Phenotypic flexibility in digestive system structure and function in migratory birds and its ecological significance

Is there a "migratory syndrome" common to all migrant birds?

Bird migration has been assumed, mostly implicitly, to represent a distinct class of animal behavior, with deep and strong homologies in the various phenotypic expressions of migratory behavior between different taxa. Here the evidence for the existence of what could be called a "migratory syndrome," a tightly integrated, old group of adaptive traits that enables birds to commit themselves to highly organized seasonal migrations, is assessed. A list of problems faced by migratory birds is listed first and the traits that migratory birds have evolved to deal with these problems are discussed. The usefulness of comparative approaches to investigate which traits are unique to migrants is then discussed. A provisional conclusion that, perhaps apart from a capacity for night-time compass orientation, there is little evidence for deeply rooted coadapted trait complexes that could make up such a migratory syndrome, is suggested. Detailed analyses of the genetic and physiological architecture of potential adaptations to migration, combined with a comparative approach to further identify the phylogenetic levels at which different adaptive traits for migration have evolved, are recommended.

Flexible remodeling of organ size during spring migration of the garden warbler (Sylvia borin)

The energetic demands of long-distance migratory birds change drastically, depending on the stage of their life cycle. Changing demands are reflected in the up and down regulation of adipose tissue and organ mass. This paper presents new data on organ size changes during different stages of spring migration of garden warblers (Sylvia borin). Phenotypic mass changes were quantified in 13 organs of birds caught in Tanzania, Ethiopia and Egypt. We also sampled birds after a simulated stopover in Egypt. Some organs increased in mass up to about 1.5-fold during migration from Tanzania to Ethiopia, while some remained unchanged or even decreased in mass. During flight across the Sahara, nearly all organ masses including heart and flight muscles were reduced. Exceptionally large reductions (approximately 50%) were observed for liver, bile, spleen, kidney and digestive tract organs. The only exceptions were the testes, which increased 4-fold in mass. During the simulated stopover in Egypt, a significant recovery was observed for kidney, liver, heart, proventriculus, and small intestine. The testes continued to increase in mass. Flexible remodeling of organ size in the course of spring migration thus comprises significant changes for all quantified organs, with a variety of organ-specific patterns. Individual organ patterns are differentially shaped by functional aspects according to the different organ requirements in the alternation of flight and stopover phases, energetics, future demands, and protein requirements. Anticipatory mechanisms account for the size change of the testes, and we suggest the same for the kidney and the gall bladder.

Digestive Organ Sizes and Enzyme Activities of Refueling Western Sandpipers (Calidris mauri): Contrasting Effects of Season and Age

We examined seasonal and age-related variation in digestive organ sizes and enzyme activities in female western sandpipers (Calidris mauri) refueling at a coastal stopover site in southern British Columbia. Adult sandpipers exhibited seasonal variation in pancreatic and intestinal enzyme activities but not in digestive system or organ sizes. Spring migrants had 22% higher total and 67% higher standardized pancreatic lipase activities but 37% lower total pancreatic amylase activity than fall migrants, which suggests that the spring diet was enriched with lipids but low in glycogen. Spring migrants also had 47% higher total intestinal maltase activity as well as 56% higher standardized maltase and 13% higher standardized aminopeptidase-N activities. Spring migrants had higher total enzymic capacity than fall migrants, due primarily to higher total lipase and maltase activities. During fall migration, the juvenile's digestive system was 10% larger than the adult's, and it was composed differently: juveniles had a 16% larger small intestine but a 27% smaller proventriculus. The juvenile's larger digestive system was associated with lower total enzymic capacity than the adult's due to 20% lower total chitinase and 23% lower total lipase activities. These results suggest that juvenile western sandpipers may process food differently from adults and/or have a lower-quality diet.

Digestive morphology and enzyme activity in the Andean toad Bufo spinulosus: hard-wired or flexible physiology?

Gut plasticity is a trait with implications on animal performance. However, and despite their importance as study models in physiology, research on gut flexibility in amphibians is scarce. In the present work, we analyse digestive adjustments of Bufo spinulosus adult individuals to cope with changes in diet quality and quantity at two organizational levels (i.e., digestive morphology and enzymes). We found that changes in gut size are related to the amount of food ingested, but not to diet composition. This is in agreement with "the gut seasonal change" hypothesis and offers a proximal explanation for this change. Digestive enzymatic activity (maltase and aminopeptidase-N) did not change with diet quality or quantity, which agrees with the hypothesis of "hard-wired physiology in adult amphibians". Both hypotheses are in agreement with the general theoretical framework of gut phenotypic flexibility when interpreted in light of amphibian natural history. In addition, our results indicate that the correlation between feeding frequency and the level of gut up-regulation proposed for interspecific comparisons may also be found at the intraspecific level.

Phenotypic flexibility in the intestinal enzymes of the African clawed frog Xenopus laevis

The intestinal plasticity of digestive enzymes of amphibian species is poorly known. The goal of this study was to characterize digestive enzyme profiles along the small intestine of adult frogs, Xenopus laevis, in response to an experimental diet. We acclimated adult X. laevis for 30 days either to carbohydrate-rich or protein-rich diets, and determined the morphology and digestive enzymes of the small intestine. We found a significant difference of aminopeptidase-N activity between carbohydrate-rich and protein-rich acclimated animals. We also found a little variation in the expression of maltase activity, which contrast with the proposed hypothesis about the existence of digestive tradeoff in vertebrates. This finding supports the adaptive modulation hypothesis and suggests that caution is called for when analyzing physiological data regarding assumed discrete trophic category of species.

Dynamic digestive responses to increased energy demands in the leaf-eared mouse (Phyllotis darwini)

A major area of interest in comparative physiology has been to understand how animals cope with changing environmental demands in time and space. The digestive system has been identified as one of the more sensitive systems to changes in environmental conditions. However, most research on this topic has evaluated these effects during peak energetic demands, which do not allow for evaluation of the dynamics of the digestive response along a more natural continuous gradient of environmental conditions. We examined phenotypic flexibility in digestive responses of the leaf-eared mouse Phyllotis darwini to increments in total energy demands (via sequential exposure to 26, 12 and 0 degrees C). Additionally, we evaluated the effect of a moderate energy demand (12 degrees C) over three different time periods (7, 17 and 27 days) on digestive traits. Moderate increases in energy demand were associated with changes in the distribution of digesta in the gut, whereas higher increases in energy demand involved increases in the tissue mass of digestive organs. Time-course analysis showed that at 12 degrees C practically all digestive variables reached stable values within 7 days, which is in agreement with empirical data and theoretical deductions from cellular turnover rates. We conclude that although the input of energy and nutrients into the digestive tract is typically periodic, many aspects of digestive physiology are likely to be flexible in response to environmental variability over both short-term (daily) and long-term (seasonal) time scales.

Corticosterone Secretion, Energetic Condition, and a Test of the Migration Modulation Hypothesis in the Hermit Thrush (Catharus Guttatus), A Short-Distance Migrant

Recent evidence suggests that migration may be extremely important in limiting populations of migratory songbirds. Understanding the relationship between changes in cor- ticosterone secretion and energy reserves during migration is essential to understanding how environmental conditions such as weather, food availability, predation pressure, and habitat quality may be affecting birds en route. Several studies have found that baseline corticosterone is often elevated, and response to capture and handling stress reduced (migration modulation hypothesis), in birds sampled during the migratory period. However, because neither corticos- terone secretion nor energetic condition within groups of birds sampled either in laboratory or in field varied significantly (Holberton et al. 1996, Holberton 1999), it was unknown whether corticosterone secretion could be modulated to track changes in energetic condition within the migratory period. Here, we investigated patterns of corticosterone secretion associated with variation in energetic condition in 18 free-living Hermit Thrushes (Catharus guttatus) sampled during autumn migration. Mean baseline corticosterone (mean ± SE) was 37.22 ± 7.70 ng mL−1 and increased during the 30-min interval up to 54.52 ± 6.01 ng mL−1. Lean birds were more likely to exhibit higher levels of baseline corticosterone and a reduced adrenocortical response, compared with birds that had greater energy reserves. Collectively, the results support the migration modulation hypothesis and illustrate that baseline and stress-induced corticosterone secretion can be modulated to meet the changing energy needs of birds during the migratory period.

Diet Quality and Food Limitation Affect the Dynamics of Body Composition and Digestive Organs in a Migratory Songbird (Zonotrichia albicollis)

Migrating songbirds interrupt their feeding to fly between stopover sites that may vary appreciably in diet quality. We studied the effects of fasting and food restriction on body composition and digestive organs in a migratory songbird and how these effects interacted with diet quality to influence the rate of recovery of nutrient reserves. Food limitation caused white-throated sparrows to reduce both lean and fat reserves, with about 20% of the decline in lean mass represented by a decline in stomach, small intestine, and liver. During refeeding on diets similar in nutrient composition to either grain or fruit, food-limited grain-fed birds ate 40% more than did control birds, and they regained body mass, with on average 60% of the increase in body mass composed of lean mass including digestive organs. In contrast, food-limited fruit-fed birds did not eat more than did control birds and did not regain body mass, suggesting that a digestive constraint limited their food intake. The interacting effects of food limitation and diet quality on the dynamics of body composition and digestive organs in sparrows suggest that the adequacy of the diet at stopover sites can directly influence the rate of recovery of body reserves in migrating songbirds and hence the pace of their migration.

Nonbreeding eastern curlews Numenius madagascariensis do not increase the rate of intake or digestive efficiency before long-distance migration because of an apparent digestive constraint

The possibility of premigratory modulation in gastric digestive performance was investigated in a long-distance migrant, the eastern curlew (Numenius madagascariensis), in eastern Australia. The rate of intake in the curlews was limited by the rate of digestion but not by food availability. It was hypothesized that before migration, eastern curlews would meet the increased energy demand by increasing energy consumption. It was predicted that (1) an increase in the rate of intake and the corresponding rate of gastric throughput would occur or (2) the gastric digestive efficiency would increase between the mid-nonbreeding and premigratory periods. Neither crude intake rate (the rate of intake calculated including inactive pauses; 0.22 g DM [grams dry mass] or 3.09 kJ min(-1)) nor the rate of gastric throughput (0.15 g DM or 2.85 kJ min(-1)) changed over time. Gastric digestive efficiency did not improve between the periods (91%) nor did the estimated overall energy assimilation efficiency (63% and 58%, respectively). It was concluded that the crustacean-dominated diet of the birds is processed at its highest rate and efficiency throughout a season. It appears that without a qualitative shift in diet, no increase in intake rate is possible. Accepting these findings at their face value poses the question of how and over what time period the eastern curlews store the nutrients necessary for the ensuing long, northward nonstop flight.

Physiological and biochemical responses to dietary protein in the omnivore passerine Zonotrichia capensis (Emberizidae)

We studied the physiological, biochemical and morphological responses of the omnivore sparrow Zonotrichia capensis, a small opportunistic passerine from Central Chile acclimated to high- and low-protein diets. After 4 weeks of acclimation to 30% (high-protein group) or 7% (low-protein group) dietary casein, we collected urine and plasma for nitrogen waste production and osmometry analysis, and measured gross renal morphology. Plasma osmolality and hematocrit were not significantly affected by dietary treatment, but urine osmolality was higher in the high-protein group than in the low-protein group. Kidney and heart masses were higher in animals acclimated to the high-protein diet. Mean total nitrogen waste was significantly higher in the high-protein group, but the proportions of nitrogen excreted as uric acid, urea and ammonia were unaffected by diet. Our data suggest that the response of Z. capensis to an increase in dietary protein content is through greater amounts of total nitrogen excretion and hypertrophy of kidney structures, without any modification of the proportion of excretory compounds.

Anatomical and Histological Changes in the Alimentary Tract of Migrating Blackcaps (Sylvia atricapilla): A Comparison among Fed, Fasted, Food‐Restricted, and Refed Birds

During northward migration, blackcaps that arrive to refuel at stopover sites in Israel's Negev Desert have reduced masses of organs that are important in food digestion and assimilation. We tested several predictions from the general hypothesis that smaller organs of digestion (small intestine and pancreas) and nutrient assimilation (liver) bring about a lower capacity to consume food and that the organs must be restored before blackcaps can feed and digest at a high rate. We used a fasting protocol to create a group of blackcaps with reduced intestine and liver mass (reduced by 45% and 36%, respectively) compared with controls fed ad lib. Because most of the small intestine's biochemical digestive capacity reside in enterocytes found on villi, we predicted and found that reduced intestinal mass in fasted blackcaps related mainly to changes in enterocytes rather than other cells and tissues such as nonabsorptive crypt cells or underlying muscle. Because migrating blackcaps that stop over to feed begin to increase in body mass only 2 d after arrival, we predicted and found a similar recovery period in blackcaps that were first fasted but then refed--the organ mass, structure, function, and ability to consume food was restored after 2 d of feeding. Another group of food-restricted blackcaps (fed at one-third ad lib. level) lost similar amounts of body mass as fasted blackcaps but had much greater capacity to consume food than fasted blackcaps, and so we predicted that they would exhibit little or no reduction in alimentary organs relative to controls fed ad lib. A surprising result was that, as in fasted blackcaps, in food-restricted blackcaps, the decreases in masses of small intestine, liver, and pancreas were proportionally greater than the decreases in body mass or in masses of nonalimentary organs (heart, pectoralis). Food restriction, like fasting, caused a decrease in amount of intestinal mucosa and an alteration in the phenotype of enterocytes. These results are thus not consistent with the general hypothesis, and although they can be rationalized by assuming that blackcaps fed ad lib. have excess digestive capacity, it may also be that the physiological process or processes limiting very high feeding rate lie elsewhere than in the digestive system.

The integration of energy and nitrogen balance in the hummingbird Sephanoides sephaniodes

Floral nectars are rich in energy but contain only traces of amino acids, and it has been hypothesized that arthropods may be an important source of energy and amino acids for hummingbirds. We studied the nitrogen requirements of hummingbirds as well as how they use small arthropods to satisfy their nitrogen and energy requirements and how organ mass varies with nitrogen intake. Non-reproductive green-backed firecrowns Sephanoides sephaniodes were maintained for 10 days on diets containing 0%, 0.96%, 1.82%, 4.11% and 11.1% nitrogen (dry-matter basis). A second group of individuals were fed with varying amounts of nitrogen-free nectar supplemented with fruit flies. Finally, non-reproductive hummingbirds were captured as a control group for analysis of organ mass and size as well as fat content. The maintenance nitrogen requirement of green-backed firecrowns determined by regression was 1.42 mg N day(-1), yet they required nearly 10 mg N day(-1) to maintain body mass. When arthropods were available, we observed that hummingbirds required approximately 150 fruit flies to maintain body mass, which corresponds to a 5% nitrogen diet. Interestingly, when nectar was restricted (to 4 ml day(-1)), or was absent, arthropods alone were not able to satisfy the body mass balance requirements of hummingbirds, suggesting that arthropods are not adequate as an energy source. In the group offered an 11.1% nitrogen diet, the size and surface of the small intestine, and liver and kidney mass increased in comparison with the control group (non-reproductive field hummingbirds) or the nitrogen-free group, suggesting a nitrogen overload. Our results are in agreement with other studies showing low nitrogen requirements by nectarivores. An important point to stress is that nitrogen digestibility declined in the 11.1% nitrogen diet, which strongly supports our nitrogen absorption saturation hypothesis.

Cost-benefit analysis of mollusc-eating in a shorebird. II. Optimizing gizzard size in the face of seasonal demands

Aiming to interpret functionally the large variation in gizzard masses of red knots Calidris canutus, we experimentally studied how the digestive processing rate is influenced by the size of the gizzard. During their non-breeding season, red knots feed on hard-shelled molluscs, which they ingest whole and crush in their gizzard. In three experiments with captive birds we tested predictions of the hypothesis that gizzard size, via the rate of shell crushing and processing, constrains intake rate in red knots (against the alternative idea that external handling times constrain intake rate). Gizzard size within individual birds was manipulated by varying the hardness of the diet on offer, and was confirmed by ultrasonography. The results upheld the "shell-crushing hypothesis" and rejected the "handling time hypothesis". Intake rates on with-shell prey increased with gizzard size, and decreased with shell mass per prey. Intake rates on soft (without shell) prey were higher than on with-shell prey and were unaffected by gizzard size. Offering prey that were heavily shelled relative to their flesh mass led to energy intake rates that were marginally sufficient to balance the daily energy budget within the time that is naturally available in a tidal system. We predicted the optimal gizzard sizes that are required to either (1) balance energy income with energy expenditure, or (2) to maximise net daily energy intake. The gizzard mass of free-living red knots in the Wadden Sea is such that it maximises daily net energy intake in spring when fuelling for migration, while it balances energy budget throughout the remainder of the year.

Phenotypic flexibility of structure and function of the digestive system of Japanese quail

Organisms adjust their phenotype to fluctuating conditions of the environment and to changing internal demands. We report flexible responses of the gizzard and the small intestine of Japanese quail to a high-fibre diet. Switching from a standard diet to a high-fibre diet results in a highly significant increase in gizzard size, intestine length, mucosal surface, thickness of the intestinal muscular layer and vascularization of the mucosa. After diet switching, increased or decreased gizzard size results from changes in cell size, i.e. smooth muscle cell hypertrophy and hypotrophy, respectively. Increased cell proliferation is not the cause of increase in gizzard size. In the small intestine, however, we found elevated levels of cell proliferation after diet switching and conclude that increased capacity (upregulation) of the small intestine is based on increased rates of mitosis in the intestinal crypts. It is highly probable that elevated levels of cell proliferation in the crypts are balanced by elevated levels of cell extrusion at the tip of intestinal villi. The lipid contents of the liver were reduced, indicating that lipid stores in the liver were mobilized to fuel the flexible response of the gastrointestinal tract. During changes of organ size in response to changes in food composition, resting metabolic rate was not altered.

Phenotypic flexibility of body composition in relation to migratory state, age, and sex in the western sandpiper (Calidris mauri)

We investigated the flexibility of body composition in relation to seasonally variable demands for endurance flight capacity and hyperphagia in a migratory shorebird. Migrating western sandpipers were sampled in spring and fall while refueling at a north temperate stopover and were compared with nonmigrating birds captured at a tropical wintering area in Panama. Sandpipers weighed 25% more at stopover, and nearly 40% of migratory mass increase consisted of lean body components. Most organs and flight muscles were 10%-100% larger during migration, and the greatest relative size increases occurred in the digestive system (including liver). Birds preparing to initiate spring migration from Panama deposited only fat, suggesting that changes in lean body components take place after migration has begun, possibly through training effects. Sex did not influence body composition. Juveniles making their first southward migration were similar to adults in structural size and body mass but had substantially enlarged alimentary tracts. Sandpipers appeared to deposit lean mass during stopover in fall but not in spring. The dramatic enlargement of the digestive system in this small species that makes short flights and fuels frequently contrasts with the reduction of digestive components in larger species that fuel only once or twice by making one or two very long flights to their destination.

Digestive response to restricted feeding in migratory yellow-rumped warblers

Smaller guts and slow initial mass gains at stopover sites have led to the idea that digestive physiology limits refueling rates in migrating birds. We tested the digestive-limitation hypothesis in yellow-rumped warblers using food restriction to simulate infrequent feeding during migration, which may cause a reduction in alimentary tract mass. Restricted birds had small intestine, pancreas, and liver masses 18%-22% lower than ad lib.-fed controls. Total activities of sucrase, maltase, aminopeptidase, and amylase were significantly lower in restricted birds, while those of trypsin and chymotrypsin were not. Only aminopeptidase mass-specific activity was significantly lower in restricted birds. Previously restricted birds were able to feed and digest at a high rate immediately following return to ad lib. feeding. Digestive efficiency did not differ between groups. These results suggest that before migration yellow-rumped warblers have some spare digestive capacity to compensate for declines in their digestive organ masses during migration.