Other evidence points to events in breeding areas (as opposed to wintering areas) as being a major cause of population fluctuations in some Neotropical migrants. For many years, some migrant warblers have undergone long-term changes in breeding densities in association with changes in caterpillar abundance. Most outbreaks of defoliating caterpillars in eastern North American hardwood forests are sporadic, occurring in one location in one year and somewhere else the next. The probability that any particular stand will experience an outbreak in any given year is low. Every now and then, however, one caterpillar species of boreal regions, the Spruce Budworm Choristoneura fumiferana, increases over a period of years to reach plague proportions over wide areas, causing extensive defoliation. Several migrant bird species show strong numerical responses to this insect, including the Bay-breasted Warbler Dendroica castanea, Blackpoll Warbler D. striata, Tennessee Warbler Vermivora peregrina, Cape May Warbler Dendroica tigrina, Black-billed Cuckoo Coccyzus erythropthalmus and Yellow-billed Cuckoo C. americanus (Kendeigh 1947, Morris et al. 1958, Crawford & Jennings 1989). In one study, Bay-breasted Warblers Dendroica castanea increased in abundance from 2.5 pairs per 10 ha in uninfested stands to 300 pairs per 10 ha during an outbreak, Blackburnian Warblers D. fusca increased from 25-30 pairs to 100-125 pairs per 10 ha, and Tennessee Warblers from 0 to 125 pairs per 10 ha (Morris et al. 1958). Because in each outbreak the numbers of the birds rose over several years in parallel with the caterpillars, it was hard to tell how much the increase was due to immigration and how much to the high local breeding success, though both were involved.
The role of previous breeding success in the annual population changes of some Neotropical migrants was confirmed in studies in the Hubbard Brooks Experimental Forest in New Hampshire (Holmes et al. 1991, 1996, Sherry & Holmes 1992, Rodenhouse & Holmes 1992). Over a period of years, and in different forest plots, warbler breeding densities fluctuated in parallel with caterpillar biomass the previous summer. There was thus a correlation between the production of young in one year and the numbers of breeders returning the next. In other words, breeding success rather than overwinter survival had most influence on the annual changes in breeding numbers, and increases in breeding density were marked by greater recruitment of yearlings (Sherry & Holmes 1988, 1992). These findings were apparent in the American Redstart Setophaga ruticilla, Black-throated Blue Warbler Dendroica caerulescens, Black-throated Green Warbler D. virens, Red-eyed Vireo Vireo olivaceus and others, so in these species the year-to-year changes in breeding success and in subsequent breeding numbers were attributable to fluctuations in summer food supplies. Elsewhere, a relationship between the annual recruitment of yearlings to a breeding population and fledgling production the previous year was noted in another Neotropical migrant, the Prairie Warbler Dendroica discolor (Nolan 1978), paralleling the Pied Flycatcher Ficedula hypoleuca and others in Europe (see Figure 26.4; Virolainen 1984, Stenning et al. 1988).
The abundance of foliage-gleaning birds in the Hubbard Brook Forest, especially of warblers and vireos, increased in the early 1970s, coincident with a major outbreak of defoliating caterpillars (Holmes 1990). Furthermore, natural and experimentally-induced declines in food (mainly caterpillar) abundance were shown to reduce the frequency of re-nesting and second-brood attempts, as well as hatching and fledging success, and nestling growth rates, while increasing the frequency of nestling starvation. The general pattern seemed to be that food was abundant for birds in this forest only during Lepidoptera irruptions, but such events occurred infrequently, perhaps once every 10-20 years in any one forest-stand. Between these outbreaks, birds probably experienced prolonged periods of food limitation, as caterpillars were scarce for several years in succession. The Neotropical migrants thus showed population trends in this forest that were different from those of residents and short-distance migrants whose breeding numbers fluctuated from year to year mainly in line with winter severity. The Neotropical migrants were able to escape the northern winters, but the residents and short-distance migrants were not.
The importance of these findings in our present context is two-fold. First, they show that events in breeding areas which affect breeding success can influence subsequent breeding densities of migrants, apparently overriding any opposing effects of events in wintering areas. Second, they show that food supply can affect breeding success and breeding numbers in addition to any effects of predation and parasitism. To what extent, then, did recent declines in the numbers of certain migrant species in North America result from recent declines in caterpillar densities? Budworm caterpillars were abundant over wide areas in the late 1940s and again in the 1970s. One would therefore expect a decline in the numbers of some species in the 1950s-1960s and again in the 1980s-1990s. In addition, since the 1950s, insecticides have been used widely in eastern Canada to suppress caterpillar outbreaks. This practice is likely not only to have obliterated the 'good food years', but also to have suppressed all insect populations in the affected areas in the years of spraying. The influence of summer food supply on migrant population trends could thus be substantial and may have contributed to the declines in some species witnessed in the 1980s-1990s.
Another factor thought to operate on breeding areas to reduce migrant numbers is summer drought, again through its influence on insects and other food supplies. Severe declines in the numbers of several Neotropical migrant species in Wisconsin and Michigan during 1986-1988 coincided with drought (Blake et al. 1992), as did similar declines in Illinois (Robinson 1992). Because the migrants do not start nesting until June, they are more vulnerable to the effects of drought than are residents and short-distance migrants that start in April or May. Such effects would be expected to be short-lived, however, and followed by population recoveries as the weather conditions improved. In addition, long-term changes in the species composition of particular forest areas would be expected from the successional changes that occur as the forest matures, making the habitat less suitable for some species and more suitable for others (for examples, see Litwin & Smith 1992, Holmes & Sherry 2001).
Further evidence for the importance of events on breeding areas for declines in Nearctic-Neotropical migrants has come from studies in wintering areas. For example, between 1972 and 1990, birds were systematically netted in the Guanica Forest in Puerto Rico (Faaborg & Arendt 1992). Over the period concerned, the numbers of wintering migrants steadily declined. Some species, such as Northern Parula Warbler Parula americana and Prairie Warbler Dendoica discolor, were common at the start, but virtually absent in later years, while other migrants, such as Black-and-white Warbler Mniolilta varia, and various resident species, maintained their numbers. As no obvious change occurred in the forest itself during the study, the declines were attributable to events in breeding areas.
Much of the evidence thus points to changes in the eastern North American breeding areas as being the main factors underlying late twentieth century declines in the numbers of some forest-dwelling Nearctic-Neotropical migrants. Such changes may not have occurred over the entire range or in every wood, but they have clearly occurred in a sufficiently large proportion of woods to put the regional populations of some species into decline. Only time will tell how long the declines will continue.
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