S15.Summary: Stopover ecology of migrant landbirds

The probability a migrant will meet its nutritional requirements and achieve safe passage is correlated with the intrinsic suitability of stopover habitat. Suitability of en route habitat depends largely on three factors: (1) foraging opportunities, (2) competition with other migrants and with resident birds, and (3) shelter against predators and adverse weather. Possibly the single most important constraint during migration is to acquire enough food to meet energetic requirements, especially for long-distance migrants that must overcome geographic barriers. Other factors besides food availability may influence the suitability of stopover habitat and affect habitat use. (1) Physical structure, such as plant species composition and foliage structure may influence how birds move through the habitat and how they capture prey. (2) Habitat extent or patchiness may be an important factor because migrants may require different threshold levels of habitat area below which they find habitat unsuitable. Suitable habitat associated with ecological barriers, for example, is often fragmented and many woodlands average only a few hectares in area. If fragments are widely dispersed, the opportunity to gain access to conditions wherein fat stores can be safely replenished would be restricted. (3) Dehydration and water economy might constrain migratory range. Lean migrants that have mobilised carbohydrate or protein sources in response to increased energy demand might experience a serious water balance problem. (4) Predation may play a role because stopover habitats undoubtedly vary in predation risk. When the best areas for depositing fat are also the most dangerous, the migrant must trade off energy gain against mortality risks. (5) Despite high oxidative capacity, migratory birds may experience muscular fatigue during sustained flights and stop to metabolise lactate and "repay" an oxygen debt, regardless of how much fat remains. Stopover would also be required for tissue repair if migrants are forced to catabolize muscle tissue to offset unexpected energy demands or if muscle fibres are damaged during sustained, long-distance flight.

Although we might expect migrants to settle in habitats on the basis of suitability, that outcome is not assured. Over the course of a season's migration, a migratory bird encounters a variety of habitats, most of them new habitats with associated new food, new competitors, and new predators. After a night's passage it finds itself in a habitat that may be very different from the one occupied the previous day, let alone the previous year. Moreover, favourable en route habitat, where migrants can rapidly accumulate energy reserves, is probably limited in an absolute sense, or effectively so because migrants have limited time to search for the "best" stopover site. Nevertheless, evidence indicates that migrants prefer certain habitats and select among alternatives during stopover, presumably in response to differential suitability.

The ecological diversity of migratory species, coupled with the often variable weather patterns that steer migratory movements, make assessment of habitat requirements and development of management strategies for migrants particularly difficult. The complexity of this issue, and the fact that the abundance of migrants found at individual stopover sites varies from year to year, tends to devalue the migratory period when developing conservation programs. Because intercontinental landbird migrants spend more of their lives in breeding and wintering habitats, these areas become natural targets for conservation efforts. Nevertheless, if the persistence of migrant populations depends on the bird's ability to find favourable conditions for survival throughout the annual cycle, factors associated with the en route ecology of migrants must figure in any analysis of population change and in the development of a comprehensive conservation "strategy" for landbird migrants.

Consider the consequences of en route habitat loss on landbird migrant populations. The density of landbird migrants will increase in remaining areas, which will intensify competition. Increased competition may reduce food availability and increase interference, thereby slowing migration, delaying arrival on breeding and wintering areas, not to mention increasing predation pressure. Increased competition may also redistribute birds among habitats, with younger, less experienced migrants forced into poorer sites where mortality rates are expected to be higher.

If mortality is concentrated in the migratory period, then factors that increased cost of migration could have a disproportionate influence on overall population levels. Thus, while individual fragmented woodlots may represent local population sinks on breeding grounds, birds in these habitats can often select alternative or more productive habitats. In contrast, the rigors of migration often place birds close to their physiological limits in unfamiliar landscapes, where they do not have the luxury of selecting alternative habitats. Therefore, a lack of suitable stopover habitat will result in death or reproductive failure for migrants and contribute substantially to future population declines.