Mapping migration

At the end of Hitchcock’s classic suspense thriller The Birds, a badly gashed Tippi Hedren is helped away from a white house on the shore of northern California’s Bodega Bay by a taciturn Rod Taylor. Every surface, every wall, every telegraph wire is covered by a menacing sea of birds, continually shifting but eerily silent. It is one of the most powerful images of 20th century cinema. 

The central MacGuffin of the film (Hitchcock’s own term for the dramatic question that drives a story) is: Why are these birds acting in such a hostile fashion? Are they seeking revenge for our mistreatment of nature, are they a presentiment of some apocalyptic Doomsday, or are they sent by God as a punishment for the evil of humankind? 

If Hitchcock were alive today he would perhaps be glad to know that if we still don’t know why the birds in the movie were attacking humankind we now at least have a method of determining where they came from. One of the greatest puzzles in ecology - the mystery of migration - is in the process of being solved. 

Migration, the regular seasonal movement of individuals to and from breeding locations, is a fundamental part of the life cycle of many species and its most conspicuous practitioners are birds. For decades migratory biology has attracted the attention of ecologists but attempts to track migrating individuals - by banding or radio-telemetry, for example - have almost always resulted in failure. 

Consider the problems: only a small subset of a very large population is ever tagged and the individual elements that are tagged typically weigh less than 50g. Is it any surprise that the recovery rate of tagged individuals is only between one-in-1000 and one-in-100,000? But in recent years all of that has changed through the use of isotopes - particularly the stable isotopes - as natural abundance tracers. The technique relies on the fact that the relative abundance of several species of stable isotopes in the tissues of animals is intimately linked to geographic location. 

The use of natural abundance isotopes as migration tracers has been stimulated partly by fears over the decline in certain species. For example, in the US, southern breeding populations of the black-throated blue warbler (Dendroica caerulescens) have been declining and the worry is that this may be due to overwintering habitat loss. 

In a recent issue of Science a group based at Dartmouth College in New Hampshire, Stanford University and the Smithsonian Institution, have managed to track the movements of populations of the black-throated blue warbler using the abundances of isotopes of hydrogen and carbon in feathers. The two stable isotopes of hydrogen (H and D) are strongly latitude dependent because the heavier D isotope tends to condense out first and fall as precipitation. 

Since water vapour enters the atmosphere in the low latitudes through evaporation, and the D-rich water condenses out first, the net result is a gradient of decreasing D abundance with increasing latitude. When this is expressed as a ratio of D to H we find that isotope ratios decline with increasing latitude (Table 1). This gross pattern can be locally modified by seasonal changes, distance from the sea, and the altitude dependence of precipitation (rain tends to fall as warm, moist air is forced over mountain ranges). 

Table 1. Isotropic species in migration and trophic studies

Isotope	Convention	Standard*	Fractionation
Hydrogen/Deuterium	δ	SMOW	Precipitation/condensation
Carbon-12/Carbon-13	δ13 C	PDB	Proportion of C3/C4 plants
Nitrogen-15/Nitrogen-14	δ15 N	Air	Marine or terrestrial plants + degree of aridity
Strontium-87/Strontium-86	δ87 Sr	NBS-987	Source rock type

*SMOW = Standard Mean Ocean Water; PDB = Pee Dee Belemnite

In the continental US the overall effect of these interactions is that the hydrogen isotope ratios tend to decrease in a north-westerly direction. Since local vegetation reflects this unique isotopic signature, all higher elements in the food chain, including the migratory species under study do so too. 

C3 plants are an ancient group of carbon-fixing green plants. They use rubisco (the most abundant enzyme on Earth) to make a three-carbon compound which is the initial, stable product of photosynthesis. However, such plants may subsequently lose up to half of this recently fixed carbon through photorespiration at night. But since the onset of the global decline in atmospheric CO₂ that started at the end of the Cretaceous period (65 million years ago) another type of photosynthetic pathway has evolved, the C4 pathway. 

C4 plants possess biochemical and anatomical mechanisms that serve to concentrate CO₂ at the site of fixation, on the chloroplasts, which greatly reduces carbon loss through respiration. Their biochemistry is based on the enzyme PEP-carboxylase that makes an initial four-carbon compound. This is then transferred to specialised cells where the CO₂ is re-released and refixed using rubisco. These different biochemistries of C3 and C4 plants result in very different stable isotope signatures. 

Stable isotope ratios are conventionally expressed in parts per thousand (per mil or o/oo) difference from an agreed standard. C3 plants tend to cluster around an average δ ¹³ C of -25o/oo, while C4 plants have a much heavier isotopic composition, with an average δ ¹³ C of about -12o/oo. Once again there are local complications but these are minor. Since C3 plants predominate at high latitudes and C4 plants are more common in the low latitudes there is a gradient of decreasing δ ¹³ C with increasing latitude. 

The Dartmouth study used these isotopic gradients to identify in detail where birds from breeding populations of the black-throated blue warbler spent their breeding season and the winter. By analysing feather keratin (which preserves the isotopic ratios at the time of moult) they found that birds overwintering on the Caribbean islands from Cuba in the west to Puerto Rico in the east came from two latitudinally distinct breeding populations on the US mainland. 

Those overwintering in the west of the Greater Antilles (Cuba and Jamaica) came predominantly from northern populations ranging from Lake Michigan to as far north as New Brunswick, Canada, while those overwintering to the east of the range (the islands of Hispaniola and Puerto Rico) came predominantly from the southern part of the warbler’s breeding range in Georgia and Virginia. Interestingly, although there is latitudinal segregation within these overwintering populations there was no longitudinal segregation in the northern populations overwintering on Cuba and Jamaica. (The southern population came from too restricted a longitudinal range to test for segregation.) 

The study hinted at an explanation for why the southern breeding populations of the black-throated blue warbler are declining in abundance. These are the birds that overwinter in the eastern portion of the Greater Antilles and it may be significant that the island of Hispaniola has undergone massive deforestation in the past few years with consequent habitat destruction. 

This approach is not restricted to migratory species that live in the US. A similar study has used nitrogen isotopes together with carbon isotopes to show that two subspecies of the Scandinavian Willow Warbler (Phylloscopus trochilus trochilus and Phylloscopus trochilus acredula), which occupy distinct latitudinal ranges in Sweden, also occupy different overwintering grounds when they migrate to Africa. 

Nitrogen isotopes, like carbon isotopes vary geographically, although here the control is more complicated. The largest differences are between marine and terrestrial plants. Marine plants have more positive δ ¹⁵ N values than terrestrial plants and these differences are reflected further up the food chain, for example in migrating song birds. But there also appears to be an effect that is rather enigmatically linked to aridity. 

With increasing dryness δ ¹⁵ N ratios tend to increase (spanning a range from <10o/oo in regions with >800mm of rainfall a year to >15o/oo in areas withThe ratio of strontium-87 to Sr-86 can also be used as a migratory tracer (although note that Sr-87 can be formed radiogenically by the decay of Rb-87 so it is not a stable isotope in the sense of, for example, C-13). In an earlier study of the black-throated blue warbler the Dartmouth team found that breeding populations could be separated according to the δ ⁸⁷ Sr ratio preserved in their bones. Birds breeding near the core of the Appalachian Mountains in Georgia and Virginia, had high isotopic ratios of around 9o/oo, while those that lived further north near the Great Lakes region and southern Canada had lower values of around 4o/oo. 

This contrast is because of the higher proportion of geologically ancient, igneous, rocks in the Appalachians relative to the low δ ⁸⁷ Sr sedimentary rocks of the Great Lakes and southern Canadian regions. It seems that even the basic geology of a region has an isotopic spoor that can reflected in the animals that live there. 

So we can see then that to use stable isotopes as tracers of migratory animals various conditions must be satisfied. In the case of annually migrating animals the tissue to be analysed must be renewed seasonally otherwise the analysis will be an average of isotopic ratios incorporated into the animal throughout its lifetime. Feathers of course are ideal. 

Secondly, the isotope ratios must be an accurate reflection of their local levels. In practice carbon, nitrogen and strontium tend to be true reflections because they do not exchange with the atmosphere after incorporation with feather keratin. Hydrogen isotopes tend to be more exchangeable. Thirdly, the geographic variability of the isotope system in question needs to have been carefully mapped if small scale differences in regional origin are to be discerned. 

The use of Sr isotopes as migratory tracers, however, is more limited than the other isotope systems so far discussed for several reasons. First, geographic Sr variability has not been as well mapped as other isotope systems; secondly its areal distribution is not as straightforward as, for example, hydrogen or carbon; and thirdly, the technique demands the sacrifice of the animal in order that its bone tissue may be sampled, which rather defeats the underlying emphasis on conservation. 

Birds are not the only animal group that is being studied using natural abundance isotopes. In some cases isotope tracking can help preserve even severely endangered species. Today isotopes are offering undreamt of insights into the ecology of several animal species. These tools will undoubtedly be refined in the future and offer the hope that we can at last understand the complex web of feeding and migratory relationships among animals and use this information to help in their conservation. 

The final scene envisaged by Hitchcock for the end of The Birds was cut for budgetary reasons. It was to be a composite matte of the Golden Gate Bridge covered in a seething mass of birds preparing to assault San Francisco. Their place of origin - the ’where’ of the movie might now be solved through the use of isotopes - but the master of suspense can rest easy; his ’why’ remains as enigmatic as ever. 

Source: Chemistry in Britain

Richard Corfield is an isotope geochemist and science writer in the department of earth sciences, Oxford University, where he directs the stable isotope laboratory.