Age-Specific Reproductive Tactics in the California Gull

I thank Nur for the opportunity to further clarify the report published in Science (Pugesek 1981). Research conducted on the Bamforth Lake California gulls was originally designed to evaluate three hypotheses which may explain age-related increases in breeding success known to occur among sea birds. Hypotheses were that breeding success increased with parental age because older parents obtained better nest sites, older birds were more efficient parents, and that older parents increased reproductive effort. As such, data published in Science (Pugesek 1981) and elsewhere (Pugesek 1983, Pugesek and Diem 1983) do not represent claims, but are carefully evaluated evidence which support the effort hypothesis. Nur's criticisms along theoretical lines hinge on an argument that the expectation of future life of a 3-yr-old gull is equal to that of a 15-yr-old gull. This statement is incorrect. Botkin and Miller (1974) estimate that if mortality among sea birds was age-independent, we could expect to find individuals in access of 150 yr old in large colonies. This is obviously not the case. Nur's "evidence" of age-independent mortality is, in fact, two theoretical papers. I refer the reader to data on kittiwakes which show increasing mortality with age (Coulson and Wooller 1976) and data which demonstrate lower expectation of future life among kittiwakes which began breeding at 4 yr old or younger compared to individuals that deferred first breeding 5 or more years (Wooller and Coulson 1977). On the Bamforth Lake colony, older gulls had a lower yearly return rate indicating a higher mortality rate compared with younger gulls. A portion of the higher mortality of older gulls was directly attributed to breeding activities (Pugesek 1983). Thus, mortality is known to increase with age in more than one sea bird species. Since mortality among the Bamforth Lake California gulls was, in part, due to breeding activities such as territorial defense, the selective basis for increased reproductive effort with age is present in this population (Pianka and Parker 1975). Nur's criticisms of my data are divided into two categories which he calls foraging efficiency and parental efficiency, and some additional comments on my measures of territorial defense. By combining the data from Fig. 1, Nur attempts to prove that old gulls spend less time foraging, take fewer foraging trips and are, therefore, more efficient foragers. His method of combining data and his interpretation are both incorrect. I reported a negative correlation between the amount of time nests were unattended and offspring feeding rate. Thus, the amount of time nests were unattended by parents does not represent foraging for offspring by both parents. It demonstrates instead that young parents have a shorter duration of parental care. The process of fledging offspring occurs gradually. Parents begin the process by leaving nests unattended for short periods. This behavior increases in frequency and duration as chicks grow older, and eventually parents abandon their nests entirely. In this study, older parents had a longer duration of parental care (Pugesek 1983). During the first forty days of chick life analysed in the Science report, none of the old parents had fledged their offspring. A few old parents left nests unattended for short periods of time. Conversely, young parents were in advanced stages of fledging by day thirty, and left chicks unattended for long periods of time. Most young parents had completely fledged offspring by the fortieth day. Measures of time nests were unattended at this stage were, therefore, sixty minutes out of every hour. Nur has computed this data as 120 min of foraging for offspring when, in fact, chicks were fledged and parents were no longer at the nest site. Foraging efficiency did not improve with age. Older parents foraged longer for offspring, rested less, and did so over a longer duration of parental care (Pugesek 1983). The greater foraging effort of older breeders was associated with higher weight loss during the breeding season compared with younger breeders (F1,226 = 4.07: P < 0.05). Young (3-5 yr old) and old (' 6 yr old) began breeding seasons at similar body weights. Both age groups lost significant amounts of body weight during the course of the breeding season. However, old gulls ended breeding seasons with significantly lower body weights compared with young gulls (Pugesek unpubl.). Had older gulls raised greater numbers of offspring because they foraged more efficiently, they would have lost similar or lower amounts of body weight than younger gulls. No evidence exists in the literature that foraging efficiency improves with age among adult breeding birds. Cross sectional studies have shown that immature sea birds are less successful at prey capture than fully mature adults (Orians 1969, Verbeek 1977, Ingolfsson and Estrella 1978, Morrison et al. 1978). These studies are limited in their ability to tell us whether foraging skills are learned. It is unknown whether the less successful immatures improve with age or whether the least successful immatures fail to survive to sexual maturity. It is also assumed that motivational levels of adults and immatures are similar. Since breeding causes substantial weight loss among adults, this assumption is probably false. Cross sectional studies are incapable of separating the above effects from learning. In any event, those