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

In the search for a solution, let us hypothesize that d^sub x^ declines sharply after the first fledglings have left the nest. Parents must tend both fledglings and nestlings after first fledging, and this may well be difficult for them to do satisfactorily. The quality of post-first-fledging care of the remaining nestlings could vary among species with different life histories or living in different environments.

First, let us consider a population in which the only difference from the situation in Table 1 is that d^sub x, 1^ is zero (Table 3)-no young remaining in the nest following first fledging survive to fledge-which is typical of nidifugous species. The greatest probability of rearing any young at all (s = 0.4270) occurs when incubation begins with the first egg. The greatest number of young reared per successful clutch (k = 4) occurs when incubation begins with the last egg. In this case, however, the greatest reproductive success overall (k* = sk= 1.3625) occurs when incubation begins with the last egg.

Next, consider a population in which d^sub x, 1^ is 0.9864 and d^sub x, 2^ is zero (Table 4)-no young remaining in the nest on the second day following first-fledging survive to fledge. The greatest probability of rearing any young at all (s = 0.4270) occurs when incubation begins with the first egg. The greatest number of young reared per successful clutch (k = 4) occurs when incubation begins with the last egg. In this case, the greatest reproductive success overall (k* = sk = 1.4642) occurs when incubation begins with the third egg.

Finally, consider a population in which both d^sub x, 1^ and d^sub x, 2^ are 0.9864 and d^sub x, 3^ is zero (Table 5)-no young remaining in the nest on the third day following first fledging survive to fledge. The greatest probability of rearing any young at all (s = 0.4270) occurs when incubation begins with the first egg. The greatest number of young reared per successful clutch (k = 4) occurs when incubation begins with the last egg. In this case, however, the greatest reproductive success overall (k* = sk = 1.5681) occurs when incubation begins with the second egg.

In every case (Tables 1-5), the probability of rearing at least one young from a clutch, s, is maximized when incubation starts with the first egg, and the number of young reared from a successful clutch, k, is maximized when incubation starts with the last egg. Both cannot be maximized simultaneously. The most successful genotype has the maximum reproduction (i.e., sk = k*). In every case, the egg with which incubation should begin depends on how long nestlings can survive in the nest after the first nestlings fledge, and this will depend on whether parents continue tending nestlings when also caring for already-fledged young. Incubation should start as early in the laying sequence as possible (large s) but not so early that post-first-fledging nestlings die because parents are tending the earlier-fledged young (reducing k).

For the Least Flycatcher, specifically, I propose that the high d^sub x^ following first fledging (0.9864) persists for no longer than two days, that is, that d^sub x, 3^ is close to zero. The hatching spread is 26.4 h for three-egg clutches to 45.8 h in five-egg clutches (Briskie and Sealy 1989). The start of incubation seems to vary with clutch size, starting with the laying of the second egg in the smallest clutch (c/3) or third egg in the largest clutch (c/5).