Consequences of Nest Desertion and Inattendance for Magellanic Penguin Hatching Success

Abstract

Advantages of shared parental care during incubation are that eggs are attended at all times, protected against predators, and incubated at the correct temperature (White and Kiney 1974). However, egg neglect and desertion by parents are frequent in many bird species (Boersma 1982, Yorio and Boersma in prep.). Nest desertion increases egg losses for several seabird species (Boersma 1976, Davis 1982, Johnstone and Davis 1990), and has been suggested as the main cause of decreased hatching success in the Magellanic Penguin (Spheniscus magellanicus; Scolaro 1984, 1990). In Magellanic Penguins, eggs abandoned by an incubating bird may be left unattended for several days until the mate returns to resume incubation. When left unattended, eggs are exposed to factors that affect their probability of survival, such as extreme temperatures and predation. Temporary desertion may result in decreased likelihood of hatching if eggs are exposed to suboptimal temperatures (Hunt 1972, Hunter et al. 1976, Boersma and Wheelright 1979, Phillips 1987). At Punta Tombo, in Argentina, penguin eggs and chicks are mainly preyed upon by Kelp Gulls (Larus dominicanus), Southern Skuas (Cat haracta [skua] antarctica), hairy armadillos (Chaetophractus villosus), and grey foxes (Dusicyon griseus). Other potential predators are Patagonian ferrets (Lyncodon magellanicus), Patagonian skunks (Conepatus humboldti), and Dolphin Gulls (Larus scoresbii; Conway 1971, Rodriguez 1983, Scolaro 1985). The goals of our study were to: (1) determine and quantify the causes of egg loss; (2) evaluate the effects of nest desertion on hatching success; and (3) assess the importance of nest desertion as a cause of predation and embryo mortality. Punta Tombo Provincial Reserve, Chubut, Argentina (44?02'S, 65?llW), has a diverse seabird colony and the largest continental colony of Magellanic Penguins (Boswall and McIver 1975, Boersma et al. 1990). Magellanic Penguins have a seasonal breeding schedule, arriving at the colony to breed in late August or early September. They lay two eggs in early October, hatch chicks in November, fledge chicks in late January and February, and molt before they migrate north in March or early April. Both sexes defend the nest site, incubate eggs and feed young. Males and females take turns incubating eggs, which hatch approximately 40 days after being laid (Boersma et al. 1990). Over 100 nests were marked in an area (site 1) of approximately 180 x 70 m, and studied during each breeding season from 1983 to 1989. At each nest, adults were banded with numbered stainless-steel flipper bands. Nests were checked daily from early September to January to monitor adult presence and nest contents. Thus, laying and hatching dates, as well as egg losses, were recorded daily. Eggs were marked with a number corresponding to their laying order and nest number. At each visit we recorded if the eggs were being incubated, left alone, or missing. Egg loss was categorized by cause: (1) predation; (2) desertion; (3) addled; (4) broken; (5) died during hatching; or (6) broken by investigators. For a more detailed analysis, the predation category was further subdivided into three categories: (a) preyed on by an avian predator (if broken egg shells were away from the nest or if eggs had a hole characteristic of avian predators); (b) preyed on by hairy armadillos (if signs of digging found in or around nest, or if egg shells were smashed and in small fragments); and (c) preyed on by an unknown predator (if eggs disappeared from nest and no signs to identify the predator were found). The laying sequence markings on the eggs allowed egg shells found outside and away from nests to be identified. Each year of the study, samples of eggs that had not hatched 10 to 15 days after the expected hatching date were opened to determine if they showed signs of development. Addled eggs were subdivided into two categories: (a) those with embryos that died after some visible development; and (b) those with no visible embryo development. The latter category included eggs that were infertile, as well as those where the embryo died at an early stage. In 1990, as a control for egg loss due to researcher disturbance during incubation, we checked 49 nests every four days in an area similar to site 1 in nest density and habitat (100 m from this study area). Neither birds nor eggs were handled, but the presence or absence of adults and eggs were recorded. Eggs found unattended for only one check and where the same parent was attending the nest on the previous and following day were considered neglected. If eggs were found unattended and adults were not seen at the nest during at least two consecutive nest checks, the nest was considered deserted. From 1983 to 1989, 306 eggs were lost from 1,346 eggs laid at 692 nests in the site 1 area (Table 1). The percent of eggs lost varied among years, ranging from 13.28% (n = 143) in 1983 to 37.00% (n = 100) in 1984. Average egg loss for the seven years of the study was