BREEDING BIOLOGY OF THE XANTUS-MURRELET

Condor 85: l 2-2 I © The Cooper Ornithological Society I983 BREEDING BIOLOGY OF THE XANTUS’ MURRELET KELVIN G. MURRAY KATHY WINNETT-MURRAY ZOE A. EPPLEY GEORGE L. HUNT, JR. AND DOUGLAS B. SCHWARTZ ABSTRACT.—Xantus’ Murrelets (Synthliboramphus [Endomychural hypoleu- cus) were studied on Santa Barbara Island, California from 1975 to 1979. Clutch initiation occurred from March to June and usually peaked in April. The two- egg clutch was 45% of the female’s weight. Eggs were laid eight days apart. Both sexes shared duties during a 34-day incubation period; partners relieved one another every three to four days. Eggs were typically left unattended in the interval between laying of successive eggs, immediately after clutch completion, and spo- radically during incubation. Breeders and non—breeders of both sexes followed similar patterns of weight change, reflecting similar patterns of colony attendance; murrelets lost weight after the egg-laying period, then gradually gained weight through the rest of the breeding season. The downy young left the nest only two nights after hatching, unfed and weighing less than 30 g. Chicks and their parents apparently moved well offshore their first night at sea. Even though most murrelets on Santa Barbara nested in concealed rock crevices, irregular nest attendance patterns left eggs vulnerable to predation, resulting in the loss of 44% of the eggs laid. Predation by deer mice (Peromyscus maniculatus) is currently the greatest risk to the breeding of these murrelets on Santa Barbara Island. The Family Alcidae is unusual for its diversity of breeding habits and the range of develop- mental patterns in its young (Sealy 1972, 1973a). Most alcids have semiprecocial young that develop thermoregulatory abilities early, but that remain in the nest until reaching about 90% of adult weight (Sealy 1973a). In contrast, the murrelets of the genus Synthliboramphus (now including Endomychura; A.O.U. I982) represent the extreme of precociality; their young go to sea within days of hatching, at about 15% of adult weight. These murrelets are also unusual in that they produce two young and typically forage far from the colony during the breeding season (Sealy 19753, 1976, Hunt et al. 1979). Sealy (I972) proposed that in alcids the eco- logical advantage of taking their young to olf- shore foraging areas is the primary selective force for precociality. With precocial young able to travel to foraging areas, feeding rates are presumably higher than if adults had to bring food back to the nest (Scott 1973). Murres (Uria spp.), which go to sea feathered and at 20% of adult weight, have higher growth rates at sea than when the adults feed them in the nest (Varoujean et al. 1979). Cody(l97l, 1973) proposed that precociality is also an adapta- tion to reduce predation on nestlings. If mor- tality at sea for young birds under parental care is low, then the two-egg clutch and precocial young of the S ynthliboramphus murrelets may allow them to rear twice as many young as other seabirds that forage offshore. This paper presents new information on the breeding biology of Xantus’ Murrelets (S. hy- poleucus). By examining aspects of their life history in relation to hypotheses for the de- velopment of precociality in the alcids, we hoped to identify some of the costs and benefits associated with this breeding pattern. Before our study, only anecdotal information on the breeding biology of Xantus‘ Murrelets was available (Lamb I909, Howell l9l7, Bent I919). Two other murrelets with similar breeding habits and precocial young have been studied: the Ancient Murrelet (S. antiquus; Sealy I976) and Craveri’s Murrelet (S. [En- domychural craveri; DeWeese and Anderson 1976). DISTRIBUTION Xantus’ Murrelets nest on ofl'shore islands be- tween central Baja California and Point Con- ception, California (A.O.U. 1957, Udvardy 1963, Jehl and Bond 1975). On the California Channel Islands only S. h. scrippsi breeds reg- ularly (Jehl and Bond 1975, Winnett et al. 1979). Santa Barbara Island is the site of the species’ largest breeding colony, which we es- [121