Kestrels' strategies for flight and hunting vary with the weather

Even so, these small raptors expend a constant amount of energy per foraging trip

Date: June 7, 2017

Source: PLOS

Kestrels adapt their flight and hunting strategies to weather conditions, including solar radiation, wind speed, and air temperature, according to a study published June 7, 2017 in the open-access journal PLOS ONE by Jesús Hernández-Pliego from Estación Biológica de Doñana, Spain, and colleagues.

Theory predicts that to maximize fitness, foraging animals should gain the most energy for the least cost of time and energy. Hernández-Pliego and colleagues tested this theory in kestrels, small insectivorous falcons that return to their nest to feed their chicks after a foraging trip. Kestrels have the options of commuting via time-saving flapping flights or energy-saving soaring-gliding flights, and of capturing prey via time-saving active hovering flights or energy-saving perch-hunting.

To explore whether the birds' behavioral decisions reflect trade-offs during the commuting and searching parts of foraging trips, the researchers tracked lesser kestrels with GPS and tri-axial accelerometers, which provide data on energy and time budgets in wild animals. The study included six kestrels (four males and two females) from two breeding colonies with in the Guadalquivir river basin of southwestern Spain.

The researchers found that kestrels' behavioral decisions varied with weather conditions. When solar radiation increased -- which strengthens thermal updrafts -- commuting birds replaced flapping with soaring-gliding. When wind speed increased -- which provides a stronger lift -- searching birds replaced perch-hunting with hovering. But kestrels also hovered more when air temperature increased possibly as warmer weather boosts the activity of large grasshoppers, the birds' preferred prey. While flight and hunting strategies varied dramatically with the weather, the researchers also found that the energy expended per foraging trip remained constant. This suggests that the birds have a fixed energy budget per foraging trip and adjust their behavior accordingly when environmental conditions vary.

Read on

Populations of most birds of prey increasing but kestrels dramatically declining

By Tim Pauling

Most species of birds of prey in Scotland are increasing, according to a new report.

The study published by Scottish Natural Heritage (SNH) shows that certain birds of prey such as goshawks, buzzards and sparrowhawks have shown signs of recovery over the past seven years due to efforts to combat persecution, habitat loss and pesticides.

However, not all species have increase and some declines have been “stark”. Numbers of kestrels, a once common and widespread bird, have decline and they are now becoming scarce in many part of Scotland.

Gordon Riddle, Scottish Raptor Group coordinator, said there was no clear evidence why kestrels were declining.

Continued ...

Kestrels, Other Urban Birds Are Stressed by Human Activity

May 10, 2013 — American kestrels, small colorful falcons often seen perched along roadways, are abundant in urban and agricultural areas. Shorter grass makes insects, snakes, mice and other prey more visible, and signposts, fences and telephone poles provide excellent perches. However a new study from scientists at Boise State University in Idaho shows that even species considered "tolerant" of human activity may be adversely impacted by human disturbance; Kestrels nesting in close proximity to roads and developed areas had elevated stress hormones and high rates of nest abandonment. The apparently favorable location, then, becomes an ecological trap. 

The peer-reviewed paper "Reproductive failure of a human-tolerant species, the American kestrel, is associated with stress and human disturbance," was published in the British Ecological Society's Journal of Applied Ecology(May 10, 2013). Boise State University graduate student Erin Strasser, now with the Rocky Mountain Bird Observatory, and Julie Heath, a professor in the Boise State Department of Biological Sciences and Raptor Research Center, conducted the research. 

Strasser and Heath conducted research along one of Idaho's major expressways, Interstate 84, and in suburban and rural areas south of the state's capital city of Boise. Since 1987, researchers from Boise State and the U.S. Geological Survey have monitored a number of nest boxes located along the area's roadways, in people's back yards, and in sagebrush-steppe habitat. 

In this study, Strasser and Heath were interested in understanding how human-dominated landscapes affect breeding kestrels, with particular attention paid to the link between disturbance, stress and nest failure. The two monitored the boxes to determine nest fate, and collected a small blood sample from adult birds. The researchers were looking at corticosterone levels, which indicate stress levels (the equivalent in humans is cortisol). Corticosterone can lead to behavioral and physiological changes that allow individuals to cope with stressful situations, while suppressing other activities such as reproduction. 

Continued: http://www.sciencedaily.com/releases/2013/05/130510102025.htm