Weather‐related mass‐mortality events in migrants

Abstract

A major perceived cost of migration in birds is the associated mortality. This mortality has proved difficult to measure and separate from mortality during stationary periods of the annual cycle. This paper reviews some major recorded mortality incidents among migratory birds attributed to inclement weather and other factors, including: (1) in-flight losses, caused by storms and other adverse weather en route, usually over water; (2) unseasonable cold weather soon after arrival in breeding areas; and (3) unseasonable cold weather before departure from breeding areas. Cold weather often kills migrants in their breeding areas, but not the local resident species which can better withstand it at those times. For migrants, cold and snow act to cut off the food supply, and can have a major selective effect on the seasonal timing of migration. Records of in-flight weather-induced mortality, involving up to hundreds or thousands of birds at a time, have affected mainly small passerines, but also larger birds, including eagles and swans. Most occurred in conditions of mist, rain or snow storms, and some involved nocturnal collisions with illuminated masts and other tall structures. Records of post-arrival mortality in breeding areas have involved mainly small insectivores (especially hirundines), but also waders and waterfowl. Such incidents, associated with cold and snow, have reduced local breeding densities from the previous year by 25-90%, depending on species and area, with up to several years required for recovery. Records of pre-departure mortality on breeding areas have mainly affected hirundines. Two major incidents in central Europe in September 1931 and 1974 killed hundreds of thousands, or even millions, of swallows and martins. After the latter incident, House Martin Delichon urbicum populations in Switzerland the following year were reduced by an estimated 25-30%. Such climatic extremes that occurred in spring or late summer in particular parts of the breeding range have been recorded at approximate mean frequencies of 2-10 per century. Average daily mortality in many bird species can clearly be much greater during migration periods than during stationary periods. Despite the heavy losses of birds on migration, it may be assumed that migration persists in the long term because the fitness costs (in terms of associated mortality) are more than offset by the fitness benefits (in terms of improved overall survival and breeding success) that accrue from migration.

Extreme variations in spring temperature affect ecosystem regulating services provided by birds during migration

“…Nevertheless, the fitness contribution of stopover sites will likely become increasingly unpredictable among years as weather exhibits more extreme variability (Winkler et al 2014). This variability may lead to greater mortality during migration (Newton 2007), to different spatial patterns of migration (Streby et al 2015), or to lower reproductive effort (e.g., number of eggs laid; Winkler et al 2002). Therefore, in addition to understanding the relationship between conditions a migratory bird experiences on the wintering grounds and their breeding season success, our results suggest that quantifying environmental conditions and bird fitness at stopover sites may provide further insights into the effects of extreme weather and climate change on avian populations.…”

Weather-induced mass deaths of the Common House Swift Apus affinis in Thengumarahada Village of the Nilgiris, southern India

“…Subsequently, these birds drowned as the storm water added more weight. Also the heavy rain saturated the plumage and increased the wing weight causing a loss of body heat (Newton 2007). After the rain, some of the live birds were collected by the study team and their wings were dried and they were allowed to fly.…”

“…Any abnormal variation in the weather condition is likely to affect the survival of the native species in their habitats. In the case of birds, inclement weather conditions can affect the feeding activities and can cause mass mortality owing to starvation (Newton 2007). Especially, for insectivorous bird species, a sufficient quantity and accessibility of food is very crucial, which in turn depends on favorable…”

A Habitat-based Wind-Wildlife Collision Model with Application to the Upper Great Plains Region

“…Adverse weather, both storms and cold weather, can cause high mortality events for bird migrants (Newton 2007). Storms can cause large migratory fallouts in unfamiliar locations with poor shelter.…”