Habitat switching by Bewick's swans: maximization of average long‐term energy gain?

Abstract

Summary 1.In a system where depletion drives a habitat shift, the hypothesis was tested that animals switch habitat as soon as the average daily net energy intake (or gain) drops below that attainable in the alternative habitat. 2. The study was performed in the Lauwersmeer area. Upon arrival during the autumn migration, Bewick's swans first feed on below-ground tubers of fennel pondweed on the lake, but subsequently switched to feeding on harvest remains in sugar beet fields. 3. The daily energy intake was estimated by multiplying the average time spent foraging per day with the instantaneous energy intake rate while foraging. In the case of pondweed feeding, the latter was estimated from the functional response and the depletion of tuber biomass. In the case of beet feeding, it was estimated from dropping production rate. Gross energy intake was converted to metabolizable energy intake using the assimilation as determined in digestion trials. The daily energy expenditure was estimated by the time-energy budget method. Energetic costs were determined using heart rate. 4. The daily gain of pondweed feeding at the median date of the habitat switch (i.e. when 50% of the swans had switched) was compared with that of beet feeding. The daily gain of beet feeding was calculated for two strategies depending on the night activity on the lake: additional pondweed feeding (mixed feeding) or sleeping (pure beet feeding). 5. The majority of the swans switched when the daily gain they could achieve by staying on the pondweed bed fell just below the average daily gain of pure beet feeders. However, mixed feeders would attain an average daily gain considerably above that of pondweed feeders. A sensitivity analysis showed that this result was robust. 6. We therefore reject the hypothesis that the habitat switch by swans can be explained by simple long-term energy rate maximization. State-dependency, predation risk, and protein requirements are put forward as explanations for the delay in habitat switch.

The role of season and social grouping on habitat use by Mute Swans (Cygnus olor) in a lowland river catchment

“…Further research could explore the mechanisms which underpin seasonal changes in the feeding profitability of different habitats and their role in driving habitat shifts. In particular, plant abundance, plant nutritional quality, and the metabolic costs associated with foraging, are all known to influence the profitability of plants to herbivores (Vickery et al 1995, Nolet et al 2002. The absence of swans in wooded habitat was consistent with the results of studies from other regions both within and outside the native range of the Mute Swan (Fuller et al 2005, Weaver et al 2012.…”

Host behaviour and physiology underpin individual variation in avian influenza virus infection in migratory Bewick's swans

“…Aquatic habitats provide Bewick's swans with abundant, high-quality food resources, longer foraging times and shelter from predation [34]. These habitats have also been associated with higher body condition prior to spring migration, as well as increased breeding success [33].…”

“…(c) Stable isotope analysis Bewick's swans forage in terrestrial and aquatic habitats throughout autumn migration and early winter [34]. In early winter, the majority of individuals rely on terrestrial food sources, with a minority also foraging on aquatic vegetation [33,35].…”

Effects of variation in food resources on foraging habitat use by wintering Hooded Cranes (Grus monacha)

“…Based on optimal foraging and ideal, free distribution (IFD) theories (Emlen 1966;Macarthur and Pianka 1966;Fretwell and Lucas 1969), the relationships of waterbirds and food resources should be close, when the waterbirds come across patches mirroring the variability of food availability (Nolet et al 2002). However, food items are complex and while some food seems abundant, it may be inaccessible (Demment and van Soest 1985;Fryxell 1991;van Beest et al 2010).…”