Rob G. Bijlsma

Avian population consequences of climate change are most severe for long-distance migrants in seasonal habitats

One consequence of climate change is an increasing mismatch between timing of food requirements and food availability. Such a mismatch is primarily expected in avian long-distance migrants because of their complex annual cycle, and in habitats with a seasonal food peak. Here we show that insectivorous long-distance migrant species in The Netherlands declined strongly (1984–2004) in forests, a habitat characterized by a short spring food peak, but that they did not decline in less seasonal marshes. Also, within generalist long-distance migrant species, populations declined more strongly in forests than in marshes. Forest-inhabiting migrant species arriving latest in spring declined most sharply, probably because their mismatch with the peak in food supply is greatest. Residents and short-distance migrants had non-declining populations in both habitats, suggesting that habitat quality did not deteriorate. Habitat-related differences in trends were most probably caused by climate change because at a European scale, long-distance migrants in forests declined more severely in western Europe, where springs have become considerably warmer, when compared with northern Europe, where temperatures during spring arrival and breeding have increased less. Our results suggest that trophic mismatches may have become a major cause for population declines in long-distance migrants in highly seasonal habitats.

Repeatability in spring arrival dates in Pied Flycatchers varies among years and sexes

Timing of arrival in long-distance migration could have fitness consequences: arrival too early impairs survival chances, whereas arrival too late reduces current reproductive success. Evolution thus may have favoured a phenotype that arrived at the optimal time. However, individuals within populations of longdistance migrant species arrive over a considerable time span, and often show consistency in whether they are early or late. This repeatability in arrival varies between studies, and we hypothesise it to be affected by conditions encountered en route or in winter. Here we report on the spring arrival dates of Pied Flycatchers Ficedula hypoleuca to their Dutch breeding sites during eight consecutive years. Our field estimates of arrival were highly accurate, as validated by geolocator data on 13 individuals. Years differed in mean arrival dates. Within years and sexes, arrival date generally spanned more than two weeks. First-year individuals arrived on average 4–5 days later than older individuals. Using repeated arrival dates of more than 500 individuals we show that (1) the overall arrival repeatabilities were similar for females and males, (2) arrival repeatabilities varied temporally, with individuals in consecutive years having sometimes moderate (R = 0.2) and sometimes rather high (>0.40) repeatabilities, and (3) individual females arrived later in their first than in their second year. In females, repeatabilities of arrival and laying dates were similar. We hypothesize that individual flycatchers have a high individual consistency in their spring migration departure date from the wintering grounds. However, previous studies suggest the expression of this individual schedule to be affected by environmental circumstances at the wintering grounds or by what is encountered en route, determining whether this variation is still present at arrival on the breeding grounds. Sexes seemed to differ in this respect, as yearto-year variation in repeatabilities of timing was explained by individual consistency in females, but not in males. We discuss the relevance of the observed variation for the potential for an evolutionary response when environments change.

Wintering White-Tailed Eagles Haliaeetus albicilla in the Netherlands: Aspects of Habitat Scale and Quality

The coastal wetlands of The Netherlands have always served as winter haunts for juvenile and immature White-tailed Eagles from breeding populations further north and east. Even as these populations were at their lowest ebb by the 1960s and 1970s, each winter a few individuals showed up, invariably favouring large wetlands with a good supply of wintering, mainly herbivorous, waterfowl. An analysis of the presence of eagles in the wetland Oostvaardersplassen showed that wintering numbers as well as the duration of individual stays increased as a function of the number and biomass of waterbirds present. During the pioneering stage of this newly reclaimed area the dynamic vegetation produced huge seed supplies that attracted vast numbers of herbivorous waterbirds. The increase in eagle numbers in the Oostvaardersplassen reserve preceded the recovery of the northern and eastern breeding populations of Whitetailed Eagles, but did not increase any further after reaching a maximum of 3–4 wintering birds, despite the fact that wintering numbers elsewhere in The Netherlands continued to rise in the wake of the increasing breeding population elsewhere in Europe. It is argued that ‘core area’ Oostvaardersplassen became saturated each winter in the 2000s. Additional eagles reaching The Netherlands spent the winter at alternative sites with smaller food supplies. In 1997–99, new waterbodies were created in the dry border zone of Oostvaardersplassen. The subsequent boost in waterbirds and fish may have triggered — in combination with the presence of undisturbed breeding habitat — the summering, and eventual breeding, of White-tailed Eagles from 2004 onwards. Water management towards improving dynamics in larger wetlands (both estuarine and riverine) may further boost food supplies for waterfowl and, hence, create suitable habitat for White-tailed Eagles elsewhere in The Netherlands.

Delayed Age at First Breeding and Experimental Removals Show Large Non-Breeding Surplus in Pied Flycatchers

Avian breeding populations have been shown to be regulated by territorial behaviour, often creating a surplus of non-breeding individuals. However, most evidence is of a male non-breeder surplus, whereas for a surplus to actually buffer a population both non-breeding males and females should be present. Here, we provide descriptive and experimental evidence for the existence of a population buffer consisting of mostly male and potentially also female Pied Flycatchers using nest box areas. First we show that local recruits often do not breed in their first year, with 23% of all recruiting males observed breeding in their first year, and 51% of females. When accounting for mortality in the years prior to observed first breeding, we estimate that only 9% of all first-year males breed locally, and 29% of first-year females. Similar percentages of first-year flycatchers skipping breeding have been observed in other study populations. We show that in the year of new establishment of our nest box plots, most known-aged flycatchers were first-year birds (77%), whereas after establishment, recruiting immigrants from the same source population were mostly older (28% first-year birds). An experimental removal of paired flycatchers from one study plot in two years (19 and 58 individuals removed) resulted in complete replacement by males and females. Male but not female replacements were younger than removed individuals. These results imply that a non-breeding surplus is present in Pied Flycatcher populations. The average later age at first-breeding in males compared to females, suggests that this non-breeding surplus is strongly male-biased. Skipping breeding in the first year(s) is not just caused by shortage of suitable nesting sites, as we observed on average 12% of males defending a nest box without pairing up with a female. Using stable isotopes ratios, we show that non-breeding first-year individuals do not stay at their African wintering grounds. Competition for nest sites is one cause for refraining from breeding, as shown by our experiments, but cannot be the sole cause, as many nest boxes remain unused in a season, and up to 20% of territorial males defend a nest box without pairing up with a female. We hypothesize that many young flycatchers arrive too late for breeding and are therefore not seen in their first year. Indeed first-year Pied Flycatchers that do breed/defend a nest box arrive on average later at the breeding grounds, and we argue that the non-observed group arrives even later. The causes of their later arrival could be the need for learning, lower quality wintering sites resulting in later departure, and/or a trade-off between low breeding success and the costs of early arrival. These could be general factors in long-distance migrants, and this pleads for a better understanding of how migration develops during ontogeny.

Demography of European Honey Buzzards Pernis apivorus

We set out to explore whether changes in fecundity and survival (or both) of European Honey Buzzards were associated with trends observed in The Netherlands. We used standardized monitoring data on numbers and reproduction from several study plots in The Netherlands over the period of 1974–2005. Survival estimates were based on recoveries of Honey Buzzards ringed throughout Europe (collated by EURING and individual ringing stations). Based on these values we computed the annual population growth rate with a time invariant population model, and used elasticity analysis to detect the parameter with the strongest influence on growth rate. Lambda was smaller than 1, suggesting a population decline, but confidence intervals around lambda were relatively wide. Elasticity of adult survival was much higher than of any other life history parameter, indicating that adult survival has potentially the greatest impact on annual population growth. Because data on reproduction varied strongly we estimated a 95% confidence interval for annual population growth by bootstrapping the reproduction data. Furthermore, we analysed the influence of stochastic food availability (i.e. wasp abundance) on reproduction, and through this demographic factor, on the population trend. The stochastic model suggests a clear population decline, which is consistent with observed declines across much of northern and western Europe (including The Netherlands). For the growth rate &lgr; to reach 1, a reproductive output of 1.16 young per pair was calculated to be necessary, much higher than the 0.53–0.88 found in two study sites in The Netherlands.