Herbert Biebach

Digestive tract function in the long-distance migratory garden warbler, Sylvia borin

Digestive tract morphology and function of captive garden warblers (Sylvia borin) were measured during four stages of their endogenous circannual rhythm: before, during and after their autumn fattening prior to migration to wintering grounds in Africa, and after a partially simulated migratory flight. Food intake increased by 33% during fattening, utilization efficiency of dry matter tended to increase, and that of energy increased significantly (P0.01). This was because digestive tract capacity (measured as dry tissue mass) increased, so that mean retention time of food remained constant before, during and after fattening (80–84 min). After a 48-h period of starvation of fattened birds to partially simulate a migratory flight, food intake was lower on the first day of refeeding than on the next 4 days, and utilization efficiency was higher on that day, at least partly because of a longer mean retention time (111 min versus 78 min on the third day). Digestive tract dry tissue mass fell by 50% during starvation, and that of the small intestine by 63%. It is concluded that the garden warbler adapts to long-distance migration without feeding by rapidly reducing the size of its digestive tract, an expensive tissue to maintain, during migration in order to save weight and energy, and possibly also to supply part of the fuel and protein required for the flight. The cost of this strategy appears to be the time taken to rebuild the gut at stopover sites with food, but the low probability of finding such a site in the Sahara Desert means that this strategy is probably optimal for garden warblers.

Sahara stopover in migratory flycatchers: fat and food affect the time program

Migrating spotted flycatchers, resting and feeding in an oasis, have longer stopover periods when fat reserves on arrival are low. In the laboratory migratory activity could likewise be suppressed by a combination of two factors: low fat reserves and the possibility of feeding.

Time-and-place learning by garden warblers, Sylvia borin

Abstract The ability of birds to learn associations between temporal and spatial changes in the daily pattern of food availability was investigated. Five captive garden warblers were trained to go from a central living room to four different feeding places, each one offering food for 3 h at different times of day. The birds reached their asymptotic level of correct choices (choosing the place with food availability) of 70% within 11 days. On test-days with food accessible in all places and at all times of day, the daily spatial and temporal patterns of feeding still persisted, showing that some kind of timing mechanism is involved.

Phenotypic organ flexibility in Garden Warblers Sylvia borin during long-distance migration

During long-distance migration, passerine birds alternate between flight and stopover phases. Flight phases can last one to three consecutive nights with rest during the days; stopover phases last one to three weeks. During long flight phases, such as across the Sahara desert, the substrate of catabolism is fat and protein in proportions of about 3 to 1. Comparison of migrating Garden Warblers Sylvia borin before and after the desert crossing shows that the protein originates primarily from the muscles of the breast and leg and from the digestive tract. Breast and leg muscles are reduced by 19%, the digestive tract by 39% in mass. A simulation of the flight phase by food deprivation over 48 h results in the same extent of organ reduction in the digestive tract but not in the breast and leg muscles, which indicates that different mechanisms are involved. It is hypothesised that during extended flights, without food intake, protein metabolites serve as intermediates in the citric cycle for the oxidation of fat. Loss of protein metabolites is at the expense of tissue proteins from the above-mentioned organs. This hypothesis does not, however, exclude other hypotheses about the functional consequences of hypertrophy and atrophy in specific organs. Flexibility in size of the breast muscle may also be a mechanism to adjust the power output to the changing power requirement during flight with change in fat load. Flexibility in the digestive system may save maintenance energy during flight at the cost of restoration during stopover.

Strategies of Trans-Sahara Migrants

Major deserts of the world, like the Sahara, the Middle Asian desert, and the central Australian desert, are potential ecological barriers for migrating birds. In terms of size, annual rainfall, and temperature the most severe of these is the Sahara, which stretches from the Atlantic coast in the west to the Saudi Arabian peninsula in the east and covers an area of 12 million km2. Mean daily maximum temperature during the migratory period in spring and autumn is between 25° and 38°C. Surface temperature may reach 70°C. During the day, relative humidity drops to 10%. Birds breeding in the west- and mid-Palearctic region and wintering in tropical Africa have to fly across the Sahara, a distance of at least 1500 km. The ecophysiological problems encountered by these birds during the desert crossing and their strategies overcoming these problems are discussed in this chapter.

The Effect of Constant Light and Phase Shifts on a Learned Time-Place Association in Garden Warblers (Sylvia borin): Hourglass or Circadian Clock?

Garden warblers are able to learn an association between time of day and feeding place. In constant dim light and constant food availability, the learned feeding pattern (successive visits to four feeding rooms for approximately 3 hr each) persisted for at least 1 day in three birds and for at least 6 days in one bird. The free-running feeding rhythm had a period of slightly greater than 23 hr. In response to a 6-hr phase advance of the light-dark cycle, the birds advanced their learned feeding pattern by 2.6 hr on the first day, whereas a 6-hr phase delay had no significant effect. These results are consistent with the hypothesis that the birds use a circadian clock rather than an hourglass mechanism of timing. This conclusion is further supported by the response of birds to forced interruptions of the daily feeding pattern (Krebs and Biebach, 1989).

Food availability affects migratory restlessness in caged garden warblers (Sylvia borin)

the territory of another toad it reacts to the arrival of waves either by retreating (response types 1 or 3) or by engaging in territorial behavior ( types2 or 4). The interaction between sender and receiver may determine the territorial boundaries between two calling male toads. Significantly the minimal territorial distance between two calling males (i.e. around 50 cm [5]) coincides with the threshold of detectability of surface-waves in the European fire-bellied toad.

Differential catabolism of muscle protein in garden warblers (Sylvia borin): flight and leg muscle act as a protein source during long-distance migration.

Abstract. Samples of flight and leg muscle tissue were taken from migratory garden warblers at three different stages of migration: (1) pre-flight: when birds face an extended flight phase within the next few days, (2) post-flight: when they have just completed an extended flight phase, and (3) recovery: when they are at the end of a stop-over period following an extended flight phase. The changes in body mass are closely related to the changes in flight (P<0.001) and leg muscle mass (P<0.001), suggesting that the skeletal muscles are involved in the protein metabolism associated with migratory flight. From pre- to post-flight, the flight and the leg muscle masses decrease by about 22%, but are restored to about 12% above the pre-flight masses during the recovery period. Biochemical analyses show that following flight a selective reduction occurred in the myofibrillar (contractile) component of the flight muscle (P<0.01). As this selective reduction accounts only for a minor part of the muscle mass changes, sarcoplasmic (non-contractile) and myofibrillar proteins of both the flight and leg muscle act as a protein source during long-distance migration. As a loss of leg muscle mass is additionally observed besides the loss in flight muscle mass, mass change seems not to be strictly associated with the mechanical power output requirements during flight. Whereas the specific content of sarcoplasmic proteins in the flight muscle is nearly twice as high as that in the leg muscle (P<0.001), the specific content of myofibrillar proteins differs only slightly (P<0.05), being comparably low in both muscles. The ratio of non-contractile to contractile proteins in the flight muscle is one of the highest observed in muscles of a vertebrate.

Great tit fat reserves under unpredictable temperatures

We tested the effects of unpredictable temperatures on fat reserves in Great Tits Parus major. During one treatment, the temperature was constant at 8.50C. In the other, temperatures fluctuated between 1.5 and 15.50 on a 24-h basis, with changes occurring just after lights-out. Residual evening weights were higher during the period of unpredictable temperatures. At the end of the period with unpredictable temperatures, more weight was gained on cold than on warm days. During the unpredictable temperature treatment, birds defecated less while eating the same amount. Nightly weight loss depended upon evening weight level, but not upon overnight temperature. Our results suggest that Great Tits use daily temperatures to predict conditions for the following night and that they regulate overnight expenditures to match reserve levels.

Effect of Clutch Size and Time of Day on the Energy Expenditure of Incubating Starlings (Sturnus vulgaris)

The steady-state energy expenditure of incubating starlings (Sturnus vulgaris) was studied at different air temperatures, at day and at night, and on different clutch sizes. During the night, energy expenditure (oxygen consumption) increases linearly from 20 down to - 10 C air temperature. During the day, the values at 25 and 10 C air temperature are 50%-60% higher than during the night but only 10% higher at -5 C. This increase is due to the digestive calorigenic effect of food at 25 and 10 C, but at -5 C the heat of the calorigenic effect substitutes for the thermoregulatory requirements. Increasing clutch size results in greater energy expenditure of the incubating starling below 20 C air temperature but has no effect in the thermoneutral zone. Each increase of clutch size by one egg increased the energy expenditure by 3%-5%. The lower critical temperature increased from 14.1 C for a clutch of two eggs to 23.6 C for a clutch of eight eggs. The consequences of different clutch sizes for the time and energy budget of free-living birds are discussed.