Partial Altitudinal Migration of a Himalayan Forest Pheasant : First Insights and Conservation Implications

Abstract

Animal migration is a complex phenomenon exhibited across many taxonomic groups and occurs in every major ecosystem on our planet. Given the sheer number of animals on the move, migration profoundly shapes, alters and regulates our environment. Though animal migration has been a subject of study for decades, the ultimate drivers and consequences of migration are still not clearly established. Bird migration in particular has received a great deal of attention from biologists. Till date, most studies have focused on long distance, cross continental latitudinal migrants. However, a significant proportion of birds undertake annual migrations along elevational gradients across mountain regions of the world. So far, only a few studies have examined altitudinal migration systems due to which altitudinal migrations remain poorly understood. This thesis therefore is the first documentation of an altitudinal migration system in the high altitude Himalayas of Bhutan with state-of-the-art accelerometer enabled GPS telemetry.I present the first detailed patterns for a complex partial altitudinal migration system for the Satyr Tragopans (Tragopan satyra) in the Bhutan Himalayas. Contrary to current perceptions, I found that altitudinal migration is not a simple up-and-down slope movement of individuals by documenting 3 main patterns of migration: 1) crossing multiple mountains; 2) descending/ascending longitudinally; 3) moving higher up in winter and lower down in summer. Migrants departed consistently across the years and much ahead of snowfall suggesting that altitudinal migrations do not occur as a response to extreme weather events. I also found that although females are more likely to migrate, no consistent patterns can be established to explain who migrates and who remains resident within sexes and therefore suggest that extant hypotheses consider both inter as well as intra-sexual differences while explaining partial migration systems. And I provide anecdotal evidence that individuals can switch strategy from being a migrant to becoming a resident.Within partial migration systems, proximate tradeoffs associated with the decision to either migrate or to remain a resident are not clearly understood. I therefore examined the possible tradeoffs related to migratory decisions in terms of energy expenditure and home range sizes. I found that winter home ranges for residents overlapped with conspecifics and were significantly larger than migrants whose home ranges occurred at discrete non-overlapping sites. Over the course of a migratory season, I did not find any significant differences between migrants and residents in energy expenditure as measured by dynamic body acceleration (DBA). Nevertheless, for migrants, I noted higher DBA scores and activity states associated with running/flying and walking during migration. Given that an individual’s migratory status does not significantly affect its overall energy expenditure despite manifesting differently in terms of space use, I suggest that fluctuating micro-habitat conditions across time may enable the maintenance of a partial migration system.Having assessed the patterns of migration and the possible proximate tradeoffs involved in such a system, I next considered the conservation requirements for partial altitudinal migrants. Both migrants and residents occupied forests all year round. Using accelerometer data, I found that migrants walk to-and-fro between summer breeding and non-breeding winter grounds taking multiple days and halting along mountain slopes across forested landscapes. I also found that females migrated in a south easterly direction while males migrated in random directions and that migrants occupied south-east facing slopes, while residents chose to remain on south-west facing slopes. I therefore suggest corridors in ideal situations would need to run in random directions and recommend that protected areas in mountainous regions include different habitat configurations (aspects) in addition to having a representation of all habitat types.The findings from my PhD thesis challenge many of the assumptions associated with altitudinal migrants and offer new perspectives to our understanding of both altitudinal and partial migration systems. In addition, by demonstrating habitat associations, migratory modes and pathways, the thesis contributes towards helping conserve such magnificent migrations which occur across the mountain regions of our planet. Taking advantage of the advances in accelerometer enabled GPS telemetry, my thesis has demonstrated that it is possible to track individuals and illustrate previously unknown patterns of migration over annual cycles. I recommend that future studies collate data across multiple years measuring explanatory variables associated with habitat quality, fecundity, mortality rates, individual condition and predation pressure, while simultaneously tracking both migrants and residents. Such a study will contribute significantly in helping understand and uncover both the proximate and ultimate causes and consequences of migrations.