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- Springer Journals
- Copyright
- Copyright © Sociedade Brasileira de Ornitologia 2014
- eISSN
- 2178-7875
- DOI
- 10.1007/bf03544257
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Abstract
Revista Brasileira de Ornitologia, 22(3), 265-269 ARTICLE September 2014 Uniformity of diet composition of Brown Skua chicks at different ages and between siblings 1,4 2 1,3 Maricel Graña Grilli , Lucas J. Marti and Diego Montalti Instituto Antártico Argentino, Balcarce 290, C1064AAF, Ciudad Autónoma de Buenos Aires, Argentina – Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Universidad CAECE, Av. de Mayo 866, C1084AAQ-Ciudad Autónoma de Buenos Aires, Argentina. División Zoología Vertebrados, Museo de La Plata, Paseo del Bosque, B1900FWA-La Plata, Argentina. Corresponding author: ggmaricel@gmail.com Received on 14 May 2014. Accepted on 17 September 2014. ABSTRACT: The diet of chicks may change according to their growth and also vary between siblings due to their competitive relationship. Diet composition of 81 Brown Skua (Stercorarius antarcticus lonnbergi) chicks was analysed from regurgitated stomach contents in order to determine changes in diet composition related to chick age and differences between siblings. Fifty-nine of the samples belonged to chicks whose age was estimated by applying culmen measurement to a regression function. The remaining 22 samples belonged to 11 sibling pairs whose stomach contents were simultaneously sampled. The diet was constituted mostly by soft tissues of penguins. Marine resources were a minor part of the diet of chicks, appearing in the third week of their life. The qualitative composition of the stomach content of siblings was similar in most cases, but mass of regurgitates differed between siblings (1.3– 90.0% = 0.5–9 g), which suggests the potential existence of aggressive food monopolization by one sibling, an issue to be elucidated with further research. KEYWORDS: Chick growth, sibling competition, Stercorarius antarcticus lonnbergi, stomach regurgitate. INTRODUCTION The Brown Skua (Stercorarius antarcticus lonnbergi) breeds on the Antarctic Peninsula and in Antarctic and The food delivered to chicks by their parents may be Sub-Antarctic islands (Ritz et al. 2008). It has opportunistic influenced by many factors, such as changes in food feeding habits taking advantage of a broad variety of available resources. Its main food resources during the availability, the status of dominance of parents or changes on their ability or opportunity to get different kinds of breeding period are penguins from nearby colonies, and resources. Changes in the nutritional requirements due to flying seabirds (Trivelpiece et al. 1980, Graña-Grilli & chick growth, as well as their acquisition of the ability to Montalti 2012). During the rearing period, lasting around feed on new kinds of food and larger meals can also lead 60 days, both parents feed chicks (Young 1994) and the hatching asynchrony between siblings, of between 1 and to variations in the food delivered to chicks (Shealer 1998, Smiseth & Amundsen 2002), not only in quantity, but 4 days, is reflected in a bigger body size of the older one also to its qualitative characteristics (Moorer 1986, Hill & (Montalti 2005, Ritz et al. 2005). Hamer 1994, Robinson & Hamer 2000, Mitrus et al. 2010). The aim of this study was to describe the diet of Hatching asynchrony gives the older chick a higher Brown Skua chicks while looking for differences in its composition at different ages and between siblings. ability to compete for food delivered by parents (Braun & Hunt 1983, Smiseth & Amundsen 2002, Smiseth et al. 2003). This higher competitiveness added to the initial difference in size at the time the second chick hatches, METHODS may determine a progressive increase in the difference in Study area size between siblings (Pugesek 1993). In turn, this reduces the probability of survival of the younger chick either during its rearing, leading to brood reduction (Seddon Sampling was carried out at Harmony Point (62º17'60''S, & van Heezik 1991, Royle & Hamer 1998, Maddox & 59º15'0''W), Nelson Island, South Shetland Islands, Weatherhead 2008), or after fledging (Ploger & Mock Antarctica, where Brown Skuas breed in sympatry with South Polar Skuas (S. maccormicki). Breeding colonies of 1986, Stenning 1996). Uniformity of diet composition of Brown Skua chicks at different ages and between siblings Maricel Graña-Grilli, Lucas J. Marti and Diego Montalti Chinstrap Penguins (Pygoscelis antarcticus) and Gentoo penguins and flying seabirds were pooled for analysis, Penguins (P. papua) are located at this site, which in 1995 despite being possible to identify them as belonging had 89,700 and 3,347 pairs, respectively (Silva et al. to each category. Similarly, krill, fish and cephalopods 1998). During the 2002–2003 season, 79 pairs of Brown remains were combined in a single category named Skua bred at Harmony Point (Author’s ump. data). “marine resources”. Sampling and sample analysis Diet of sibling chicks Between 19 January and 11 February 2003, samples of Regurgitated stomach contents were sampled from 11 stomach content were obtained from 81 Brown Skua pairs of sibling chicks, in both chicks simultaneously. chicks by stimulating their regurgitation reflex through Culmen measurements were not recorded for these abdominal massage. Samples were analyzed to identify chicks precluding us from estimating their ages. Samples food items and classify them by comparison with obtained were weighed and their qualitative composition reference material. was examined and both mass and composition were compared between siblings. Diet according to chick age Temporal change in diet Fifty-nine samples were obtained from chicks for which their culmen was also measured (precision 0.01 mm). In order to test for changes in diet throughout the study That measurement was later used for age estimation by period, regardless of chick age, all 59 samples took from means of a regression model developed from culmen chicks of different ages were classified in three groups, measurements taken from 15 known-age Brown Skua covering eight consecutive calendar days each (19/01– chicks, between 1 and 57 days old, from Laurie Island 26/01, n = 24; 27/01–03/02, n = 15; 04/02–11/02, (South Orkney Islands), whose growth was monitored n = 20) and an analysis of contingency was performed. every three days during the 1993–1994 breeding season (Montalti 2005). From a total of 206 measurements, the regression function obtained was: RESULTS Age = 2.240170–0.420258*Culmen+0.026932*Culmen and showed good fit to the data (R = 0.97). The application Diet according to chick age of the function on the same data that generated it gave a correct age assignation to 15.3% of the measurements Penguin remains found belonged to soft tissue (muscle and a difference between the calculated and actual age and viscera), bone of chicks, complete body parts, and down feathers (all them grouped as indigestible chick of 1 and 2 days to 32.6% and 23.7% of measurements, respectively (difference mean = 2 d, SD = 1.74 d). The remains); adult bones and skin with feathers, and egg. In largest differences between calculated and actual ages the case of other seabird species, remains of egg and of a were 8 and 9 days for 2 and 1 measurements, respectively, Greater Sheathbill, Chionis alba, were found. The latter the three cases belonging to chicks over 50 days old. This was found in a single sample and therefore, it was not included in the contingency table. model allowed determining that ages of chicks sampled at Harmony Point were between 4 and 43 days old. Soft tissue of penguins predominated in all age Samples were grouped according to the age of classes (Figure 1). There was an exclusive presence of chicks, defined in weeks, covering a total of six weeks. The penguin remains during the first week, with a progressive frequency of occurrence of resources in stomach contents appearance of other items in the following weeks, and the incorporation of marine resources in the third week of life was calculated for the following categories: penguin colonies – soft tissue, indigestible remains of penguin of chicks (Figure 1). Those marine resources were found chicks, feathers, bone and egg; eggs of flying seabirds; and in the stomach content of 6 chicks sampled in dates that marine resources – including krill (Euphausia superba), covered all the sampling period (between 20 January and fish, and cephalopods. 4 February). In 4 of those samples the marine remains belonged to krill in advanced digestion condition. Data analysis was carried out by contingency tables, grouping data for two consecutive weeks, obtaining three There were no significant differences in the age categories defined by fortnights: first (n = 14), second composition of the diet in the three chicks’ stages (n = 25), and third fortnight (n = 20). Fortnight blocks ( = 6.74, df = 10, P = 0.75). were used because the condition of 20% of the expected Diet of sibling chicks frequencies higher than 5 at every class was not fulfilled for chi-squared test (Quinn & Keough 2002), precluding analysis in weekly blocks. For the same reason, eggs of Regurgitated wet mass obtained was variable between Revista Brasileira de Ornitologia, 22(3), 2014 Uniformity of diet composition of Brown Skua chicks at different ages and between siblings Maricel Graña-Grilli, Lucas J. Marti and Diego Montalti sibling chicks. In five of the pairs of siblings, the difference Penguin remains were almost the only food item was between 60.9% (9–23 g) and 90.0% (1–10 g), while found in stomach contents of siblings and in 8 out of 11 in the other four pairs it was between 1.3% (38–38.5 g) pairs, the items found were coincident in both siblings and 13.3% (26–30 g). (Table 1). FIGURE 1. Frequency of occurrence of food items found in stomach contents (n = 59) of Brown Skua chicks (Stercorarius antarcticus lonnbergi) of different weeks of age. TABLE 1. Food items found in the stomach contents of sibling Brown Skua chicks (Stercorarius antarcticus lonnbergi) from Nelson Island, South Shetland Islands during the 2002-2003 breeding season. A and B denote different chicks in the same brood. Food items Nest Chick Soft Tissue Chick Feather Chick Bone Adult Feather Bone Egg Algae Ax x Bx x Ax Bx x Ax x x Bx x x Ax x x x Bx x x x Ax x x x Bx x x x Ax x x Bx x x Ax x Bx x x Ax x Bx x Ax x Bx x Ax x Bx x A xxx Bx x Revista Brasileira de Ornitologia, 22(3), 2014 Uniformity of diet composition of Brown Skua chicks at different ages and between siblings Maricel Graña-Grilli, Lucas J. Marti and Diego Montalti Temporal change in diet behaviour consists of older chick throwing out the younger sib from the territory, causing its death by predation or The comparison of items found when samples were starvation (Young & Millar 2003). However, parents can classified according to calendar date did not show control food delivered to chicks, either by reducing the significant differences between the three periods sampled effects of competition between siblings (Ricklefs 1982), ( = 8.60, df = 10, P = 0.57). or enhancing differences in food acquisition by selecting the older one during feeding (Braun & Hunt 1983). Sampling of siblings was carried out on both chicks at a DISCUSSION nest, therefore ejection from the nest is not an option in our study, and the similarity in the kind of items found We addressed in the current study only one aspect among between siblings would suggest that there is no parent several ones that can determine changes in diet of chicks, discrimination between them when delivering food. This such as changes in food availability or feeding status of rose the possibility of parents intervening to reduce the parents. The lack of differences among the three compared effect of competition between chicks, thus increasing periods sampled would suggest that the kind of food that fledging rates and consequently their own fitness skuas offer to their chicks would be determined by the On the other hand, the stomach contents obtained availability more than by changes in the specific needs of from many sibling pairs had important differences chicks related to their age. in mass. Unfortunately, our work does not allow us In agreement with previous, studies, remains of soft to determine the amount of food delivered to each tissue of penguins predominated in the diet of Brown chick after each foraging trip and whether the feeding Skua chicks throughout their nesting period (Reinhardt frequency is similar for both chicks or not. Those 1997, Graña-Grilli et al. 2011). Sampling covered a differences in mass found in the stomach contents could period of 24 days in which penguin chicks would be at be the result of aggressive food monopolization by one the crèche stage (chicks are big and get grouped under of the siblings, or differences in the period elapsed the care of a few adults while other parents forage at sea) since the last time each chick was fed. Alternatively it and at the onset of the fledging stage (Williams 1995). could result from sampling biases, such as obtaining an At those stages, penguin chicks have low vulnerability to incomplete sample of the total stomach content (Barrett the attack of skuas due to their big size and self-defense et al. 2007). Dietary differences between chicks would be ability (Burton 1968). However, the occurrence of viscera most likely reflected in its quantity rather than quality, and muscle in stomach contents would indicate that but the impossibility to distinguish among those possible parent skuas were able to get high quality food from causes for mass differences leaves the question open nearby penguin colonies. Viscera and muscle are the first about food monopolization by older siblings in skuas. parts ingested by skuas (Young 1994) and may be selected Further research on the feeding of chicks could clarify as high quality food to be given to chicks. the characteristics of the relationship between siblings The appearance of marine resources in stomach and the role of parental provisioning on them. contents of older chicks only would suggest that there is a need for supplementation of the diet that may lead parents to look for other kinds of food, apart from the ACKNOWLEDGEMENTS ones obtained at the penguin colonies, or that availability of marine foods changed through the season. However, The sampling for this study was done thanks to the the high degree of digestion of krill remains suggests that financial support of the Instituto Antártico Argentino its occurrence may be due to secondary ingestion, i.e. and under the permits given by the Program of from digestive tracts of penguins on which the skuas fed. Environmental Management and Tourism, observing the The lack of information about feeding status of guidelines of the Madrid Protocol. We thank to J. Harada parents, as owners of feeding territories or not, which can for improving the English. be a determinant of breeding success (Ens et al. 1992), precludes us from making inferences about the possibility that no territorial pairs may need to look for food sources REFERENCES other than penguins. In the same way, the age of parents, Barrett, R. T., Camphuysen, C. J., Anker-Nilssen, T., Chardine, J. which also has an influence on breeding success (Pugesek W., Furness, R. 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Journal
Ornithology Research – Springer Journals
Published: Sep 1, 2014
Keywords: Chick growth; sibling competition; Stercorarius antarcticus lonnbergi; stomach regurgitate