Thesis abstract
In many migratory populations, only some individuals migrate each year whereas others are resident; this is known as partial migration. Empirical studies of the causes and consequences of altitudinal partial migration are few, and the costs and benefits of the different life-history strategies involved require further study. In this thesis, I investigate the effect of altitude and other factors on the timing of breeding, reproductive success and parental care in a partially migrant species; the dipper (Cinclus cinclus), a specialised riparian passerine that is thought to be declining in the UK. Most dipper populations comprise partial migrants that move to higher altitudes to breed and sedentary residents at lower elevations, but little is known about the effects of elevational gradients on the life-history traits and reproductive success of these birds. Here, I used mixed effect models to analyse field data from a long-term study population and: (1) examine the effects of altitude, female age and size on lay date, nestling mass, fledging success, number of fledglings and annual productivity; and (2) investigate the effect of altitude and prey availability on parental provisioning rate. Compared to residents, migrant dippers bred later and were less likely to have a second brood, possibly influenced by the time taken to migrate and establish breeding territories but were equally likely to fledge young, perhaps because of lower risk of flooding and predation. Altitude had no effect on nestling mass, number of fledglings or annual productivity. Provisioning rates decreased with increasing altitude despite no apparent difference in prey availability and this suggests increased time travelling to foraging sites possibly compensated by reduced handling costs and increased prey loads. Altitudinal migration is likely to be a tactic driven by competition for lowland nest sites. Annual productivity of subordinate migrants was unaffected by migration and this is likely to be because single broods are offset by fewer nests lost to flooding and predation, and compensatory provisioning strategies match those of residents. Together, these results provide insights into the impact of elevation on breeding ecology in birds and suggest that there are adaptive benefits to both altitudinal migration and residency. Age may not affect timing of breeding but older females are more likely to produce heavier nestlings, fledge broods, and have a higher number of fledglings. This may be explained by previous breeding experience and more investment in parental care. However, a decline in productivity with age suggests senescence. Further research is needed to determine the underlying causes of migration and residency, and to establish whether the net fitness benefits of the two behaviours are similar.