Migratory connectivity: joining the dots

I had a fun day a few weeks ago curating the @biotweeps Twitter account on behalf of the BOU, discussing some ideas about migratory connectivity (hashtag #MigConnectivity). Whilst Twitter provides a good platform for immediate engagement, it doesn’t exactly promote longevity of information; the (insightful, profound & hard-hitting) points I was trying to make have quickly sunk to the bottom of the biotweeps news feed.

This seems a bit of shame, so I wanted to make a slightly more permanent record of these ideas.

[Caveat: I’m mostly focusing on long-distance migrant land-birds breeding in the northern hemisphere (especially Europe) and ‘wintering’ in the tropics (especially sub-Saharan Africa)]

>>Migrants in decline

There’s pretty convincing evidence that, on average, long distance migrant birds are currently declining relative to resident or short-distance migrants. This is not to say that long-distance migration has always been a necessarily ‘risky’ strategy; it presumably evolved for a reason, as an adaptation for the exploitation of environments which only have sufficient resources during part of the year. But migratory populations are somehow, at present, coping less well with environmental change than resident ones.

vickery_fig

In Europe, long-distance migrants are declining relative to short-distance migrants and residents. These are aggregate trends for birds recorded in 25 countries as parts of the Pan-European Common Bird Monitoring Scheme. From Vickery et al. 2013

Notably, migrants aren’t declining across the board. Species wintering in the ‘humid zone’ of Africa (i.e. the verdant tropical and sub-tropical forests) are currently doing particularly badly, whereas a few decades previously those wintering in the ‘arid zone’ (i.e. the greenish thorny bit south of the Sahara) were faring worse.

Even within a species, there’s variation in the health of migrant populations. This is really striking (or at least really well documented) here in the UK where migrants are, on the whole, doing worse than residents, but many migrants are doing better in the north and west (Scotland) compared to the south and east (England).

migrant-bbs-maps

Here are some cherry-picked examples of migrant birds (Willow Warbler, Cuckoo and House Martin) which are doing better in the NW (blue colours) than the SE (red colours). Maps from the BTO’s Breeding Bird Survey.

An understanding of ‘migratory connectivity’ – the connections between breeding and non-breeding populations – is obviously important for understanding these declines. If we want to protect British migrants throughout their annual cycle, or predict the population consequences of environmental change, it’s important to know where ‘our’ migrant populations spend the non-breeding season.

In this ‘loose-sense’, migratory connectivity is pretty uncontroversial and not that interesting (though the patterns revealed by studies of migratory connectivity obviously are).

I’m more interested in a ‘stricter’ definition, in which migratory connectivity is (somehow) measurable and falls along a continuum from weak to strong. ‘Weak’ connectivity reflects individuals from a particular breeding population sharing a large non-breeding area with individuals from other populations, whilst ‘strong’ connectivity reflects the use of discrete non-breeding areas by specific breeding populations. These labels are a bit counter-intuitive; weak connectivity means that a specific breeding populations is only weakly connected to any given non-breeding population (though the network of breeding and non-breeding populations has many connections!)

webster_fig

‘Strong’ connectivity (left) reflects the use of discrete non-breeding areas by specific breeding populations, whilst ‘weak’ connectivity (right) reflects individuals from a particular breeding population sharing a large non-breeding area with individuals from other populations. Figure from Webster et al. 2002

>>Connectivity: spreading and mixing

There are, by my reckoning, two main components to migratory connectivity – spreading and mixing. Spreading can be thought of as a population-level trait; how much do individuals from a particular breeding population spread out during the non-breeding season? Mixing occurs between populations; how much do individuals from different breeding populations mix during the non-breeding season? Both mixing and spreading are driven by the movements of individual migrants, which is why tracking is so important.

The above definition suggests that when a breeding population spreads out over a wide non-breeding area, it will mix/overlap with other breeding populations. Conversely, populations which have relatively low spread will remain spatially segregated during the non-breeding season.

But high spread and mixing don’t always need to coincide. Take the Eleonora’s Falcon, in which the entire species spends the non-breeding season in Madagascar; individuals from any particular breeding population don’t spread out that far, but they mix extensively with individuals from other populations. Equally, in a species which spreads over a massive non-breeding range, individuals from any particular breeding population can spread out quite far without mixing with individuals from other populations.

So, spreading and mixing represent separate components of migratory connectivity. But why are they important?

>>The importance of connectivity

Let’s go back to Scottish and English migrants.

Measuring how much they mix during the non-breeding season tells us about whether they’re exposed to similar threats / environmental change. My guess would be that, for most species, Scottish and English migrants will overlap completely overwinter – migratory mixing should be high. If Scottish and English migrants face the same conditions over winter, then problems in Africa, in isolation, can’t be responsible for the differing health of these populations.

If, on the other hand, mixing was low – with Scottish and English migrants ending up in different parts of Africa – then it would be worth exploring whether English populations are experiencing ‘worse’ conditions, which could be driving their decline. Actually, this describes pretty well what might be happening in British cuckoos; declining southern populations tend to migrate via Spain, which is more dangerous than the Italian route favoured by northern populations (it’s a bit more complicated than this, but you get the gist). This demonstrates that connectivity isn’t all about ‘winter’ sites; routes and stopovers are important too.

Spreading matters as well. British cuckoos show (remarkably, I think) low spread, with most individuals honing in on the Congo basin. This means that any local negative environmental change (or positive conservation action) should have a pretty concentrated effect on British cuckoos. Conversely, for a population with high spread which migrates to sites spread across half of Africa, the effect of any local environmental change will be more diffuse.

These thought experiments, I think, make intuitive sense. But so far there’s very little evidence that variation in the amount of mixing or spreading actually translates into differences in conservation status. No one has really tested it yet because there still isn’t enough tracking data to compare across a range of populations / species.

>>(Some gaps in my thinking)

The discussion above misses a few important things, which I might come back to another time:

  • Timing. For individuals from two non-breeding populations to truly ‘mix’ during the non-breeding season, they must co-occur in space and time.
  • I’m really interested in what determines the relative size of a species’ non-breeding area, relative to its breeding range. I guess this has a lot to do with evolutionary history.
  • Something about scale
  • What’s a population anyway?!

 

 

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