RUNNING ON EMPTY: THE WONDER OF CAPITAL BREEDING

Most people know that birds fatten up before migration to give them extra fuel for the journey. But did you know that some birds actually carry enough reserves with them to get them through their entire breeding cycle, too? Welcome to the wonder of capital breeding!

WHAT IS CAPITAL BREEDING?

Capital breeding refers to using existing or stored reserves to reproduce at a date later than when the bird consumed the food. The language comes from the world of finance: ‘capital’ is cash or wealth that you already have – that is built up, accumulated, locked-in, and not released or spent until an (often much) later date, while ‘income’ is cash that you receive and spend directly without squirrelling it away.

Carrying reserves (‘capital’) comes with a cost – increased reserves mean increased flight costs per unit travelled because the bird is heavier. All else aside, it doesn’t make sense for a bird to carry more reserves than are necessary unless it provides serious benefits – who wants to make a trip of thousands of kilometres more difficult than it needs to be? 

Why would a bird carry extra reserves all the way through a migration? 

GETTING THE JUMP ON BREEDING

Some birds – like artic geese – arrive in their breeding grounds before food is available. Being able to lay eggs, incubate and raise young on body reserves allows them to get a move on and start breeding before food becomes available for the season. 

Having young earlier in the season means that capital breeders can hatch eggs and raise young but still have time to prepare themselves for their return migration, giving them better chances of survival on the return trip. It also makes them less vulnerable to delays in food availability caused by the weather or disturbance. 

Breeding earlier in the season is a benefit for the chicks too. Research has shown that birds born later in the feeding season fare worse when adverse conditions hit during winter, so the earlier the chicks are born, the more likely they are to survive any hardships that come their way.

In some birds, like Lesser Snow Geese, eggs actually start to form during migration, allowing the birds to begin the breeding cycle just a few days after arriving, completing the whole cycle on body reserves alone: these are the true capital breeders of the bird world. 

Only after the chicks hatch do birds like Lesser Snow Geese and Canada Geese feed normally again and begin to build their own reserves back up again for their winter migration. 

WHAT DOES IT TAKE?

How much fuel a bird requires in order to engage in capital breeding depends on the bird’s sex, weight, the size of its clutch and how much additional food (if any) it is going to take in once at the breeding ground or during the breeding period. 

Often, the weight increase required for migration and capital breeding is substantial. In Canada Geese that winter in Minnesota, females increased their body weight by 36% before migrating (males by 26%). Egg laying requires not only fat, but protein and mineral reserves for eggshell calcium, so these need to be included in the capital too. This gives the female geese the reserves they need to produce eggs and raise young, and males the reserves they need to defend the nests.

ISOTOPE ANALYSIS

A fascinating way to find out whether birds are reproducing based on reserves or on food eaten at the breeding site is by isotope analysis of their eggs. 

Foods from different areas of the world have different isotope ratios (the abundance of different chemical elements), and for a while these ratios are apparent in the birds and the eggs, giving us clues as to where the bird last fed or where the reserves used to create the egg were taken on. 

This type of analysis tells us that some ducks like Mallards and Shovelers also depend partly on reserves built up in wintering areas for their breeding season, as do some waders, like Turnstones and Red Knots. 

CAPITAL VS INCOME

The opposite of a capital breeder is an income breeder – a bird that fuels its breeding season wholly with food eaten at the breeding site, relying on the allocation of resources directly to reproduction at the time that they are taken on. 

This means the bird has less weight to carry during migration, making the flight easier, but means that it’s vitally important that food is available when the bird arrives at the breeding grounds and it also often means that the bird has to leave the nest during the incubation period to seek food, increasing the risk of predation.  Sometimes, it means that the bird reproduces later in the feeding season, which could give chicks a lower chance of survival if adverse conditions hit in winter, and could inhibit the parents’ ability to prepare themselves for the winter season.

A SLIDING SCALE (OF WONDER)

Like most things in life, though, it’s more of a scale than a black and white either/or. The extent to which a species or individual bird relies on capital reserves for breeding depends on many factors, such as how close the last stop over site is to the breeding area (as this dictates how far the bird will have to carry the extra fuel), the availability of food at the breeding ground, the length of the feeding season, and the size of the bird (in larger birds, clutch weight is a smaller proportion of body weight, so larger birds are more likely to rely on body reserves for egg formation). 

Some birds even adapt their strategy year to year, depending on the success of the approach they took in the previous breeding season. 

Carrying extra fuel for thousands of kilometres allows capital breeders, or breeders on that end of the scale, to get in earlier on the breeding game and have their chicks ready for migration before it gets cold. The trade off is the extra energy required to transport the reserves – but what an amazing feat it allows!

RESOURCES

On the science of capital breeding: Newton, I., 2020. Bird Migration. William Collins

On the differences between the alternative methods of fuelling breeding: Jönsson, K. Ingemar. “Capital and Income Breeding as Alternative Tactics of Resource Use in Reproduction.” Oikos, vol. 78, no. 1, 1997, pp. 57–66. JSTOR, http://www.jstor.org/stable/3545800. 

On the comparative success of capital and income breeding seasonally and in different species of birds: Sainmont, Julie & Andersen, Ken & Varpe, Øystein & Visser, Andy. (2014). Capital versus Income Breeding in a Seasonal Environment. The American naturalist. 184. 466-476. 10.1086/677926.