For every season, tern, turn

Nothing is permanent.

As an evolutionary biologist I am very used to the concept of change, especially slow change over long periods. As an animal behaviour scientist I am used to the concept for very rapid change over short periods. Some of my career has been spent in reconciling the two, e.g. in penguins.

Finding the right place to lay your eggs and raise your chicks is very important. This is a black-fronted tern nest. Image from Courtney Hamblin.

Behaviour seems inherently plastic. Behaviour constantly changes. When I brush my teeth I don’t move the brush in exactly the same way that I did the previous day. There are subtle variations. I might start on the top left. Or the bottom right. I might do the sides of the teeth before the back.

The evolutionary biologist in me might think that with so much variation and plasticity, that behaviour should be unable to shape evolution where changes are measured in generations (not seconds). However, the fact that I brush my teeth morning and night is very predictable. I might vary when it occurs but usually it would be around 8am and 11pm. Also, tooth hygeine behaviour is something that is common throughout the human species (and presumably other primates). So some aspects of behaviour are a more long-term than others.

One group of behaviours that we might expect to be more permanent over time (and generations) are those concerning reproduction. It matters to the evolutionary fitness of a species that individuals are able to meet other members of their species, assess individuals and mate successfully. That is why salmon fight their way upstream to spawn in particular rivers and seabirds migrate half way around the globe to return to specific colonies to lay their eggs.

The Waimakariri River, a braided river. Image from Adrian Paterson.

Returning constantly to a particular areas is known as breeding site philopatry. Why would individuals do this? First, their parents successfully bred in this area. Second, the conditions were obviously good as the offspring they produced have survived well enough to return and breed themselves. Many, if not most, species have some faithfulness to particular sites for breeding. They might spend the rest of the year exploring and exploiting other locations for food, where survival rates are better, but they will return to this known area for breeding.

What if your physical breeding site changes from one year to the next? Does returning to that site make any sense? If your patch of rain forest burns down or a developer reclaims your wetland for a subdivision then perhaps this is not the best idea.

All habitats change if you give them enough time but there are some habitats that naturally change between breeding seasons. Braided rivers are one such habitat.

Braided rivers are networks of river channels that are separated by small islands. They often form where there are coarse sediments and relatively high altitude gradients. The channel courses change with any minor flood event. What was an island one week may be connected to the bank of the river the next.

Braided rivers, although rare worldwide, are a key habitat in New Zealand, especially on the east coast of the South Island. Many species have adapted to living in this high energy, dynamic habitat. One such species is the black-fronted tern (Chilidonias albostriatus).

A young tern goes about its foraging. Image from Courtney Hamblin.

As land use has changed there has been a gradual loss of area for braided rivers as introduced weed plants have invaded and begun stabilising these habitats. Introduced mammal predators have also flooded this landscape. These factors have led to a serious decline in the population of the black-fronted tern, such that it requires active conservation.

The changing nature of the braided river habitat complicates conservation efforts. A typical strategy for bird conservation is to enhance and protect breeding areas to allow the greatest number of adults to mate and chicks to survive. Courtney Hamblin, a Masters student from Lincoln University, set out to determine just how often the terns returned to particular breeding areas.

Courtney has published a paper in Notornis, with her supervisors Adrian Paterson, James Ross and Richard Maloney. Courtney used black-fronted tern data gathered over twelve years from nine South Island rivers. Volunteers and Department of Conservation workers made observations about where terns bred along these entire rivers over that time.

Terns came back to similar areas at least some of the time. Courtney divided each river up in 300 m segments and randomised where terns returned to in order to create a null distribution. Even without site philopatry, terns will occasionally come back to the same section or close by next season by chance. Courtney wanted to make sure we were not being mislead by this.

River channels often change in braided rivers. Image from Adrian Paterson.

Courtney was able to show that for seven rivers the terns came back to the same sites in different breeding seasons no more than predicted by chance. Only in the Ashley and Tasman Rivers were they more likely to come back to similar areas in some years.

From a conservation point this is important to know. There is no point in putting weed removal and predator control into a breeding area if the terns are unlikely to come back there. It might be more useful to predict where the birds might select to breed (difficult) or work out how to entice them to particular areas (less difficult, see Courtney’s other work in social attraction of terns).

Understanding how species respond and behave in braided rivers will allow us to increase our conservation success for these species, as we return the tern to its former numbers. It may not be a permanent solution, but it should last a long time.

Leave a Reply