Scientific name: Synthliboramphus hypoleuca
Until Recently: Endomychura hypoleuca
Spanish: Pato Nocturno

Any white-breasted murrelet seen about the islands of Southern California is likely to be the Xantus' Murrelet, a species named after a Hungarian naturalist, Janos Xantus, in 1859. ⁶⁴ In 1939, Green and Arnold indicated that the species was divisible into two subspecies, one breeding on Guadalupe Island (Endomychura hypoleuca hypoleuca ), the other breeding on rocky islands near the shore of Southern California and northern Baja California (Endomychura hypoleuca scrippsi ). ²²⁵ There is little difference in the appearance of the two subspecies and the casual observer would probably fail to see the distinctions through binoculars. Clinal differences in size are noted by museum specialists; the Guadalupe bird averaging 12.6 grams less in body weight than the northern race. On San Benitos Island about half way down Baja California, the Xantus' Murrelet overlaps with Craveri's Murrelet and here the Xantus' Murrelet bears closer resemblance to the Craveri's. Here Xantusı Murrelets are thinner billed than their most northern populations but stouter billed than the Guadalupe birds.

Xantus' Murrelet has a white underwing as its species name, hypoleuca, suggests. Early descriptions of the two species, Xantus' and Craveri's were confused, indicating that only one species should be named. However, more recent investigations show that the Xantus's Murrelet and Craveri's Murrelet act like distinct species even at the point of range overlap. For instance, 20 to 30 percent of the murrelets around San Benitos Island are Craveri's Murrelets which breed earlier than the Xantus' making them a valid, reproductively isolated species. 300 However, their shrill clear whistles are so similar that it is audibly difficult to distinguish between the two species.

Little precise information on the biology of Xantusı Murrelets was available until Kelvin Murray and Kathy Winnett-Murray studied them from 1975 to 1979 on Santa Barbara Island where a colony with an estimated of population of up to 10,000 birds breed. ⁴¹¹ Xantusı and Craverisı Murrelets are the most southerly breeding species of auks. The Japanese Murrelet breeds in similar low latitudes on the otherside of the Pacific.

Mating in the Xantus' Murrelet occurs, during darkness, at sea near the breeding island in March or early April. On land nest-site selection behavior also takes place nocturnally and with minimal calling. Vocalizations are also heard between pairs exchanging incubation and brooding duties. Besides this, little is known about their communicative behavior.

At the northern limits of their range, on Santa Barbara Islands, pre-egg laying activity begins the last week of March. ⁴¹¹ Two eggs are usually laid in late March or early April, but eggs may be found until the last week of June in nesting nooks among loose rocks, cliff crannies, or under vegetation. Although varying from year to year, the peak of the egg-laying period is 27 April to 10 May on the more northerly islands, and a little earlier on Guadalupe.^{300, 411} Although the eggs are glossier than other auk eggs no two are alike in color. They vary from dark chocolate brown to a light blue with a sprinkling of spots, but most are greenish drab with a wide variation of lavender and brown markings.

Eggs average 37.2 grams which makes them the heaviest egg in proportion to the adult weight as compared with of all other alcids, reaching 22.3 percent of the adult weight.( Ancient Murrelet's eggs come close to this figure). Since the normal clutch contains two eggs this represents 45 percent of the female's weight.⁴¹¹ As is also true for the Ancient Murrelet and some petrels, the Xantus' Murrelet may leave its first egg unattended for up to 16 days before the second egg is laid, but more frequently only two days elapse before the second egg appears.^{212, 411} Then both eggs may be unattended for indefinite periods before incubation begins.

Additionally, frequent absence from the nest for up to several days occurs during incubation. Such egg neglect may be related to increased foraging time requirements, or in some instances to heat stress during excessively hot weather. It may also assist in establishing synchronous hatching of both eggs, which ensures that both chicks will fledge together. A disadvantage is that a chick which survives and hatches from a neglected egg weighs less than one from an egg which was more continuously incubated.⁴¹⁰ Males usually begin the incubation shifts, which average three days, but may be as long as six days. The incubation period ranges from 27 to 44 days with an average of 33.6 days.⁴¹¹

In the spring of 1975, Eppley and others reported that they had observed fifty-four nests of Xantus' Murrelet from 18 April until they were vacated. They discovered that 54 percent of the eggs hatched successfully and that all of the resulting chicks, twenty-nine in number, safely left their island.¹⁸⁰

Within hours after hatching, the chicks are vigorous, active, and covered with thick down. Being black above and white below, they look like miniature adults. Within 24 to 48 hours both chicks leave the nest, without being fed, to join their calling parents in the sea. Kelvin and Kathy Murray described their moments of departure. ⁴¹¹

"Chicks were normally escorted from the nest by both parents on the night of departure. Following a period of intense vocalizations, the family emerged from the nest together. Parents typically led the chicks down the slope for less than 2 m before flying out to sea. Left on their own, the chicks made their way directly to the cliff edge where they jumped or were blown off the cliff into the surf 75 m or more below."

Although the chick weighs only 15 percent of the adult weight, their legs are almost adult size by the time they enter the water and thus they swim as well as their parents. ⁴¹¹ It has been suggested that the chickıs down may saturate easily and that it may dry out by periodically climbing on its parent's back, but this suggestion has not been firmly documented. Adults convoy the young out to sea where they may be recognized as family groups until the juveniles can no longer be distinguished from the parents. Since they usually retain the same nest site and mate from year to year the species apparently mates for life -- each pair remaining together at sea throughout the year.

Xantus' Murrelets may wander after breeding to the cooler waters of the north. They have been sighted several times 104 to 208 kilometers offshore along the Oregon coast, in 16° Celsius water.⁵²⁴

The adult murrelet tends to fly rather than dive to escape the approach of a boat. Their flight is strong and rapid. Food habits are not well known for this species. Apparently, it usually feeds on small fish, crustaceans, and other planktonic forms. Off the Oregon coast Xantus' Murrelets have been seen feeding on saury (Cololabis saira ).⁵²⁴ No data are available on the survival rate after the chicks reach the sea. One would expect that fish may take a few young birds, but as they are good swimmers, it can be expected that once in the water most survive to adulthood. Storms however, undoubtedly also take their toll.

Predation on this species is probably less of a problem at sea than it is on land during the breeding season. Foxes and feral cats prevent its breeding on some islands and it is known that mice eat unattended eggs and chicks. Kelvin Murray discovered that Peromyscus on Santa Barbara Island gnaw holes in and lap out the contents, of more than 30 percent of the eggs and that total mouse predation accounts for 44 percent of all egg losses on that island.^{410, 411} Eggs laid among sharp rocks tend to crack making nests in these situations less successful. Barn and Burrowing Owls are also predators of the Xantus' Murrelet and remains of chicks have been found at the nests of Western Gulls.