This post was contributed by Jim Sedinger, coauthor of a recent paper in The Wilson Journal of Ornithology about the effect of predation on nest site selection by Black Brants.
It was late in the evening, but at the Tutakoke River Black Brant colony on the Bering Sea coast of the Yukon-Kuskokwim Delta (YKD) in western Alaska, where the sun sets for only about three hours each day during the nesting season, it was still light. I was checking the status of brant nests and recording the unique leg band codes of nesting individuals. In the part of the colony where I was working, many of the nests are on raised areas topped with grasses and sedges that are spread throughout mudflats otherwise by monthly high tides, which we called “mud islands.” As I approached one particular mud island I could see a male with the band code T3E peeking around the edge of the mud island that held his mate’s nest. When I kneeled down at the nest to check the eggs, he walked around behind me, walked up my back, and stood on my head. I had been expecting this, because this particular male’s mate had nested on the same mud island for the previous eight years, and his behavior had been the same every time someone approached his nest. I don’t know what caused this male to adopt this unusual (some might even say weird) approach to defending his nest, but from his perspective, his method was successful: every time he performed his ritual, his mate’s clutch of eggs was left intact when we left.
Brant have a huge range of personalities, just like people, and I and many others have enjoyed these different personalities over the 39 years of the Tutakoke brant study. The graduate students (who spent a lot more time at the site than I did) always had favorites—the female who wouldn’t get off her nest unless we nudged her a bit, or the pair who were always defending their nest together (brant pairs typically mate for life). Because brant are highly faithful to their neighborhoods in the colony, we usually knew where to find them, just like T3E. These special birds, however, are in trouble.
Although brant are relatively faithful to nesting areas within the colony, we have observed a steady shift away from the part of the colony most susceptible to predation by arctic foxes (south of the Tutakoke River) and into the area north of the river where nests are less vulnerable to fox predation. We believe that this movement to the north side of the Tutakoke River is a response to the difference between the two areas in predation risk. Brant are too small to defend their nests against foxes, and because foxes don’t eat eggs right away—they cache them for later use—a few foxes can sometimes destroy a large proportion of the nests in the colony. While brant may gain some measure of protection by changing their nesting location, they cannot escape the full impact of high predation years when large numbers of nests are destroyed.
When fox predation rates are high, of course, most nests fail and few young are produced. High predation years also have longer term effects on both individuals and the population as a whole. Brant family groups are socially dominant over pairs without young and nonbreeding brant, as is true in other geese. As a result, successful nesting adults are in family groups that occupy better winter habitat and are more likely to breed the next year, while skipping breeding substantially reduces the probability of breeding the next year, and only about half of the females that skip breeding are ever able to return to the breeding population before they die. Pairs whose nests fail suffer a similar fate. So, fox predation has a substantial impact on individual fitness that extends well beyond the loss of a single year’s breeding effort. High levels of fox predation also cause reductions in the number of brant recruited into the breeding population for many years after the high predation event. The reasons for the long-term effect are a little complicated, but it starts with the food that brant goslings need to grow. Brant have precocial young that are mobile and feed themselves on key sedges shortly after hatch. Brant parents rear their goslings on areas that extend up to 30 kilometers from the colony. When brant families are abundant, their grazing activity (and even walking on the vegetation) causes taller sedges, which goslings can’t eat, to be replaced by “grazing lawns” that provide essential food for growing goslings because of the much higher protein content of the plants. However, when most nests fail due to fox predation, nearly all adult brant leave for higher-latitude areas where they molt their flight feathers. As a result, little grazing occurs and grazing lawns revert back to the taller growth form. Consequently, there is less food for goslings for several years following a major predation year. After three years of high fox predation in the early 1980s, grazing lawns were still increasing in area in 1997, 12 years after the last year of low nest success. This tells us that high levels of fox predation can reduce food for goslings for several years, resulting in lower growth rates and smaller goslings when they fledge.
When fledglings grow slowly and are small at fledging, this has important fitness consequences at both the individual and population level, so a single year of low nest success can negatively affect the brant population for years afterward. Small goslings survive are less likely to survive their first year than larger goslings, and if they do survive they are less likely to breed and lay smaller clutches if they do. All of these consequences of insufficient food cause low recruitment of new individuals into the breeding population, which is also reflected in declining age-ratios in the entire Black Brant population in fall.
Adding to the problems with recruitment created by fox predation on the YKD, we suspect that the effects of climate change on wintering areas (along the Pacific Coast of North America from Mexico to Alaska) are also impacting population dynamics. A couple of pieces of information support this idea. First, there has been a large shift of wintering brant out of Mexico and an increase in wintering brant in Alaska, which we would expect if warming is negatively impacting the eelgrass that brant eat in winter at the southern end of their wintering range. Second, first-year survival of brant has been cut in half for both brant from the YKD and the Arctic coast of Alaska. We might expect this for brant from the YKD because of the declines in growth rate and fledging size described earlier. Brant goslings from the Arctic, however, have plenty of food, are large at fledging, and have not declined in size over the past 25 years. While it is true that goslings from the Arctic survive their first year at about twice the rate of goslings from the YKD, the fact that first-year survival of goslings from both areas have declined suggests that conditions on wintering areas shared by both populations have gotten worse.
High rates of predation a few times a decade, combined with the carry-over effects on food for goslings and changes in wintering habitats, are resulting in population decline in the global Black Brant population. The YKD was historically the beating heart of the brant breeding population and likely supported more than 40,000 nests, with an additional 100,000+ nonbreeding brant living across the Arctic, extending from the Lena River Delta in Russia to the Queen Maude Gulf in Canada. Numbers on the YKD have declined by 50% since the early 1980s, and while a few thousand new nests have appeared on the Arctic coast of Alaska, the overall size of the breeding population has declined. Decline in the size of the breeding population has been associated with declining numbers of brant young in fall and the number of brant in the global population. At present, managers do not appear any closer to solving either the problem with winter habitat or the problem with high predation on YKD breeding areas. The Black Brant population risks continuing decline.
Anthony, R. M., P. L. Flint and J. S. Sedinger. 1991. Arctic fox removal improves nesting success of Black Brant. Wildlife Society Bulletin 19:176-184.
Person, B. T., M. P. Herzog, R. W. Ruess, J. S. Sedinger, R. M. Anthony, and C. A. Babcock. 2003. Feedback dynamics of grazing lawns: coupling vegetation change with animal growth. Oecologia 135:583-592.
Riecke, T. V., A. G. Leach, D. Gibson, and J. S. Sedinger. 2018. Parameterizing the robust design in the BUGS language: lifetime carry-over effects of environmental conditions during growth on a long-lived bird. Methods in Ecology and Evolution 9: 2294-2305.
Sedinger, J. S., C. A. Nicolai, A. W. VanDellen, A. G. Leach, H. M. Wilson, and R. M. Anthony. 2016. Predation and reduced grazing interact to reduce recruitment and population growth in a small herbivore. Condor 118:433-444.
Sedinger, J. S., J. L. Schamber, D. H. Ward, C. A. Nicolai, and B. Conant. 2011. Carryover effects associated with winter location affect fitness, social status, and population dynamics in a long distance migrant. American Naturalist 178:E110-E123.
Uher-Koch, B. D., J. A. Schmutz, H. M. Wilson, R. M. Anthony, T. L. Day, T. F. Fondell, B. T. Person, and J. S. Sedinger. 2019. Ecosystem scale loss of grazing habitat impacted by abundance of dominant herbivores. Ecosphere 10:e02767.
Van Dellen, A. W., and J. S. Sedinger. 2021. Nest density and competing risks, a long-term investigation of Black Brant (Branta bernicla nigricans) nest survival. Wilson Journal of Ornithology 132:379-387.