Back in September I wrote a blog in which I introduced my research work on the wheatear (Oenanthe oenanthe). Today I’m going to delve a bit further into this work, focusing on how food availability during the breeding season affects survival.
As I mentioned previously, the wheatear is a migrant songbird that undertakes some of the most extraordinary migrations of a bird of this size. Overwintering in sub-Saharan Africa, wheatears travel hundreds or thousands of kilometres to reach their summer breeding grounds. The birds spend the winter in the dry Sahel zone of Africa, where they must find enough food to survive until spring migration. Spring migration commences with a period of fuelling: food intake rates increase and the birds build up flight muscle and lay down fat, which will be used as energy to power long periods of flight. On the way, wheatears stop at several locations in order to rest and refuel, before finally arriving at their destination.
Timing of arrival is important; arrive too soon and there may be insufficient food for the birds to maintain body weight. Arrive too late and the best territories will have gone and it may be impossible to lay eggs quickly enough to ensure that the young are fed and fledged at the time when there are plentiful supplies of the insects that they eat. It is this issue of food availability that I investigated during my PhD. By supplementary feeding randomly selected pairs of wheatears and comparing their breeding success, survival and migratory fuelling with randomly selected “control” pairs (i.e. not given extra food), I could see what would be the likely outcome of changes in natural food supplies. Over 100 pairs of wheatears bred each year on the tiny island of Fair Isle, my study site.
An earlier study on black-throated blue warblers (Dendroica caerulescens) in the Americas by Sillett and Holmes found that over 85% of mortality occurred during migration. Of course this might not be the case for all other migratory songbirds, but it is reasonable to assume that it is likely to be pretty close. I was interested in seeing whether changes in food supply at the breeding grounds could affect annual survival rates – the proportion of birds surviving from year to year. The impact of such changes could still be manifest during migration as they might affect the condition of birds setting out on autumn migration and hence their ability to survive the long and perilous journey to Africa.
Manipulating food supply was straightforward, if laborious. Almost every day from when wheatears paired up until they had left in the autumn, I or an assistant would visit each fed territory and fill up a bowl with mealworms. The extra food was an experimental simulation of increased food availability that may occur through factors such as climate change or improved land management. A food reduction experiment would have been controversial but if a response to increased food was found, this would show that food was a limiting factor and so, by proxy, a reduction would have had a negative impact. Measuring survival involves following individual birds. I did this by capturing them and fitting them with their own unique combination of colour rings that could be read from a distance with binoculars or a telescope. Each year I searched Fair Isle for all colour ringed birds so that I could work out which ones made it back. I also knew whether they had been supplementary fed the previous year, either as an adult or as a juvenile.
Both males and females contribute to offspring care. Breeding therefore entails a personal cost to both of them, which could affect their likelihood of surviving to the next season.
Although the food-supplemented parents did not raise any extra young per nest than the control pairs, some of them did go on to raise an extra brood, or – in the case of some males – raise a second brood with an additional female while also helping with the first. Young were no more likely to survive if their parents had additional food available. This suggests that at the time when most pairs had chicks to feed, food was plentiful and was not limiting chick development (food limitation of chick development can’t be ruled out, however). Once fledged, however, young that had access to supplementary food were 1.56 times more likely to survive to the next season – a huge increase.
As for the adults, extra food led to a 1.22 times increase in survival rates. It seems as though the supplementary feeding may have helped the adults maintain their own condition, which in turn would help them see out the migration and wintering periods.
Food availability drops off substantially late in the breeding season, which is the reason why migratory birds leave for distant wintering grounds in the first place. This study suggests that young birds are disproportionately affected; with extra food they survived nearly as well as adults. Environmental changes that affect food availability are therefore likely to affect young birds to a greater extent than adults; what will this mean for wheatear populations?