reproduction hypothesis, synchronous hatching, viability hypothesis.

Una Nueva Ecuación para Calcular el Éxito Reproductivo de Nidadas en Función del Día en que Comienza la Incubación: Algunas Implicaciones

RESUMEN.-Muchas especies de aves comienzan a incubar antes de poner el último huevo de la nidada, lo que generalmente resulta en una eclosión y salida del nido asincrónica de los volantones. Un patrón de incubación favorecido por la selección natural debería llevar a un éxito reproductivo mayor que los patrones de incubación alternativos. Clark y Wilson (1981) y Hussel (1985) han propuesto ecuaciones para calcular el éxito reproductivo de una nidada en función del inicio de la incubación. Ambas ecuaciones predicen una frecuencia mayor de inicio adelantado de la incubación de lo que ocurre en la naturaleza. Aquí presento una nueva ecuación en la que los parámetros relevantes que determinan el éxito reproductivo en función del inicio de la incubación son (1) la probabilidad de que cualquier polluelo deje el nido y (2) la tasa de supervivencia diaria posterior a la salida del primer volantón del nido para el resto de los polluelos. Esta ecuación es aplicable universalmente y da cuenta de una mayor frecuencia de inicio tardío, más que de un inicio adelantado, de la incubación en la secuencia de puesta de la mayoría de las aves. Utilizo esta ecuación para evaluar varias hipótesis acerca de la evolución de los patrones de incubación.

(ProQuest Information and Learning: ... denotes formulae omitted.)

EACH SPECIES OF bird characteristically begins incubation with a particular egg in the laying sequence. In anatids, phasianids, and other nidifugous species, incubation begins with the laying of the last egg. Rarely, it begins with the first egg, as in the Green-rumped Parrotlet (Forpus passerinus) reported by Beissinger and Waltman (1991). Most commonly, incubation begins with the penultimate egg, especially in passerines (Lack 1968, Clark and Wilson 1981). As a consequence, hatching and fledging may vary from complete synchrony to complete asynchrony. To study incubation, hatching, and fledging patterns quantitatively, Clark and Wilson (1981) and Hussell (1985) introduced equations for calculating the reproductive success of a clutch as a function of the start of incubation.

Clark and Wilson (1981) proposed that the degree of hatching asynchrony is related to the ratio of post-hatching to prehatching daily nest-failure rates. The greater the ratio, the more synchronous should be the start of incubation and hatching. Hussell (1985) suggested instead that the relevant probabilities were those before incubation started (i.e., the egg-laying stage) and after the first young left the nest (the fledging stage). An alternative hypothesis, presented here, is that parent birds have difficulty in simultaneously caring for both fledglings and nestlings and that, therefore, the relevant daily survival probabilities are those occurring after first fledging. I refer to this as the "successful reproduction" hypothesis.

The basic equations of Clark and Wilson (1981) and of Hussell (1985) are applicable only to species that have constant daily survival rates during each of the egg-laying, incubation, nestling, and fledging stages of a breeding episode. Several other factors have been added to the basic equations, and these will be discussed below. These equations assume no effect on survival of the nestlings remaining in the nest following the first departure of their siblings. It seems possible that post-first-fledging daily survival rates should fall off, because parents would be tending already-fledged young, which are moving about in forest or field. Here, I present a general equation, which is applicable to any species, for calculating the success of a clutch as a function of the day on which incubation starts. I will use this equation to examine the effect of a reduction in parental attendance on reproductive success because of reduced care of nestlings remaining in the nest after the first nestling fledges.