FACTORS DETERMINING A CLUTCH SIZE REDUCTION IN CALIFORNIA GULLS (LARUS CALIFORNICUS) : A MULTI-HYPOTHESIS APPROACH

Abstract

In the thirty-five years since David Lack first highlighted the importance of clutch size, a large number of hypotheses have been proposed relating clutch size variation to various environmental and demographic factors. Despite a great deal of both empirical and theoretical work on clutch size, there has been very little effort to test many of the competing hypotheses in explaining a clutch size difference between two populations of the same species. I have taken the latter approach in an effort to explain a clutch size reduction in the California Gull (Larus californicus) population at Mono Lake, California. I compared the breeding biologies of the gulls at Mono Lake and at Great Salt Lake, Utah, collecting data for three breeding seasons at Mono Lake and one breeding season at Great Salt Lake. These data included measurements of the conditions of 60 adults, growth and mortality measurements for approximately 900 chicks, 4450 nest-hours of parental care observations, and the results of egg-removal experiments on 40 females. I tested seven hypotheses to explain the clutch size reduction: age structure, egg predation, bet-hedging, effort reallocation, most productive brood size, parental mortality, and pre-egg food limitation. Each of these hypotheses is described in detail in the introduction. The pre-egg food limitation hypothesis is best able to explain the clutch size reduction at Mono Lake, although the egg-removal experiments show that the resource limitation is relative and not absolute. Clutch size variation at each site need not be viewed as the result of fine-scaled evolutionary adjustment, although the general clutch size decision machinery is presumably molded by selection. Future research must focus on the details of this clutch size decision machinery and its application to the concept of reproductive effort.

Egg Production in a Coastal Seabird, the Glaucous-Winged Gull (Larus glaucescens), Declines during the Last Century

“…The actual cost of egg production to breeding birds in general is controversial [4] but for gulls at least there is good evidence that food input, particularly in the form of protein, affects egg size and clutch number [3], [6], [22], [29], [50], [51]. California gulls ( L. californicus ) breeding at Mono Lake, California have been reduced to laying 2-egg clutches since the early 1900s (with eggs also smaller than those from other populations); this is apparently due to regional food shortages [52]. In red-billed gulls ( L. novaehollandiae ), egg and clutch size over 41 years were positively correlated with the availability of their preferred prey, the euphausiid Nyctiphanes australis [53].…”

“…Thus, glaucous-winged gulls may be unable to use alternative food sources (e.g., trash) to buffer against consistent shortages of natural foods during certain periods of their breeding cycle, and could be undergoing an ongoing trade-off of their own survival against production of offspring. It is possible that the study population may be shifting toward a modal 2-egg clutch, as has occurred in another food-limited population of gulls in the 20 th century [52]. Experimentally testing whether gulls in the Salish Sea respond to increased high-quality fish prey by increasing egg or clutch size would provide more conclusive evidence for or against natural food supply as a mechanism driving observed trends.…”

The Relationship between Reproduction and Survival in Known‐Aged California Gulls

“…One major difference of our studies is that the Blacklegged Kittiwakes studied by Coulson nest in manmade habitats. Winkler (1985) found that California Gulls feeding offspring from food gathered at a garbage dump had larger clutches and higher survival rates compared to another population that depended on natural foods. Winkler (1985) found that California Gulls feeding offspring from food gathered at a garbage dump had larger clutches and higher survival rates compared to another population that depended on natural foods.…”

Life-History Strategy of the Herring Gull: Changes in Survival and Fecundity in a Population Subjected to Various Feeding Conditions

“…Capture-recapture data for male and female herring gulls colour-ringed between 1983 and 1989 on Treberon islandYear next recaptured Released Newly marked19841985198619891990 …”